转载自：http://zjutkz.net/2016/04/07/万能的APT！编译时注解的妙用/

本篇文章会带你了解什么是注解，注解的用法和分类，并且从knight和butterKnife的使用方式和源码着手让你更加了解编译时注解的好处。

不要脸的推广

在开始这篇文章之前，请允许我不要脸的推广一下我写的一个库，使用编译时注解＋apt＋javapoet去完成白天/夜间模式的切换，现在鲁棒性还不强，我还会继续维护，欢迎大家star和fork。

注解，字面意思是注释，解释，这个东西在jdk 5被添加进来之后发挥了很大的用处。

@Override
public void onCreate() {
    super.onCreate();
}

上面这个场景大家几乎每天都会看到吧，上面的那个@Override就是一个注解，它是什么意思呢？

**
 * Annotation type used to mark methods that override a method declaration in a
 * superclass. Compilers produce an error if a method annotated with @Override
 * does not actually override a method in a superclass.
 *
 * @since 1.5
 */
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}

注释写的很清楚了。哦对了，这里要区分两个概念，注解是我们这篇文章的主角，而注释则是用来标注代码用的，如果你高兴你可以这样。

//                            _ooOoo_  
//                           o8888888o  
//                           88" . "88  
//                           (| -_- |)  
//                            O\ = /O  
//                        ____/`---'\____  
//                      .   ' \\| |// `.  
//                       / \\||| : |||// \  
//                     / _||||| -:- |||||- \  
//                       | | \\\ - /// | |  
//                     | \_| ''\---/'' | |  
//                      \ .-\__ `-` ___/-. /  
//                   ___`. .' /--.--\ `. . __  
//                ."" '< `.___\_<|>_/___.' >'"".  
//               | | : `- \`.;`\ _ /`;.`/ - ` : | |  
//                 \ \ `-. \_ __\ /__ _/ .-` / /  
//         ======`-.____`-.___\_____/___.-`____.-'======  
//                            `=---='  
//  
//         .............................................  
//                  佛祖镇楼                  BUG辟易  
//          佛曰:  
//                  写字楼里写字间，写字间里程序员；  
//                  程序人员写程序，又拿程序换酒钱。  
//                  酒醒只在网上坐，酒醉还来网下眠；  
//                  酒醉酒醒日复日，网上网下年复年。  
//                  但愿老死电脑间，不愿鞠躬老板前；  
//                  奔驰宝马贵者趣，公交自行程序员。  
//                  别人笑我忒疯癫，我笑自己命太贱；  
//                  不见满街漂亮妹，哪个归得程序员？  

public class MyClass{

}

是不是很炫酷，哈哈哈。好了好了，言归正传，大家在看到上面@Override的代码的时候，如果不了解注解，是不是对上面所写的东西一头雾水呢？@Target是什么鬼？@Retantion又是啥，怎么注解里还用注解呢？没关系，下面让我带大家一起走进注解的世界。（如果有的同学对注解有所了解了，可以选择性的跳过这一段）

了解注解

注解的定义：

注解（也被成为元数据）为我们在代码中添加信息提供了一种形式化的方法，使我们可以在稍后某个时刻非常方便地使用这些数据。 ——————摘自《Thinking in Java》

简单来说注解的作用就是将我们的需要的数据储存起来，在以后的某一个时刻（可能是编译时，也可能是运行时）去调用它。

注解的使用方式：

首先如果你想定义一个注解，那么你必须在类的开头使用@interface这个限定符，像这样

1 2	public @interface MyAnnotation { }

这样你就完成了一个最简单的注解，可以去使用它了，就像这样

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2
3

@MyAnnotation
public class TestClass {
}

但是这样的注解并没有什么卵用，因为它什么也没做啊，定义出来干嘛呢？好玩吗？肯定不是，想要注解发挥它的作用，你必须要再写那么一点点代码。首先，刚刚说过，注解的作用是保存一些数据，那怎么保存呢？你可以在你的注解中定义一个属性，像这样

public @interface MyAnnotation {

    String value();
}

然后你就可以这样去修饰你想要注解的东西

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3

@MyAnnotation("TestClass")
public class TestClass {
}

这样，通过MyAnnotation这个注解，TestClass类就保存了”TestClass”这样一个String值，但是要怎么拿到这个值呢？是不是也可以用同样的方式去注解一个方法或者一个变量呢？

还记得上面提到的@Override吗？在这个类的上面还有两个注解@Target和@Retantion，它们被成为元注解，什么是元注解呢？元注解就是修饰注解的注解，意思就是你想要选择你的注解在编译期还是运行期其作用，就选择使用@Target，你想要选择你的注解去注解的东西是一个类，方法还是变量，就使用@Retantion。下面看看这两个元注解的具体内容。

Retantion

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public @interface Retention {
    RetentionPolicy value();
}

public enum RetentionPolicy {
    /**
     * Annotation is only available in the source code.
     */
    SOURCE,
    /**
     * Annotation is available in the source code and in the class file, but not
     * at runtime. This is the default policy.
     */
    CLASS,
    /**
     * Annotation is available in the source code, the class file and is
     * available at runtime.
     */
    RUNTIME
}

Retention注解有一个属性value，是RetentionPolicy类型的，Enum RetentionPolicy是一个枚举类型，
这个枚举决定了Retention注解应该如何去保持，也可理解为Rentention 搭配 RententionPolicy使用。

RetentionPolicy有3个值：CLASS,RUNTIME,SOURCE

用@Retention(RetentionPolicy.CLASS)修饰的注解，表示注解的信息被保留在class文件(字节码文件)中当程序编译时，但不会被虚拟机读取在运行的时候；
用@Retention(RetentionPolicy.SOURCE )修饰的注解,表示注解的信息会被编译器抛弃，不会留在class文件中，注解的信息只会留在源文件中；
用@Retention(RetentionPolicy.RUNTIME )修饰的注解，表示注解的信息被保留在class文件(字节码文件)中当程序编译时，会被虚拟机保留在运行时，
所以他们可以用反射的方式读取。
RetentionPolicy.RUNTIME 可以让你从JVM中读取Annotation注解的信息，以便在分析程序的时候使用.

Target

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public @interface Target {
    ElementType[] value();
}

public enum ElementType {
    /**
     * Class, interface or enum declaration.
     */
    TYPE,
    /**
     * Field declaration.
     */
    FIELD,
    /**
     * Method declaration.
     */
    METHOD,
    /**
     * Parameter declaration.
     */
    PARAMETER,
    /**
     * Constructor declaration.
     */
    CONSTRUCTOR,
    /**
     * Local variable declaration.
     */
    LOCAL_VARIABLE,
    /**
     * Annotation type declaration.
     */
    ANNOTATION_TYPE,
    /**
     * Package declaration.
     */
    PACKAGE
}

它有一个属性ElementType也是枚举类型，值就是上面代码中写的这样。

如@Target(ElementType.METHOD) 修饰的注解表示该注解只能用来修饰在方法上。其他同理。

这样大家清楚了吧？如果你想要你的注解在运行时起作用，并且只能修饰一个类，那你可以这样定义

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface MyAnnotation {

    String value();
}

注解的分类：

从取值的方式上来说，注解可以分成两类——编译时注解和运行时注解。

运行时注解

运行时注解表示你只能在程序运行时去操作它，那怎么样才能在运行时去操作呢？很简单，使用反射。

有一个非常非常著名的开源库就用到了运行时注解，那就是retrofit,Github上star数破万，现今最火的网络库，特别是结合RxJava来使用。如果大家想了解retrofit内部的源码，大家可以参考这篇文章。

好了，说完retrofit，大家有没有想自己动手写一个注解框架呢？这里再给大家推荐一篇文章，hongyang大神写的，看完相信大家会对运行时注解有很好的理解。

编译时注解

既然运行时注解这么屌，那为什么我们这篇文章的主角是编译时注解呢？因为运行时注解毕竟是在程序运行时去进行操作的，用到了反射，在效率方面嘛。。。都懂的，所以，编译时注解势在必行！

上面在介绍运行时注解的时候提到了retrofit，那有没有什么第三方库是使用编译时注解的呢？那真是多了去了。。。dagger，butterKnife，包括谷歌官方推荐的data binding库，都用到了编译时注解，而且，和retrofit不同的是，retrofit的核心是动态代理（个人理解，不喜勿喷。。）而像dagger，dagger2，butterKnife这些库，核心就是编译时注解＋APT＋动态生成字节码！可见其强大之处。

打造使用编译时注解完成的一个换肤框架——Knight

编译时注解基本使用方式

上面的文章带大家了解了什么注解以及注解的基本使用方式，下面来讲讲今天的主题——编译时注解。

编译时注解，顾名思义，就是编译时去处理的注解，要怎么做呢？下面我一步一步教大家。

一. 新建一个Android Studio工程

这步没什么好说的，跳过跳过。

二. 在工程中创建一个java library并且让我们的app module去依赖这个java库

注意一定要是java library不能是android library，因为如果你新建的是一个android library，javax中的一些你是不能使用的，所以也就没办法完成编译时注解了。

