图像在列表视图中消失

Question

我的应用程序使用ListView来显示来自url的一些图像和标题。但无论我做什么，图像在向上或向下滚动后都会从ListView消失，并且没有人再次看到它们。 我该怎么做？我是Android新手，所以我会很高兴看到任何建议。

我的代码是在这里：

AndroidManifest：

<?xml version="1.0" encoding="utf-8"?> 
<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
    package="com.example.forwork"> 
    <uses-permission android:name="android.permission.INTERNET" /> 
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" /> 

    <application 
     android:allowBackup="true" 
     android:icon="@mipmap/ic_launcher" 
     android:label="@string/app_name" 
     android:supportsRtl="true" 
     android:theme="@style/AppTheme"> 
     <activity 
      android:name=".MainActivity" 
      android:configChanges="keyboardHidden|orientation|screenSize"> 
      <intent-filter> 
       <action android:name="android.intent.action.MAIN" /> 

       <category android:name="android.intent.category.LAUNCHER" /> 
      </intent-filter> 
     </activity> 
    </application> 

</manifest> 

MainActivity.java：

import android.app.Activity; 
import android.content.Context; 
import android.content.res.Configuration; 
import android.graphics.Bitmap; 
import android.graphics.BitmapFactory; 
import android.net.ConnectivityManager; 
import android.net.NetworkInfo; 
import android.os.AsyncTask; 
import android.os.Bundle; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.ArrayAdapter; 
import android.widget.ImageView; 
import android.widget.ListView; 
import android.widget.TextView; 
import android.widget.Toast; 

import org.apache.http.HttpStatus; 
import org.json.JSONArray; 
import org.json.JSONException; 
import org.json.JSONObject; 

import java.io.BufferedReader; 
import java.io.InputStream; 
import java.io.InputStreamReader; 
import java.lang.ref.WeakReference; 
import java.net.HttpURLConnection; 
import java.net.URL; 
import java.util.Objects; 
import java.util.concurrent.ExecutionException; 

public class MainActivity extends Activity{ 

    public final static int SIZE=30; 

    public String[] titles = new String[SIZE];   //array for titles 
    public String[] keys = new String[SIZE];   //array for id of news 
    public String[] image_url = new String[SIZE];  //array for image urls to download 
    public String data=""; 
    //that's for parsing 
    JSONObject news = null; 
    String title = ""; 
    JSONObject prefs = null; 
    JSONObject inner = null; 
    JSONObject elements = null; 
    JSONObject image =null; 
    String pic = ""; 
    JSONObject json = null; 
    JSONArray collection = null; 
    JSONObject documents = null; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     if (isInternetThere(this))           //if internet is not absent 
      try { 
       ListView my_list_view = (ListView) findViewById(R.id.list_view); //custom list view 
       CustomList adapter = new CustomList(this,titles,image_url);   //custom adapter 
       my_list_view.setAdapter(adapter); 
       Parse p = new Parse();            //instance of my async task for getting json 
       data = p.execute().get(); 
      } catch (InterruptedException | ExecutionException e) { 
       e.printStackTrace(); 
      } 
     else { 
      Toast.makeText(this, "No internet connection", Toast.LENGTH_LONG).show(); 
      finish(); 
     } 

     try { 
      json = new JSONObject(data);         //parsing json 
      collection = json.getJSONArray("collection");     //new's ids 
      documents = json.getJSONObject("documents");     //new's info 
      for (int i=0; i<keys.length;i++){ 
       keys[i]=collection.getString(i); 
       news = documents.getJSONObject(keys[i]); 
       title = news.getString("title");       //title 
       titles[i]=title;           //put title into list 
       prefs = news.getJSONObject("prefs"); 
       inner = prefs.getJSONObject("inner"); 
       elements = inner.getJSONObject("elements"); 
       image = elements.getJSONObject("image"); 
       if (image.has("small_url"))         //whether there is a picture? 
        pic = image.getString("small_url"); 
       else pic = ""; 
       image_url[i] = pic; 
      } 
     } catch (JSONException e) { 
      e.printStackTrace(); 
     } 
    } 

    @Override 
    public void onConfigurationChanged(Configuration newConfig) {   //it is just for orientation monitoring 
     super.onConfigurationChanged(newConfig); 
     if (newConfig.orientation == Configuration.ORIENTATION_LANDSCAPE) { 
      Toast.makeText(this, "landscape", Toast.LENGTH_SHORT).show(); 
     } else if (newConfig.orientation == Configuration.ORIENTATION_PORTRAIT) { 
      Toast.makeText(this, "portrait", Toast.LENGTH_SHORT).show(); 
     } 
    } 

    public class Parse extends AsyncTask<Void,Void,String> {    //my async task #1 that just get a json from url 

     @Override 
     protected String doInBackground(Void... params) { 
      String str= null; 
      try { 
       str = readJSON(); 
      } catch (Exception e) { 
       e.printStackTrace(); 
      } 
      return str; 
     } 

     public String readJSON() {           //function that get json 
      HttpURLConnection urlConnection = null; 
      BufferedReader reader = null; 
      String resultJson = "";           //this will be a string with json 
      try { 
       URL url = new URL("https://meduza.io/api/v3/search?chrono=news&page=0&per_page=30&locale=ru");  //url with 30 news 

       urlConnection = (HttpURLConnection) url.openConnection(); 
       urlConnection.setRequestMethod("GET"); 
       urlConnection.connect(); 