三. 在java library中创建一个注解类

@Retention(RetentionPolicy.CLASS)
@Target(ElementType.METHOD)
public @interface MyAnnotation {

    String value();
}

根据我们上面讲到的知识，@Retention(RetentionPolicy.CLASS)表示这是一个在编译器使用的注解，@Target(ElementType.METHOD)表示它可以注解的范围是方法，String value();表示它接受一个String类型的参数。我们可以这样去修饰一个方法。

public class MainActivity extends AppCompatActivity {

    @MyAnnotation("onCreate")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    @MyAnnotation("print method")
    private String print(){
        return "print!";
    }
}

四. 在java library中创建一个类继承自AbstractProcessor

public class MyProcessor extends AbstractProcessor {

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        return false;
    }
}

五.在java library工程的src/main目录上创建resources文件夹

六. 在resources文件夹中创建META-INF包，在META-INF包中创建services包

七. 在services包下创建javax.annotation.processing.Processor文件

八. 在javax.annotation.processing.Processor文件中写下你继承自AbstractProcessor类的那个子类的全完整路径

最终截图如下。

好了，至此，准备工作就做完了。这样，在我们的编译器在编译的时候就会去执行我们的processor类中的process方法，我们可以在这个方法中拿到和注解相关的所有东西。这就是所有编译时注解框架的基础。这里我举个最简单的例子吧，打印出我们注解的方法的名称，参数，返回值和注解的值。

//表示支持的注解类
@SupportedAnnotationTypes("com.example.MyAnnotation")
public class MyProcessor extends AbstractProcessor {

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        //取出每一个用MyAnnotation注解的元素
        for(Element element : roundEnv.getElementsAnnotatedWith(MyAnnotation.class)){
            //如果这个元素是一个方法
            if(element.getKind() == ElementKind.METHOD){
                //强转成方法对应的element，同理，如果你的注解是一个类，那你可以强转成TypeElement
                ExecutableElement executableElement = (ExecutableElement)element;

                //打印方法名
                System.out.println(executableElement.getSimpleName());

                //打印方法的返回类型
                System.out.println(executableElement.getReturnType().toString());

                //获取方法所有的参数
                List<? extends VariableElement> params = executableElement.getParameters();
                //逐个打印参数名
                for(VariableElement variableElement : params){
                    System.out.println(variableElement.getSimpleName());
                }

                //打印注解的值
                System.out.println(executableElement.getAnnotation(MyAnnotation.class).value());
            }
            System.out.println("------------------------------");
        }
        return false;
    }
}

这个类的注释已经很清楚了，相信大家都能看懂。好了，现在大家可以尝试着编译一下，在命令行中使用gradle构建

如果在编译时出现了如下错误

这说明你的java版本有问题，其实很好确定原因的，”bad class file magic or version”，说明子节码里的魔数或者版本号出了问题。

解决的方案就是首先你得确保的你java环境是1.8以下的，并且在你对应的java library的build.gradle中添加sourceCompatibility = “1.7”这句话。

这样再执行build就不会有问题了～

好了，执行build之后让我们看看terminal中都输出了什么。

看到了吗！最后那一堆输出，依次打印上前面我们注解上的两个方法的名字，返回类型，参数名称和注解值，另外说一个细节，看看输出内容的上面一句话，compileDebugJavaWithJavac，javac！这证实了我们的注解确实是在编译的时候去执行的。

好了，上面介绍了编译时注解的简单使用方法，下面开始探究我们的夜间模式框架——Knight。

Knight使用介绍

首先先确定Knight是什么，它是一个用来实现白天/夜间模式切换的框架，现在Android上实现这种业务逻辑的方法其实有不少了，但是我总结了下，有些是直接重置了theme，然后调用activity的recreate()方法；有些是侵入了activity的源码，反射替换了layoutInflater，然后在xml中去定义资源。我觉得两者都有各自不好的地方，前者不好的地方是它会重新刷新activity的生命周期，这样就要自己保存数据，不然会被清空；后者不好的地方在于侵入性太强了，而Knight的特点在于使用注解的方式去保存资源，使用比较简单，低侵入性，并且不会刷新activity的生命周期，先已支持插件式换肤。下面来看看如此使用吧。

我们直接取一个简单的界面来演示。

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/container"
    android:orientation="vertical"
    android:background="#ffffff"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Button
        android:id="@+id/change"
        android:text="change"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:onClick="change"/>

    <TextView
        android:id="@+id/tv"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="presented by zjutkz!"
        android:textSize="25sp"
        android:textColor="#000000"/>

    <ImageView
        android:id="@+id/iv"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:scaleType="centerCrop"
        android:src="@drawable/nav_header_day"/>
</LinearLayout>

它的界面展示是这样的

我们想要的效果是点击按钮之后切换到夜间模式，整个背景变黑，字颜色变白，然后再换个图片。来看看我们的activity

public class TestActivity extends AppCompatActivity{

    @Knight(resName = "backgroundColor",nightResId = {R.color.night_background},dayResId = {R.color.day_background})
    @Bind(R.id.container)
    LinearLayout container;

    @Bind(R.id.change)
    Button btn;

    @Knight(resName = "background,textColor",nightResId = {R.color.night_background,R.color.night_text_color},dayResId = {R.color.day_background,R.color.day_text_color})
    @Bind(R.id.tv)
    TextView tv;

    @Knight(resName = "src",nightResId = {R.drawable.nav_header_night},dayResId = {R.drawable.nav_header_day})
    @Bind(R.id.iv)
    ImageView iv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_test);

        ButterKnife.bind(this);
    }

    public void change(View view){
        //KnightUtil.changeToDay(this);
        KnightUtil.changeToNight(this);
    }
}

这里我结合了butterKnife一起使用，注意如果你也和我一样使用了butterKnife，请在对应的build.gradle中加上这样一句代码：

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packagingOptions {
    exclude 'META-INF/services/javax.annotation.processing.Processor'
}

可以看到我们的直接在对应的view上添加了@Knight注解，注解里面的值也很好懂，想要替换什么就写什么，注意现在Knight支持四个元素——src，background，backgroundColor和textColor，你只要在注解的第一个值中设置，后面两个值就是对应的夜晚/白天的资源，现在对于颜色只支持在resource文件中定义。当然，Knight也支持注解自定义的方法，具体用法可以去看我github上的demo。

好了，让我们看看点击button以后的界面。

具体我们的Knight是怎么做的呢？这里我不做分析了，大家可以自己去我的github上看代码，基本的原理我上一节已经讲了，里面用到的一个核心依赖javapoet，大家可以去github上查看使用方式，具体的api风格还是比较清晰的，相信大家都能看懂。

butterKnife源码分析

说到butterKnife，大家一定不会陌生，作为Android中最火的几个库之一，它的作用基本就是用来代替传统的findViewById和setOnClickListener等操作，让程序员们不用写大量重复的代码，节省时间。

由于butterKnife的注解类型太多，比如有最常用的Bind，OnClick，还有OnLongClick，OnItemClick等等，这里我就不每一个都分析了，就以OnClick为例，其他的都是异曲同工。

首先来看看butterKnife的使用方式。

你在xml中写了这样一段布局

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context="zjutkz.com.annotationtest.MainActivity">


    <Button
        android:id="@+id/btn"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="button"/>

</android.support.design.widget.CoordinatorLayout>

然后是java代码

public class MainActivity extends AppCompatActivity {

    @Bind(R.id.btn)
    Button btn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ButterKnife.bind(this);
    }
}

可以看到简单的使用Bind注解并且在开始的时候调用bind()函数就完成了findViewById的工作，是不是很省事呢。

@Bind注解

下面我们来看看Bind注解具体是什么

@Retention(CLASS) @Target(FIELD)
public @interface Bind {
  /** View ID to which the field will be bound. */
  int[] value();
}

很简单对吧，有了上节的基础，这个注解很容易看懂。@Retention(CLASS)表示它是一个编译时注解，@Target(FIELD)表示它只接受注解参数，而int[] value();则表示它有一个参数是int型的数组，这个int型的数组对应到具体的代码就是R.id.xx。

processor源码

看完Bind注解具体的内容，下面就让我们看看butterKnife的processor都做了什么

package butterknife.internal;

import android.view.View;
import butterknife.Bind;
import butterknife.BindBool;
import butterknife.BindColor;
import butterknife.BindDimen;
import butterknife.BindDrawable;
import butterknife.BindInt;
import butterknife.BindString;
import butterknife.OnCheckedChanged;
import butterknife.OnClick;
import butterknife.OnEditorAction;
import butterknife.OnFocusChange;
import butterknife.OnItemClick;
import butterknife.OnItemLongClick;
import butterknife.OnItemSelected;
import butterknife.OnLongClick;
import butterknife.OnPageChange;
import butterknife.OnTextChanged;
import butterknife.OnTouch;
import java.io.IOException;
import java.io.PrintWriter;
import java.io.StringWriter;
import java.io.Writer;
import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.BitSet;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.Filer;
import javax.annotation.processing.ProcessingEnvironment;
import javax.annotation.processing.RoundEnvironment;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.AnnotationMirror;
import javax.lang.model.element.Element;
import javax.lang.model.element.ExecutableElement;
import javax.lang.model.element.Modifier;
import javax.lang.model.element.TypeElement;
import javax.lang.model.element.VariableElement;
import javax.lang.model.type.ArrayType;
import javax.lang.model.type.DeclaredType;
import javax.lang.model.type.TypeKind;
import javax.lang.model.type.TypeMirror;
import javax.lang.model.type.TypeVariable;
import javax.lang.model.util.Elements;
import javax.lang.model.util.Types;
import javax.tools.JavaFileObject;

import static javax.lang.model.element.ElementKind.CLASS;
import static javax.lang.model.element.ElementKind.INTERFACE;
import static javax.lang.model.element.ElementKind.METHOD;
import static javax.lang.model.element.Modifier.PRIVATE;
import static javax.lang.model.element.Modifier.STATIC;
import static javax.tools.Diagnostic.Kind.ERROR;

public final class ButterKnifeProcessor extends AbstractProcessor {
  public static final String SUFFIX = "$$ViewBinder";
  public static final String ANDROID_PREFIX = "android.";
  public static final String JAVA_PREFIX = "java.";
  static final String VIEW_TYPE = "android.view.View";
  private static final String COLOR_STATE_LIST_TYPE = "android.content.res.ColorStateList";
  private static final String DRAWABLE_TYPE = "android.graphics.drawable.Drawable";
  private static final String NULLABLE_ANNOTATION_NAME = "Nullable";
  private static final String ITERABLE_TYPE = "java.lang.Iterable<?>";
  private static final String LIST_TYPE = List.class.getCanonicalName();
  private static final List<Class<? extends Annotation>> LISTENERS = Arrays.asList(//
      OnCheckedChanged.class, //
      OnClick.class, //
      OnEditorAction.class, //
      OnFocusChange.class, //
      OnItemClick.class, //
      OnItemLongClick.class, //
      OnItemSelected.class, //
      OnLongClick.class, //
      OnPageChange.class, //
      OnTextChanged.class, //
      OnTouch.class //
  );

  private Elements elementUtils;
  private Types typeUtils;
  private Filer filer;

  @Override public synchronized void init(ProcessingEnvironment env) {
    super.init(env);

    elementUtils = env.getElementUtils();
    typeUtils = env.getTypeUtils();
    filer = env.getFiler();
  }

  @Override public Set<String> getSupportedAnnotationTypes() {
    Set<String> types = new LinkedHashSet<String>();

    types.add(Bind.class.getCanonicalName());

    for (Class<? extends Annotation> listener : LISTENERS) {
      types.add(listener.getCanonicalName());
    }

    types.add(BindBool.class.getCanonicalName());
    types.add(BindColor.class.getCanonicalName());
    types.add(BindDimen.class.getCanonicalName());
    types.add(BindDrawable.class.getCanonicalName());
    types.add(BindInt.class.getCanonicalName());
    types.add(BindString.class.getCanonicalName());

    return types;
  }

  @Override public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {
    Map<TypeElement, BindingClass> targetClassMap = findAndParseTargets(env);

    for (Map.Entry<TypeElement, BindingClass> entry : targetClassMap.entrySet()) {
      TypeElement typeElement = entry.getKey();
      BindingClass bindingClass = entry.getValue();

      try {
        JavaFileObject jfo = filer.createSourceFile(bindingClass.getFqcn(), typeElement);
        Writer writer = jfo.openWriter();
        writer.write(bindingClass.brewJava());
        writer.flush();
        writer.close();
      } catch (IOException e) {
        error(typeElement, "Unable to write view binder for type %s: %s", typeElement,
            e.getMessage());
      }
    }

    return true;
  }

  private Map<TypeElement, BindingClass> findAndParseTargets(RoundEnvironment env) {
    Map<TypeElement, BindingClass> targetClassMap = new LinkedHashMap<TypeElement, BindingClass>();
    Set<String> erasedTargetNames = new LinkedHashSet<String>();

    // Process each @Bind element.
    for (Element element : env.getElementsAnnotatedWith(Bind.class)) {
      try {
        parseBind(element, targetClassMap, erasedTargetNames);
      } catch (Exception e) {
        logParsingError(element, Bind.class, e);
      }
    }

    // Process each annotation that corresponds to a listener.
    for (Class<? extends Annotation> listener : LISTENERS) {
      findAndParseListener(env, listener, targetClassMap, erasedTargetNames);
    }

    // Process each @BindBool element.
    for (Element element : env.getElementsAnnotatedWith(BindBool.class)) {
      try {
        parseResourceBool(element, targetClassMap, erasedTargetNames);
      } catch (Exception e) {
        logParsingError(element, BindBool.class, e);
      }
    }

    // Process each @BindColor element.
    for (Element element : env.getElementsAnnotatedWith(BindColor.class)) {
      try {
        parseResourceColor(element, targetClassMap, erasedTargetNames);
      } catch (Exception e) {
        logParsingError(element, BindColor.class, e);
      }
    }

    // Process each @BindDimen element.
    for (Element element : env.getElementsAnnotatedWith(BindDimen.class)) {
      try {
        parseResourceDimen(element, targetClassMap, erasedTargetNames);
      } catch (Exception e) {
        logParsingError(element, BindDimen.class, e);
      }
    }

    // Process each @BindDrawable element.
    for (Element element : env.getElementsAnnotatedWith(BindDrawable.class)) {
      try {
        parseResourceDrawable(element, targetClassMap, erasedTargetNames);
      } catch (Exception e) {
        logParsingError(element, BindDrawable.class, e);
      }
    }

    // Process each @BindInt element.
    for (Element element : env.getElementsAnnotatedWith(BindInt.class)) {
      try {
        parseResourceInt(element, targetClassMap, erasedTargetNames);
      } catch (Exception e) {
        logParsingError(element, BindInt.class, e);
      }
    }

    // Process each @BindString element.
    for (Element element : env.getElementsAnnotatedWith(BindString.class)) {
      try {
        parseResourceString(element, targetClassMap, erasedTargetNames);
      } catch (Exception e) {
        logParsingError(element, BindString.class, e);
      }
    }

    // Try to find a parent binder for each.
    for (Map.Entry<TypeElement, BindingClass> entry : targetClassMap.entrySet()) {
      String parentClassFqcn = findParentFqcn(entry.getKey(), erasedTargetNames);
      if (parentClassFqcn != null) {
        entry.getValue().setParentViewBinder(parentClassFqcn + SUFFIX);
      }
    }

    return targetClassMap;
  }

  private void logParsingError(Element element, Class<? extends Annotation> annotation,
      Exception e) {
    StringWriter stackTrace = new StringWriter();
    e.printStackTrace(new PrintWriter(stackTrace));
    error(element, "Unable to parse @%s binding.\n\n%s", annotation.getSimpleName(), stackTrace);
  }

  private boolean isInaccessibleViaGeneratedCode(Class<? extends Annotation> annotationClass,
      String targetThing, Element element) {
    boolean hasError = false;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

    // Verify method modifiers.
    Set<Modifier> modifiers = element.getModifiers();
    if (modifiers.contains(PRIVATE) || modifiers.contains(STATIC)) {
      error(element, "@%s %s must not be private or static. (%s.%s)",
          annotationClass.getSimpleName(), targetThing, enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

    // Verify containing type.
    if (enclosingElement.getKind() != CLASS) {
      error(enclosingElement, "@%s %s may only be contained in classes. (%s.%s)",
          annotationClass.getSimpleName(), targetThing, enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

    // Verify containing class visibility is not private.
    if (enclosingElement.getModifiers().contains(PRIVATE)) {
      error(enclosingElement, "@%s %s may not be contained in private classes. (%s.%s)",
          annotationClass.getSimpleName(), targetThing, enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

    return hasError;
  }

  private boolean isBindingInWrongPackage(Class<? extends Annotation> annotationClass,
      Element element) {
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();
    String qualifiedName = enclosingElement.getQualifiedName().toString();

    if (qualifiedName.startsWith(ANDROID_PREFIX)) {
      error(element, "@%s-annotated class incorrectly in Android framework package. (%s)",
          annotationClass.getSimpleName(), qualifiedName);
      return true;
    }
    if (qualifiedName.startsWith(JAVA_PREFIX)) {
      error(element, "@%s-annotated class incorrectly in Java framework package. (%s)",
          annotationClass.getSimpleName(), qualifiedName);
      return true;
    }

    return false;
  }

  private void parseBind(Element element, Map<TypeElement, BindingClass> targetClassMap,
      Set<String> erasedTargetNames) {
    // Verify common generated code restrictions.
    if (isInaccessibleViaGeneratedCode(Bind.class, "fields", element)
        || isBindingInWrongPackage(Bind.class, element)) {
      return;
    }