       InputStream inputStream = urlConnection.getInputStream();  //stream for getting json 
       StringBuffer buffer = new StringBuffer();      //buffer for json saving 

       reader = new BufferedReader(new InputStreamReader(inputStream));  //it just reads 

       String line; 
       while ((line = reader.readLine()) != null) {       //line by line.. 
        buffer.append(line);            //...and add it into buffer 
       } 

       resultJson = buffer.toString();         //result is string got from buffer 

      } catch (Exception e) { 
       e.printStackTrace(); 
      } 
      return resultJson; 
     } 
    } 

    public class DownloadImage extends AsyncTask<String,Void,Bitmap>{    //my async task #2 for downloading image from url using Bitmap 
     private final WeakReference<ImageView> imgv;        //it means that.. 
     public DownloadImage(ImageView imageView){ 
      imgv = new WeakReference<ImageView>(imageView);      //the object can be deleted by garbage collector without any trubles 
     }                   //it helps to avoid memory leaks 

     @Override 
     protected Bitmap doInBackground(String... strings) { 
      return down_Bit(strings[0]); 
     } 

     @Override 
     protected void onPostExecute(Bitmap res){ 
      if (isCancelled()){ 
       res=null; 
      } 

      if(imgv != null){              //if the link is actual... 
       ImageView iv = imgv.get();           //we get it 
       if (iv != null){             //if the object is exists... 
        if (res != null)            //and image exests... 
         iv.setImageBitmap(res);          //we show it 

       } 
      } 
     } 

     private Bitmap down_Bit(String url){          //it is for downloading images 
      HttpURLConnection urlConnection = null; 
      try{ 
       URL uri = new URL(url); 
       urlConnection = (HttpURLConnection) uri.openConnection(); 
       int statusCode = urlConnection.getResponseCode(); 
       if (statusCode != HttpStatus.SC_OK)         //if our url is not right(for ex. "http" instead of "https" 
        return null; 

       InputStream is = urlConnection.getInputStream(); 
       if (is != null){ 
        Bitmap bitmap = BitmapFactory.decodeStream(is);     //if input stream exists we get the Bitmap from it.... 
        return bitmap;             //and return it 
       } 
      } catch (Exception e){ 
       e.printStackTrace(); 
      } finally { 
       if (urlConnection != null) 
        urlConnection.disconnect(); 
      } 
      return null; 
     } 
    } 

    public class CustomList extends ArrayAdapter<String>{       //that is my custom adapter named CustomList. probably for not misunderstanding 
     private Activity context; 
     private String[] item_name; 
     private String[] imgurl; 

     public CustomList(Activity context,String[] item_name, String[] imgurl){  //constructor 
      super(context,R.layout.my_listview_item,item_name); 
      this.context=context;    //activity 
      this.item_name=item_name;   //there is will be the titles 
      this.imgurl=imgurl;     //there is will be the images 
     } 

     @Override 
     public int getCount(){ 
      return item_name.length; 
     } 

     @Override 
     public String getItem(int position){ 
      return item_name[position]; 
     } 

     @Override 
     public long getItemId(int position){ 
      return position; 
     } 

     public View getView(int position, View view, ViewGroup parent) { 
      LayoutInflater inflater = context.getLayoutInflater(); 
      View rowView = inflater.inflate(R.layout.my_listview_item,null,true); 

      TextView txtTitle = (TextView) rowView.findViewById(R.id.txt); 


      if (imgurl[position] != "") {            //if there is a url of an image(because there some news without it) 
       rowView.findViewById(R.id.img).setVisibility(View.VISIBLE);    //show the picture slot 
       imgurl[position] = "https://meduza.io" + imgurl[position];    //and make the url absolute, not relative 
       new DownloadImage((ImageView) rowView.findViewById(R.id.img)).execute(imgurl[position]);  //and download the picture using the absolute url 
      } 
      else 
       rowView.findViewById(R.id.img).setVisibility(View.GONE);    //if there is no picture, just hide the slot 
      txtTitle.setText(item_name[position]);          //it is title 

      return rowView; 
     } 
    } 

    public boolean isInternetThere(Context context) {         //it is for internet connection testing 
     ConnectivityManager manager = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE); 
     NetworkInfo networkInfo = manager.getActiveNetworkInfo(); 
     if (networkInfo != null && networkInfo.isConnected()) { 
      return true; 
     } else { 
      return false; 
     } 
    } 
} 

Answer 1

我认为您使用的是ListView。在滚动时ListView，Android重新使用列表项，所以当你滚动图像开始消失。

因此，您需要做的第一项更改是使用RecyclerView而不是ListView。 RecyclerView实施将迫使您实施ViewHolder模式，这将使您的滚动体验更好！检查此列表smooth scrolling.

最后，要根据需要让您的图片正常工作，建议您使用Picasso。这是可用的最佳图像加载库。您不需要维护列表中元素的位置，并且图像将根据需要运行。

我建议一个图书馆实施图像加载最佳实践可能需要很多时间。 The best practices.你必须保持图像缓存，有效保持BitMap大小等。

希望这些点指导你实现你的图像滚动列表最好的方式:)干杯！