    TypeMirror elementType = element.asType();
    if (elementType.getKind() == TypeKind.ARRAY) {
      parseBindMany(element, targetClassMap, erasedTargetNames);
    } else if (LIST_TYPE.equals(doubleErasure(elementType))) {
      parseBindMany(element, targetClassMap, erasedTargetNames);
    } else if (isSubtypeOfType(elementType, ITERABLE_TYPE)) {
      error(element, "@%s must be a List or array. (%s.%s)", Bind.class.getSimpleName(),
          ((TypeElement) element.getEnclosingElement()).getQualifiedName(),
          element.getSimpleName());
    } else {
      parseBindOne(element, targetClassMap, erasedTargetNames);
    }
  }

  private void parseBindOne(Element element, Map<TypeElement, BindingClass> targetClassMap,
      Set<String> erasedTargetNames) {
    boolean hasError = false;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

    // Verify that the target type extends from View.
    TypeMirror elementType = element.asType();
    if (elementType.getKind() == TypeKind.TYPEVAR) {
      TypeVariable typeVariable = (TypeVariable) elementType;
      elementType = typeVariable.getUpperBound();
    }
    if (!isSubtypeOfType(elementType, VIEW_TYPE) && !isInterface(elementType)) {
      error(element, "@%s fields must extend from View or be an interface. (%s.%s)",
          Bind.class.getSimpleName(), enclosingElement.getQualifiedName(), element.getSimpleName());
      hasError = true;
    }

    // Assemble information on the field.
    int[] ids = element.getAnnotation(Bind.class).value();
    if (ids.length != 1) {
      error(element, "@%s for a view must only specify one ID. Found: %s. (%s.%s)",
          Bind.class.getSimpleName(), Arrays.toString(ids), enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

    if (hasError) {
      return;
    }

    int id = ids[0];
    BindingClass bindingClass = targetClassMap.get(enclosingElement);
    if (bindingClass != null) {
      ViewBindings viewBindings = bindingClass.getViewBinding(id);
      if (viewBindings != null) {
        Iterator<FieldViewBinding> iterator = viewBindings.getFieldBindings().iterator();
        if (iterator.hasNext()) {
          FieldViewBinding existingBinding = iterator.next();
          error(element, "Attempt to use @%s for an already bound ID %d on '%s'. (%s.%s)",
              Bind.class.getSimpleName(), id, existingBinding.getName(),
              enclosingElement.getQualifiedName(), element.getSimpleName());
          return;
        }
      }
    } else {
      bindingClass = getOrCreateTargetClass(targetClassMap, enclosingElement);
    }

    String name = element.getSimpleName().toString();
    String type = elementType.toString();
    boolean required = isRequiredBinding(element);

    FieldViewBinding binding = new FieldViewBinding(name, type, required);
    bindingClass.addField(id, binding);

    // Add the type-erased version to the valid binding targets set.
    erasedTargetNames.add(enclosingElement.toString());
  }

  private void parseBindMany(Element element, Map<TypeElement, BindingClass> targetClassMap,
      Set<String> erasedTargetNames) {
    boolean hasError = false;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

    // Verify that the type is a List or an array.
    TypeMirror elementType = element.asType();
    String erasedType = doubleErasure(elementType);
    TypeMirror viewType = null;
    FieldCollectionViewBinding.Kind kind = null;
    if (elementType.getKind() == TypeKind.ARRAY) {
      ArrayType arrayType = (ArrayType) elementType;
      viewType = arrayType.getComponentType();
      kind = FieldCollectionViewBinding.Kind.ARRAY;
    } else if (LIST_TYPE.equals(erasedType)) {
      DeclaredType declaredType = (DeclaredType) elementType;
      List<? extends TypeMirror> typeArguments = declaredType.getTypeArguments();
      if (typeArguments.size() != 1) {
        error(element, "@%s List must have a generic component. (%s.%s)",
            Bind.class.getSimpleName(), enclosingElement.getQualifiedName(),
            element.getSimpleName());
        hasError = true;
      } else {
        viewType = typeArguments.get(0);
      }
      kind = FieldCollectionViewBinding.Kind.LIST;
    } else {
      throw new AssertionError();
    }
    if (viewType != null && viewType.getKind() == TypeKind.TYPEVAR) {
      TypeVariable typeVariable = (TypeVariable) viewType;
      viewType = typeVariable.getUpperBound();
    }

    // Verify that the target type extends from View.
    if (viewType != null && !isSubtypeOfType(viewType, VIEW_TYPE) && !isInterface(viewType)) {
      error(element, "@%s List or array type must extend from View or be an interface. (%s.%s)",
          Bind.class.getSimpleName(), enclosingElement.getQualifiedName(), element.getSimpleName());
      hasError = true;
    }

    if (hasError) {
      return;
    }

    // Assemble information on the field.
    String name = element.getSimpleName().toString();
    int[] ids = element.getAnnotation(Bind.class).value();
    if (ids.length == 0) {
      error(element, "@%s must specify at least one ID. (%s.%s)", Bind.class.getSimpleName(),
          enclosingElement.getQualifiedName(), element.getSimpleName());
      return;
    }

    Integer duplicateId = findDuplicate(ids);
    if (duplicateId != null) {
      error(element, "@%s annotation contains duplicate ID %d. (%s.%s)", Bind.class.getSimpleName(),
          duplicateId, enclosingElement.getQualifiedName(), element.getSimpleName());
    }

    assert viewType != null; // Always false as hasError would have been true.
    String type = viewType.toString();
    boolean required = isRequiredBinding(element);

    BindingClass bindingClass = getOrCreateTargetClass(targetClassMap, enclosingElement);
    FieldCollectionViewBinding binding = new FieldCollectionViewBinding(name, type, kind, required);
    bindingClass.addFieldCollection(ids, binding);

    erasedTargetNames.add(enclosingElement.toString());
  }

  private void parseResourceBool(Element element, Map<TypeElement, BindingClass> targetClassMap,
      Set<String> erasedTargetNames) {
    boolean hasError = false;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

    // Verify that the target type is bool.
    if (element.asType().getKind() != TypeKind.BOOLEAN) {
      error(element, "@%s field type must be 'boolean'. (%s.%s)",
          BindBool.class.getSimpleName(), enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

    // Verify common generated code restrictions.
    hasError |= isInaccessibleViaGeneratedCode(BindBool.class, "fields", element);
    hasError |= isBindingInWrongPackage(BindBool.class, element);

    if (hasError) {
      return;
    }

    // Assemble information on the field.
    String name = element.getSimpleName().toString();
    int id = element.getAnnotation(BindBool.class).value();

    BindingClass bindingClass = getOrCreateTargetClass(targetClassMap, enclosingElement);
    FieldResourceBinding binding = new FieldResourceBinding(id, name, "getBoolean");
    bindingClass.addResource(binding);

    erasedTargetNames.add(enclosingElement.toString());
  }

  private void parseResourceColor(Element element, Map<TypeElement, BindingClass> targetClassMap,
      Set<String> erasedTargetNames) {
    boolean hasError = false;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

    // Verify that the target type is int or ColorStateList.
    boolean isColorStateList = false;
    TypeMirror elementType = element.asType();
    if (COLOR_STATE_LIST_TYPE.equals(elementType.toString())) {
      isColorStateList = true;
    } else if (elementType.getKind() != TypeKind.INT) {
      error(element, "@%s field type must be 'int' or 'ColorStateList'. (%s.%s)",
          BindColor.class.getSimpleName(), enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

    // Verify common generated code restrictions.
    hasError |= isInaccessibleViaGeneratedCode(BindColor.class, "fields", element);
    hasError |= isBindingInWrongPackage(BindColor.class, element);

    if (hasError) {
      return;
    }

    // Assemble information on the field.
    String name = element.getSimpleName().toString();
    int id = element.getAnnotation(BindColor.class).value();

    BindingClass bindingClass = getOrCreateTargetClass(targetClassMap, enclosingElement);
    FieldResourceBinding binding = new FieldResourceBinding(id, name,
        isColorStateList ? "getColorStateList" : "getColor");
    bindingClass.addResource(binding);

    erasedTargetNames.add(enclosingElement.toString());
  }

  private void parseResourceDimen(Element element, Map<TypeElement, BindingClass> targetClassMap,
      Set<String> erasedTargetNames) {
    boolean hasError = false;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

    // Verify that the target type is int or ColorStateList.
    boolean isInt = false;
    TypeMirror elementType = element.asType();
    if (elementType.getKind() == TypeKind.INT) {
      isInt = true;
    } else if (elementType.getKind() != TypeKind.FLOAT) {
      error(element, "@%s field type must be 'int' or 'float'. (%s.%s)",
          BindDimen.class.getSimpleName(), enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

    // Verify common generated code restrictions.
    hasError |= isInaccessibleViaGeneratedCode(BindDimen.class, "fields", element);
    hasError |= isBindingInWrongPackage(BindDimen.class, element);

    if (hasError) {
      return;
    }

    // Assemble information on the field.
    String name = element.getSimpleName().toString();
    int id = element.getAnnotation(BindDimen.class).value();

    BindingClass bindingClass = getOrCreateTargetClass(targetClassMap, enclosingElement);
    FieldResourceBinding binding = new FieldResourceBinding(id, name,
        isInt ? "getDimensionPixelSize" : "getDimension");
    bindingClass.addResource(binding);

    erasedTargetNames.add(enclosingElement.toString());
  }

  private void parseResourceDrawable(Element element, Map<TypeElement, BindingClass> targetClassMap,
      Set<String> erasedTargetNames) {
    boolean hasError = false;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

    // Verify that the target type is Drawable.
    if (!DRAWABLE_TYPE.equals(element.asType().toString())) {
      error(element, "@%s field type must be 'Drawable'. (%s.%s)",
          BindDrawable.class.getSimpleName(), enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

    // Verify common generated code restrictions.
    hasError |= isInaccessibleViaGeneratedCode(BindDrawable.class, "fields", element);
    hasError |= isBindingInWrongPackage(BindDrawable.class, element);

    if (hasError) {
      return;
    }

    // Assemble information on the field.
    String name = element.getSimpleName().toString();
    int id = element.getAnnotation(BindDrawable.class).value();

    BindingClass bindingClass = getOrCreateTargetClass(targetClassMap, enclosingElement);
    FieldResourceBinding binding = new FieldResourceBinding(id, name, "getDrawable");
    bindingClass.addResource(binding);

    erasedTargetNames.add(enclosingElement.toString());
  }

  private void parseResourceInt(Element element, Map<TypeElement, BindingClass> targetClassMap,
      Set<String> erasedTargetNames) {
    boolean hasError = false;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

    // Verify that the target type is int.
    if (element.asType().getKind() != TypeKind.INT) {
      error(element, "@%s field type must be 'int'. (%s.%s)", BindInt.class.getSimpleName(),
          enclosingElement.getQualifiedName(), element.getSimpleName());
      hasError = true;
    }

    // Verify common generated code restrictions.
    hasError |= isInaccessibleViaGeneratedCode(BindInt.class, "fields", element);
    hasError |= isBindingInWrongPackage(BindInt.class, element);

    if (hasError) {
      return;
    }

    // Assemble information on the field.
    String name = element.getSimpleName().toString();
    int id = element.getAnnotation(BindInt.class).value();

    BindingClass bindingClass = getOrCreateTargetClass(targetClassMap, enclosingElement);
    FieldResourceBinding binding = new FieldResourceBinding(id, name, "getInteger");
    bindingClass.addResource(binding);

    erasedTargetNames.add(enclosingElement.toString());
  }

  private void parseResourceString(Element element, Map<TypeElement, BindingClass> targetClassMap,
      Set<String> erasedTargetNames) {
    boolean hasError = false;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

    // Verify that the target type is String.
    if (!"java.lang.String".equals(element.asType().toString())) {
      error(element, "@%s field type must be 'String'. (%s.%s)",
          BindString.class.getSimpleName(), enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

    // Verify common generated code restrictions.
    hasError |= isInaccessibleViaGeneratedCode(BindString.class, "fields", element);
    hasError |= isBindingInWrongPackage(BindString.class, element);

    if (hasError) {
      return;
    }

    // Assemble information on the field.
    String name = element.getSimpleName().toString();
    int id = element.getAnnotation(BindString.class).value();

    BindingClass bindingClass = getOrCreateTargetClass(targetClassMap, enclosingElement);
    FieldResourceBinding binding = new FieldResourceBinding(id, name, "getString");
    bindingClass.addResource(binding);

    erasedTargetNames.add(enclosingElement.toString());
  }

  /** Returns the first duplicate element inside an array, null if there are no duplicates. */
  private static Integer findDuplicate(int[] array) {
    Set<Integer> seenElements = new LinkedHashSet<Integer>();

    for (int element : array) {
      if (!seenElements.add(element)) {
        return element;
      }
    }

    return null;
  }

  /** Uses both {@link Types#erasure} and string manipulation to strip any generic types. */
  private String doubleErasure(TypeMirror elementType) {
    String name = typeUtils.erasure(elementType).toString();
    int typeParamStart = name.indexOf('<');
    if (typeParamStart != -1) {
      name = name.substring(0, typeParamStart);
    }
    return name;
  }

  private void findAndParseListener(RoundEnvironment env,
      Class<? extends Annotation> annotationClass, Map<TypeElement, BindingClass> targetClassMap,
      Set<String> erasedTargetNames) {
    for (Element element : env.getElementsAnnotatedWith(annotationClass)) {
      try {
        parseListenerAnnotation(annotationClass, element, targetClassMap, erasedTargetNames);
      } catch (Exception e) {
        StringWriter stackTrace = new StringWriter();
        e.printStackTrace(new PrintWriter(stackTrace));

        error(element, "Unable to generate view binder for @%s.\n\n%s",
            annotationClass.getSimpleName(), stackTrace.toString());
      }
    }
  }

  private void parseListenerAnnotation(Class<? extends Annotation> annotationClass, Element element,
      Map<TypeElement, BindingClass> targetClassMap, Set<String> erasedTargetNames)
      throws Exception {
    // This should be guarded by the annotation's @Target but it's worth a check for safe casting.
    if (!(element instanceof ExecutableElement) || element.getKind() != METHOD) {
      throw new IllegalStateException(
          String.format("@%s annotation must be on a method.", annotationClass.getSimpleName()));
    }

    ExecutableElement executableElement = (ExecutableElement) element;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

    // Assemble information on the method.
    Annotation annotation = element.getAnnotation(annotationClass);
    Method annotationValue = annotationClass.getDeclaredMethod("value");
    if (annotationValue.getReturnType() != int[].class) {
      throw new IllegalStateException(
          String.format("@%s annotation value() type not int[].", annotationClass));
    }

    int[] ids = (int[]) annotationValue.invoke(annotation);
    String name = executableElement.getSimpleName().toString();
    boolean required = isRequiredBinding(element);

    // Verify that the method and its containing class are accessible via generated code.
    boolean hasError = isInaccessibleViaGeneratedCode(annotationClass, "methods", element);
    hasError |= isBindingInWrongPackage(annotationClass, element);

    Integer duplicateId = findDuplicate(ids);
    if (duplicateId != null) {
      error(element, "@%s annotation for method contains duplicate ID %d. (%s.%s)",
          annotationClass.getSimpleName(), duplicateId, enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

    ListenerClass listener = annotationClass.getAnnotation(ListenerClass.class);
    if (listener == null) {
      throw new IllegalStateException(
          String.format("No @%s defined on @%s.", ListenerClass.class.getSimpleName(),
              annotationClass.getSimpleName()));
    }

    for (int id : ids) {
      if (id == View.NO_ID) {
        if (ids.length == 1) {
          if (!required) {
            error(element, "ID-free binding must not be annotated with @Nullable. (%s.%s)",
                enclosingElement.getQualifiedName(), element.getSimpleName());
            hasError = true;
          }

          // Verify target type is valid for a binding without an id.
          String targetType = listener.targetType();
          if (!isSubtypeOfType(enclosingElement.asType(), targetType)
              && !isInterface(enclosingElement.asType())) {
            error(element, "@%s annotation without an ID may only be used with an object of type "
                    + "\"%s\" or an interface. (%s.%s)",
                    annotationClass.getSimpleName(), targetType,
                enclosingElement.getQualifiedName(), element.getSimpleName());
            hasError = true;
          }
        } else {
          error(element, "@%s annotation contains invalid ID %d. (%s.%s)",
              annotationClass.getSimpleName(), id, enclosingElement.getQualifiedName(),
              element.getSimpleName());
          hasError = true;
        }
      }
    }

    ListenerMethod method;
    ListenerMethod[] methods = listener.method();
    if (methods.length > 1) {
      throw new IllegalStateException(String.format("Multiple listener methods specified on @%s.",
          annotationClass.getSimpleName()));
    } else if (methods.length == 1) {
      if (listener.callbacks() != ListenerClass.NONE.class) {
        throw new IllegalStateException(
            String.format("Both method() and callback() defined on @%s.",
                annotationClass.getSimpleName()));
      }
      method = methods[0];
    } else {
      Method annotationCallback = annotationClass.getDeclaredMethod("callback");
      Enum<?> callback = (Enum<?>) annotationCallback.invoke(annotation);
      Field callbackField = callback.getDeclaringClass().getField(callback.name());
      method = callbackField.getAnnotation(ListenerMethod.class);
      if (method == null) {
        throw new IllegalStateException(
            String.format("No @%s defined on @%s's %s.%s.", ListenerMethod.class.getSimpleName(),
                annotationClass.getSimpleName(), callback.getDeclaringClass().getSimpleName(),
                callback.name()));
      }
    }

    // Verify that the method has equal to or less than the number of parameters as the listener.
    List<? extends VariableElement> methodParameters = executableElement.getParameters();
    if (methodParameters.size() > method.parameters().length) {
      error(element, "@%s methods can have at most %s parameter(s). (%s.%s)",
          annotationClass.getSimpleName(), method.parameters().length,
          enclosingElement.getQualifiedName(), element.getSimpleName());
      hasError = true;
    }

    // Verify method return type matches the listener.
    TypeMirror returnType = executableElement.getReturnType();
    if (returnType instanceof TypeVariable) {
      TypeVariable typeVariable = (TypeVariable) returnType;
      returnType = typeVariable.getUpperBound();
    }
    if (!returnType.toString().equals(method.returnType())) {
      error(element, "@%s methods must have a '%s' return type. (%s.%s)",
          annotationClass.getSimpleName(), method.returnType(),
          enclosingElement.getQualifiedName(), element.getSimpleName());
      hasError = true;
    }

    if (hasError) {
      return;
    }

    Parameter[] parameters = Parameter.NONE;
    if (!methodParameters.isEmpty()) {
      parameters = new Parameter[methodParameters.size()];
      BitSet methodParameterUsed = new BitSet(methodParameters.size());
      String[] parameterTypes = method.parameters();
      for (int i = 0; i < methodParameters.size(); i++) {
        VariableElement methodParameter = methodParameters.get(i);
        TypeMirror methodParameterType = methodParameter.asType();
        if (methodParameterType instanceof TypeVariable) {
          TypeVariable typeVariable = (TypeVariable) methodParameterType;
          methodParameterType = typeVariable.getUpperBound();
        }

        for (int j = 0; j < parameterTypes.length; j++) {
          if (methodParameterUsed.get(j)) {
            continue;
          }
          if (isSubtypeOfType(methodParameterType, parameterTypes[j])
              || isInterface(methodParameterType)) {
            parameters[i] = new Parameter(j, methodParameterType.toString());
            methodParameterUsed.set(j);
            break;
          }
        }
        if (parameters[i] == null) {
          StringBuilder builder = new StringBuilder();
          builder.append("Unable to match @")
              .append(annotationClass.getSimpleName())
              .append(" method arguments. (")
              .append(enclosingElement.getQualifiedName())
              .append('.')
              .append(element.getSimpleName())
              .append(')');
          for (int j = 0; j < parameters.length; j++) {
            Parameter parameter = parameters[j];
            builder.append("\n\n  Parameter #")
                .append(j + 1)
                .append(": ")
                .append(methodParameters.get(j).asType().toString())
                .append("\n    ");
            if (parameter == null) {
              builder.append("did not match any listener parameters");
            } else {
              builder.append("matched listener parameter #")
                  .append(parameter.getListenerPosition() + 1)
                  .append(": ")
                  .append(parameter.getType());
            }
          }
          builder.append("\n\nMethods may have up to ")
              .append(method.parameters().length)
              .append(" parameter(s):\n");
          for (String parameterType : method.parameters()) {
            builder.append("\n  ").append(parameterType);
          }
          builder.append(
              "\n\nThese may be listed in any order but will be searched for from top to bottom.");
          error(executableElement, builder.toString());
          return;
        }
      }
    }

    MethodViewBinding binding = new MethodViewBinding(name, Arrays.asList(parameters), required);
    BindingClass bindingClass = getOrCreateTargetClass(targetClassMap, enclosingElement);
    for (int id : ids) {
      if (!bindingClass.addMethod(id, listener, method, binding)) {
        error(element, "Multiple listener methods with return value specified for ID %d. (%s.%s)",
            id, enclosingElement.getQualifiedName(), element.getSimpleName());
        return;
      }
    }

    // Add the type-erased version to the valid binding targets set.
    erasedTargetNames.add(enclosingElement.toString());
  }

  private boolean isInterface(TypeMirror typeMirror) {
    if (!(typeMirror instanceof DeclaredType)) {
      return false;
    }
    return ((DeclaredType) typeMirror).asElement().getKind() == INTERFACE;
  }

  private boolean isSubtypeOfType(TypeMirror typeMirror, String otherType) {
    if (otherType.equals(typeMirror.toString())) {
      return true;
    }
    if (typeMirror.getKind() != TypeKind.DECLARED) {
      return false;
    }
    DeclaredType declaredType = (DeclaredType) typeMirror;
    List<? extends TypeMirror> typeArguments = declaredType.getTypeArguments();
    if (typeArguments.size() > 0) {
      StringBuilder typeString = new StringBuilder(declaredType.asElement().toString());
      typeString.append('<');
      for (int i = 0; i < typeArguments.size(); i++) {
        if (i > 0) {
          typeString.append(',');
        }
        typeString.append('?');
      }
      typeString.append('>');
      if (typeString.toString().equals(otherType)) {
        return true;
      }
    }
    Element element = declaredType.asElement();
    if (!(element instanceof TypeElement)) {
      return false;
    }
    TypeElement typeElement = (TypeElement) element;
    TypeMirror superType = typeElement.getSuperclass();
    if (isSubtypeOfType(superType, otherType)) {
      return true;
    }
    for (TypeMirror interfaceType : typeElement.getInterfaces()) {
      if (isSubtypeOfType(interfaceType, otherType)) {
        return true;
      }
    }
    return false;
  }

  private BindingClass getOrCreateTargetClass(Map<TypeElement, BindingClass> targetClassMap,
      TypeElement enclosingElement) {
    BindingClass bindingClass = targetClassMap.get(enclosingElement);
    if (bindingClass == null) {
      String targetType = enclosingElement.getQualifiedName().toString();
      String classPackage = getPackageName(enclosingElement);
      String className = getClassName(enclosingElement, classPackage) + SUFFIX;

      bindingClass = new BindingClass(classPackage, className, targetType);
      targetClassMap.put(enclosingElement, bindingClass);
    }
    return bindingClass;
  }

  private static String getClassName(TypeElement type, String packageName) {
    int packageLen = packageName.length() + 1;
    return type.getQualifiedName().toString().substring(packageLen).replace('.', '$');
  }

  /** Finds the parent binder type in the supplied set, if any. */
  private String findParentFqcn(TypeElement typeElement, Set<String> parents) {
    TypeMirror type;
    while (true) {
      type = typeElement.getSuperclass();
      if (type.getKind() == TypeKind.NONE) {
        return null;
      }
      typeElement = (TypeElement) ((DeclaredType) type).asElement();
      if (parents.contains(typeElement.toString())) {
        String packageName = getPackageName(typeElement);
        return packageName + "." + getClassName(typeElement, packageName);
      }
    }
  }

  @Override public SourceVersion getSupportedSourceVersion() {
    return SourceVersion.latestSupported();
  }

  private void error(Element element, String message, Object... args) {
    if (args.length > 0) {
      message = String.format(message, args);
    }
    processingEnv.getMessager().printMessage(ERROR, message, element);
  }

  private String getPackageName(TypeElement type) {
    return elementUtils.getPackageOf(type).getQualifiedName().toString();
  }

  private static boolean hasAnnotationWithName(Element element, String simpleName) {
    for (AnnotationMirror mirror : element.getAnnotationMirrors()) {
      String annotationName = mirror.getAnnotationType().asElement().getSimpleName().toString();
      if (simpleName.equals(annotationName)) {
        return true;
      }
    }
    return false;
  }

  private static boolean isRequiredBinding(Element element) {
    return !hasAnnotationWithName(element, NULLABLE_ANNOTATION_NAME);
  }
}

这么长的源码，没关系，我们先看process函数

public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {
  Map<TypeElement, BindingClass> targetClassMap = findAndParseTargets(env);

  for (Map.Entry<TypeElement, BindingClass> entry : targetClassMap.entrySet()) {
    TypeElement typeElement = entry.getKey();
    BindingClass bindingClass = entry.getValue();

    try {
      JavaFileObject jfo = filer.createSourceFile(bindingClass.getFqcn(), typeElement);
      Writer writer = jfo.openWriter();
      writer.write(bindingClass.brewJava());
      writer.flush();
      writer.close();
    } catch (IOException e) {
      error(typeElement, "Unable to write view binder for type %s: %s", typeElement,
          e.getMessage());
    }
  }

  return true;
}

先调用了findAndParseTargets()方法

private Map<TypeElement, BindingClass> findAndParseTargets(RoundEnvironment env) {
  Map<TypeElement, BindingClass> targetClassMap = new LinkedHashMap<TypeElement, BindingClass>();
  Set<String> erasedTargetNames = new LinkedHashSet<String>();

  // Process each @Bind element.
  for (Element element : env.getElementsAnnotatedWith(Bind.class)) {
    try {
      parseBind(element, targetClassMap, erasedTargetNames);
    } catch (Exception e) {
      logParsingError(element, Bind.class, e);
    }
  }

  // Process each annotation that corresponds to a listener.
  for (Class<? extends Annotation> listener : LISTENERS) {
    findAndParseListener(env, listener, targetClassMap, erasedTargetNames);
  }

  // Process each @BindBool element.
  for (Element element : env.getElementsAnnotatedWith(BindBool.class)) {
    try {
      parseResourceBool(element, targetClassMap, erasedTargetNames);
    } catch (Exception e) {
      logParsingError(element, BindBool.class, e);
    }
  }

  // Process each @BindColor element.
  for (Element element : env.getElementsAnnotatedWith(BindColor.class)) {
    try {
      parseResourceColor(element, targetClassMap, erasedTargetNames);
    } catch (Exception e) {
      logParsingError(element, BindColor.class, e);
    }
  }

  // Process each @BindDimen element.
  for (Element element : env.getElementsAnnotatedWith(BindDimen.class)) {
    try {
      parseResourceDimen(element, targetClassMap, erasedTargetNames);
    } catch (Exception e) {
      logParsingError(element, BindDimen.class, e);
    }
  }

  // Process each @BindDrawable element.
  for (Element element : env.getElementsAnnotatedWith(BindDrawable.class)) {
    try {
      parseResourceDrawable(element, targetClassMap, erasedTargetNames);
    } catch (Exception e) {
      logParsingError(element, BindDrawable.class, e);
    }
  }

  // Process each @BindInt element.
  for (Element element : env.getElementsAnnotatedWith(BindInt.class)) {
    try {
      parseResourceInt(element, targetClassMap, erasedTargetNames);
    } catch (Exception e) {
      logParsingError(element, BindInt.class, e);
    }
  }

  // Process each @BindString element.
  for (Element element : env.getElementsAnnotatedWith(BindString.class)) {
    try {
      parseResourceString(element, targetClassMap, erasedTargetNames);
    } catch (Exception e) {
      logParsingError(element, BindString.class, e);
    }
  }

  // Try to find a parent binder for each.
  for (Map.Entry<TypeElement, BindingClass> entry : targetClassMap.entrySet()) {
    String parentClassFqcn = findParentFqcn(entry.getKey(), erasedTargetNames);
    if (parentClassFqcn != null) {
      entry.getValue().setParentViewBinder(parentClassFqcn + SUFFIX);
    }
  }

  return targetClassMap;
}

我们只看和Bind有关的

// Process each @Bind element.
for (Element element : env.getElementsAnnotatedWith(Bind.class)) {
  try {
    parseBind(element, targetClassMap, erasedTargetNames);
  } catch (Exception e) {
    logParsingError(element, Bind.class, e);
  }
}

调用了parseBind()方法

private void parseBind(Element element, Map<TypeElement, BindingClass> targetClassMap,
    Set<String> erasedTargetNames) {
  // Verify common generated code restrictions.
  if (isInaccessibleViaGeneratedCode(Bind.class, "fields", element)
      || isBindingInWrongPackage(Bind.class, element)) {
    return;
  }

  TypeMirror elementType = element.asType();
  if (elementType.getKind() == TypeKind.ARRAY) {
    parseBindMany(element, targetClassMap, erasedTargetNames);
  } else if (LIST_TYPE.equals(doubleErasure(elementType))) {
    parseBindMany(element, targetClassMap, erasedTargetNames);
  } else if (isSubtypeOfType(elementType, ITERABLE_TYPE)) {
    error(element, "@%s must be a List or array. (%s.%s)", Bind.class.getSimpleName(),
        ((TypeElement) element.getEnclosingElement()).getQualifiedName(),
        element.getSimpleName());
  } else {
    parseBindOne(element, targetClassMap, erasedTargetNames);
  }
}

在parseBind()方法中调用了parseOne()和parseMany()方法，从parseBind()方法中的判断中可以看出，如果被注解的对象是数组或者list就调用parseMany()，否则调用parseOne()，那我们就看parseOne()方法。

private void parseBindOne(Element element, Map<TypeElement, BindingClass> targetClassMap,
      Set<String> erasedTargetNames) {
    boolean hasError = false;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

    // Verify that the target type extends from View.
    TypeMirror elementType = element.asType();
    if (elementType.getKind() == TypeKind.TYPEVAR) {
      TypeVariable typeVariable = (TypeVariable) elementType;
      elementType = typeVariable.getUpperBound();
    }
    if (!isSubtypeOfType(elementType, VIEW_TYPE) && !isInterface(elementType)) {
      error(element, "@%s fields must extend from View or be an interface. (%s.%s)",
          Bind.class.getSimpleName(), enclosingElement.getQualifiedName(), element.getSimpleName());
      hasError = true;
    }

    // Assemble information on the field.
    int[] ids = element.getAnnotation(Bind.class).value();
    if (ids.length != 1) {
      error(element, "@%s for a view must only specify one ID. Found: %s. (%s.%s)",
          Bind.class.getSimpleName(), Arrays.toString(ids), enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

    if (hasError) {
      return;
    }

    int id = ids[0];
    BindingClass bindingClass = targetClassMap.get(enclosingElement);
    if (bindingClass != null) {
      ViewBindings viewBindings = bindingClass.getViewBinding(id);
      if (viewBindings != null) {
        Iterator<FieldViewBinding> iterator = viewBindings.getFieldBindings().iterator();
        if (iterator.hasNext()) {
          FieldViewBinding existingBinding = iterator.next();
          error(element, "Attempt to use @%s for an already bound ID %d on '%s'. (%s.%s)",
              Bind.class.getSimpleName(), id, existingBinding.getName(),
              enclosingElement.getQualifiedName(), element.getSimpleName());
          return;
        }
      }
    } else {
      bindingClass = getOrCreateTargetClass(targetClassMap, enclosingElement);
    }

    String name = element.getSimpleName().toString();
    String type = elementType.toString();
    boolean required = isRequiredBinding(element);

    FieldViewBinding binding = new FieldViewBinding(name, type, required);
    bindingClass.addField(id, binding);

    // Add the type-erased version to the valid binding targets set.
    erasedTargetNames.add(enclosingElement.toString());
  }

前面是一堆取值＋判断，重点是这一句。

int[] ids = element.getAnnotation(Bind.class).value();
    if (ids.length != 1) {
      error(element, "@%s for a view must only specify one ID. Found: %s. (%s.%s)",
          Bind.class.getSimpleName(), Arrays.toString(ids), enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

取出Bind注解中的值，也就是各种R.id.xxx，如果长度不为1则报错，因为这是parseOne()方法嘛。

后面的内容就是从map缓存中出去对应的值并且把被Bind注解的变量的名字，类型，id值等等保存起来，如果看过Knight源码的同学应该对这一段轻车熟路了，逻辑都差不多。

让我们回到process()方法

public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {
  Map<TypeElement, BindingClass> targetClassMap = findAndParseTargets(env);

  for (Map.Entry<TypeElement, BindingClass> entry : targetClassMap.entrySet()) {
    TypeElement typeElement = entry.getKey();
    BindingClass bindingClass = entry.getValue();

    try {
      JavaFileObject jfo = filer.createSourceFile(bindingClass.getFqcn(), typeElement);
      Writer writer = jfo.openWriter();
      writer.write(bindingClass.brewJava());
      writer.flush();
      writer.close();
    } catch (IOException e) {
      error(typeElement, "Unable to write view binder for type %s: %s", typeElement,
          e.getMessage());
    }
  }

  return true;
}

在通过findAndParseTargets()方法保存了对应的值以后，调用了bindingClass.brewJava()方法，让我们跟踪进去看一看。

String brewJava() {
  StringBuilder builder = new StringBuilder();
  builder.append("// Generated code from Butter Knife. Do not modify!\n");
  builder.append("package ").append(classPackage).append(";\n\n");

  if (!resourceBindings.isEmpty()) {
    builder.append("import android.content.res.Resources;\n");
  }
  if (!viewIdMap.isEmpty() || !collectionBindings.isEmpty()) {
    builder.append("import android.view.View;\n");
  }
  builder.append("import butterknife.ButterKnife.Finder;\n");
  if (parentViewBinder == null) {
    builder.append("import butterknife.ButterKnife.ViewBinder;\n");
  }
  builder.append('\n');

  builder.append("public class ").append(className);
  builder.append("<T extends ").append(targetClass).append(">");

  if (parentViewBinder != null) {
    builder.append(" extends ").append(parentViewBinder).append("<T>");
  } else {
    builder.append(" implements ViewBinder<T>");
  }
  builder.append(" {\n");

  emitBindMethod(builder);
  builder.append('\n');
  emitUnbindMethod(builder);

  builder.append("}\n");
  return builder.toString();
}

各种手写代码有木有，其中有一句

1	builder.append(" implements ViewBinder<T>");

这表明我们动态生成的一个java类是要实现ViewBinder的，至于ViewBinder是什么，我们后面再说。

为什么jake大神不用同为square开源的javapoet呢？我也不清楚。。。

到这我们就简单分析完了processor这个类，主要就是通过保存注解相关的内容并且通过javaFileObject去生成java文件。那这个java文件是怎么调用的呢？还记得butterKnife怎么用的吗？

1	ButterKnife.bind(this);

让我们看看bind()函数做了什么

public static void bind(Activity target) {
  bind(target, target, Finder.ACTIVITY);
}

static void bind(Object target, Object source, Finder finder) {
    Class<?> targetClass = target.getClass();
    try {
      if (debug) Log.d(TAG, "Looking up view binder for " + targetClass.getName());
      ViewBinder<Object> viewBinder = findViewBinderForClass(targetClass);
      if (viewBinder != null) {
        viewBinder.bind(finder, target, source);
      }
    } catch (Exception e) {
      throw new RuntimeException("Unable to bind views for " + targetClass.getName(), e);
    }
  }

先不管这个Finder.ACTIVITY是什么鬼，可以看到bind()函数中调用了findViewBinderForClass

private static ViewBinder<Object> findViewBinderForClass(Class<?> cls)
    throws IllegalAccessException, InstantiationException {
  ViewBinder<Object> viewBinder = BINDERS.get(cls);
  if (viewBinder != null) {
    if (debug) Log.d(TAG, "HIT: Cached in view binder map.");
    return viewBinder;
  }
  String clsName = cls.getName();
  if (clsName.startsWith(ANDROID_PREFIX) || clsName.startsWith(JAVA_PREFIX)) {
    if (debug) Log.d(TAG, "MISS: Reached framework class. Abandoning search.");
    return NOP_VIEW_BINDER;
  }
  try {
    Class<?> viewBindingClass = Class.forName(clsName + ButterKnifeProcessor.SUFFIX);
    //noinspection unchecked
    viewBinder = (ViewBinder<Object>) viewBindingClass.newInstance();
    if (debug) Log.d(TAG, "HIT: Loaded view binder class.");
  } catch (ClassNotFoundException e) {
    if (debug) Log.d(TAG, "Not found. Trying superclass " + cls.getSuperclass().getName());
    viewBinder = findViewBinderForClass(cls.getSuperclass());
  }
  BINDERS.put(cls, viewBinder);
  return viewBinder;
}

这么多的逻辑，总的来说其实就是通过cls的类型去从map中得到对应的ViewBinder。还记得ViewBinder吗，我们刚刚动态生成的那个java类就是继承自ViewBinder的，我们这里取出的也就是这个ViewBinder。那我们动态生成的java文件在哪儿呢？在/build/generated/source/apt/debud/xxxx/目录下，xxx是你对应的包名。当然你想要看到有对应的java文件，需要在整个工程的build.gradle下添加

1	classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'

在对应的module下添加

1	apply plugin: 'com.neenbedankt.android-apt'

我们以这样一个简单的activity为例

public class MainActivity extends AppCompatActivity {

    @Bind(R.id.btn)
    Button btn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.bind(this);
    }
}

来看看动态生成的java类是怎么样的

// Generated code from Butter Knife. Do not modify!
package zjutkz.com.annotationtest;

import android.view.View;
import butterknife.ButterKnife.Finder;
import butterknife.ButterKnife.ViewBinder;

public class MainActivity$$ViewBinder<T extends zjutkz.com.annotationtest.MainActivity> implements ViewBinder<T> {
  @Override public void bind(final Finder finder, final T target, Object source) {
    View view;
    view = finder.findRequiredView(source, 2131492969, "field 'btn'");
    target.btn = finder.castView(view, 2131492969, "field 'btn'");
  }

  @Override public void unbind(T target) {
    target.btn = null;
  }
}

大家可以对照上面的brewJava()方法去看。

看到bind方法了吗？这就是我们activity中调用ButterKnife.bind(this);这个函数以后，最终就会走到这个方法中。调用了finder的两个方法，下面让我们看看finder是什么。

public enum Finder {
  VIEW {
    @Override protected View findView(Object source, int id) {
      return ((View) source).findViewById(id);
    }

    @Override public Context getContext(Object source) {
      return ((View) source).getContext();
    }
  },
  ACTIVITY {
    @Override protected View findView(Object source, int id) {
      return ((Activity) source).findViewById(id);
    }

    @Override public Context getContext(Object source) {
      return (Activity) source;
    }
  },
  DIALOG {
    @Override protected View findView(Object source, int id) {
      return ((Dialog) source).findViewById(id);
    }

    @Override public Context getContext(Object source) {
      return ((Dialog) source).getContext();
    }
  };

首先可以看到Finder是一个枚举，然后来看看findRequiredView()方法做了什么

public <T> T findRequiredView(Object source, int id, String who) {
  T view = findOptionalView(source, id, who);
  if (view == null) {
    String name = getContext(source).getResources().getResourceEntryName(id);
    throw new IllegalStateException("Required view '"
        + name
        + "' with ID "
        + id
        + " for "
        + who
        + " was not found. If this view is optional add '@Nullable' annotation.");
  }
  return view;
}

public <T> T findOptionalView(Object source, int id, String who) {
  View view = findView(source, id);
  return castView(view, id, who);
}

@SuppressWarnings("unchecked") // That's the point.
public <T> T castView(View view, int id, String who) {
  try {
    return (T) view;
  } catch (ClassCastException e) {
    if (who == null) {
      throw new AssertionError();
    }
    String name = view.getResources().getResourceEntryName(id);
    throw new IllegalStateException("View '"
        + name
        + "' with ID "
        + id
        + " for "
        + who
        + " was of the wrong type. See cause for more info.", e);
  }
}
protected abstract View findView(Object source, int id);

findRequiredView()方法中调用了findOptionalView()方法，而findOptionalView()方法中调用了findView()方法，而findView()是一个抽象方法。还记得刚刚看到的Finder的定义吗？

VIEW {
  @Override protected View findView(Object source, int id) {
    return ((View) source).findViewById(id);
  }

  @Override public Context getContext(Object source) {
    return ((View) source).getContext();
  }
},
ACTIVITY {
  @Override protected View findView(Object source, int id) {
    return ((Activity) source).findViewById(id);
  }

  @Override public Context getContext(Object source) {
    return (Activity) source;
  }
},
DIALOG {
  @Override protected View findView(Object source, int id) {
    return ((Dialog) source).findViewById(id);
  }

  @Override public Context getContext(Object source) {
    return ((Dialog) source).getContext();
  }
};

在每一个枚举量中重写了findView()方法。

而我们Activity.bind()方法中调用的是Finder.ACTIVITY

1
2
3

public static void bind(Activity target) {
  bind(target, target, Finder.ACTIVITY);
}

就会调用对应的

1
2
3

protected View findView(Object source, int id) {
  return ((Activity) source).findViewById(id);
}

完成了findViewById。

为什么要这样设计呢？想象一下findViewById的使用场景，可能是Activity，可能是Fragment，可能是Dialog，可能是View等等等等，如果一股脑儿只写一个bind()方法的话，势必要有大量的强转和if判断，而jake大神采用了这样的方式，让代码的可读性和易维护性大大增强，真的要膜拜他了，真大神啊！！！

好了，到这儿butterKnife的源码也基本简单的分析完了，总的来说就是使用了编译时注解的方式，配合上动态生成java类去帮助大家完成一些工作，比如findViewById。

通过分析Knight和butterKnife的使用和源码，相信大家对编译时注解已经有了一定的了解，可以说这是一个非常使用的框架，配合上javapoet这样的动态生成java子节码的框架可以写出很多“高大上”的代码。大家不妨自己尝试一下！

下期预告

最近MVC，MVP，MVVM的讨论十分之火，这三种框架到底有什么区别呢？有没有什么好的框架能让我们高效的使用它们呢？另外，其实很多人对MVVM，data binding的理解都是存在重大误区的！至于是什么，see you next week.

万能的APT！编译时注解的妙用