Android应用篇 - 从源码角度来理解 View 的绘制流程
上一篇文章从几个场景分析了下 View 的绘制流程，这篇文章我们来 read the fuck code!
本文源代码基于 Android 7.0。
目录：
- handleResumeActivity()
- performTraversals()
- performMeasure()
- performLayout()
- performDraw()
1. handleResumeActivity()
/base/core/java/android/app/ActivityThread.java
onResume() 之后才是 Activity 真正可见和可交互的状态，那就从 ActivityThread.handleResumeActivity() 作为入口。
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
// ...
if (r.activity.mVisibleFromClient) {
// 添加视图
r.activity.makeVisible();
}
// ...
}狂删一通代码， 因为这不是讲 AMS 或者 WMS，我们这边只看调用链。makeVisible() 这便来到了 Activity 世界：
/base/core/java/android/app/Activity.java
void makeVisible() {
// 如果Window没有被添加，则addView添加decorView
if (!mWindowAdded) {
ViewManager wm = getWindowManager();
// 进入WindowManagerGlobal.addView，这边是添加DecorView，根view
wm.addView(mDecor, getWindow().getAttributes());
mWindowAdded = true;
}
// 设置decorView可见
mDecor.setVisibility(View.VISIBLE);
}wm.addView() 最终会调用到 WindowManagerGlobal.addView()，这个前面的 Framework 相关的文章有写过。
/base/core/java/android/view/WindowManagerGlobal.java
// 添加View
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
ViewRootImpl root;
View panelParentView = null;
synchronized (mLock) {
// 每次调用global.addView()都会创建一个ViewRootImpl，它是decorView与WMS沟通的桥梁
root = new ViewRootImpl(view.getContext(), display);
// 设置LayoutParams
view.setLayoutParams(wparams);
// 加到mViews, mRoots, mParams集合中
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
}
// do this last because it fires off messages to start doing things
try {
// ViewRootImpl设置View
root.setView(view, wparams, panelParentView);
} catch (RuntimeException e) {
// ...
}
}ViewRootImpl 实例化出来了，实例化出来然后被加到 mRoots (一个存放 ViewRootImpl 实例的 Arraylist) 中，同时 decorView 被添加进 mViews 中，最后我们看到调用了 root.setView() 方法。
/base/core/java/android/view/ViewRootImpl.java
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
//...
requestLayout();
//...
}好熟悉的方法名，看看它里面做了什么：
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
// ...
}
}
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
// ...
performTraversals();
// ...
}
}
- checkThread()：只能在主线程更新 UI。
- performTraversals()。
接下来的篇幅将从 performTraversals() 这个方法作为入口来写。
2. performTraversals()
/base/core/java/android/view/ViewRootImpl.java
private void performTraversals() {
// ...
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
// 测量
performMeasure(childWidthMeasureSpec,childHeightMeasureSpec);
// 布局
performLayout(lp, mWidth, mHeight);
// 绘制
performDraw();
// ...
}接着分三个小节来展开说这三个方法。
3. performMeasure()
/base/core/java/android/view/ViewRootImpl.java
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
}
}盗用一张图 - View 树的 measure 流程，画图太费时费力了：
这里调用了 mView.measure(childWidthMeasureSpec, childHeightMeasureSpec) 这个方法，那么这里的 mView 是什么东西呢，当然是从根视图开始 measure 了，所以就是 decorView 了。decorView 是个 FrameLayout，所以我们就来看 FrameLayout 的measure() 方法。FrameLayout 里面没有 measure()，只有 onMeasure()，measure() 方法在其父类 View 里，measure() 方法中会调用 onMeasure()：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
// ...
}
}
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
// setMeasuredDimension()
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
count = mMatchParentChildren.size();
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
final int width = Math.max(0, getMeasuredWidth()
- getPaddingLeftWithForeground() - getPaddingRightWithForeground()
- lp.leftMargin - lp.rightMargin);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
width, MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
final int childHeightMeasureSpec;
if (lp.height == LayoutParams.MATCH_PARENT) {
final int height = Math.max(0, getMeasuredHeight()
- getPaddingTopWithForeground() - getPaddingBottomWithForeground()
- lp.topMargin - lp.bottomMargin);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
height, MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}遍历了父视图下面的子视图，然后调用 measureChildWithMargins() 方法来进行测量子视图，这个方法位于 ViewGroup 类中：
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
根据父视图的大小还有 mode，以及自己的 Padding和 Margin 还有 LayoutParams 来测量，先看下这里面的方法getChildMeasureSpec()：
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
根据 mode 和 size 测量出自己的大小之后，会把 mode 和 size 调用 makeMeasureSpec 继续合成。
继续看 measureChildWithMargins() 方法最后会在调用 View 的 measure()，如果这个 View 还是个 ViewGoup 的话，那就会递归调用 measure() 方法，如果是 View 的话，那就会调用 View 的 measure()，也就是会调用到 onMeasure()。
4. performLayout()
/base/core/java/android/view/ViewRootImpl.java
测量完成后，就开始来布局了。
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
// ...
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
// ...
}和 measure() 一样，也是递归调用。看看 FrameLayout 的 layout()，在 FrameLayout 的父类 View 中：
public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
}
// ...
}
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}里面会调用 onLayout()，onLayout() 是一个空实现，子类可以在里面做一些自己的事，来看看 FrameLayout 的 onLayout()：
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
void layoutChildren(int left, int top, int right, int bottom,
boolean forceLeftGravity) {
final int count = getChildCount();
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
int gravity = lp.gravity;
if (gravity == -1) {
gravity = DEFAULT_CHILD_GRAVITY;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT:
default:
childLeft = parentLeft + lp.leftMargin;
}
switch (verticalGravity) {
case Gravity.TOP:
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
childTop = parentTop + lp.topMargin;
}
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
递归调用过程，和 measure() 流程一样。
5. performDraw()
盗图，画图好累，码字好累 - draw 流程图：
/base/core/java/android/view/ViewRootImpl.java
private void performDraw() {
// ...
try {
draw(fullRedrawNeeded);
} finally {
mIsDrawing = false;
}
// ...
}
private void draw(boolean fullRedrawNeeded) {
// ...
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
// ...
}
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
// ...
mView.draw(canvas);
// ...
}
和上面的 measure、layout 一样，来看看 View 的 draw()：
public void draw(Canvas canvas) {
// 画背景
if (!dirtyOpaque) {
drawBackground(canvas);
}
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
// 回调 onDraw()
if (!dirtyOpaque) onDraw(canvas);
// 分发 draw()
dispatchDraw(canvas);
// 绘制 foreground
onDrawForeground(canvas);
return;
}
// ... 后面情况也是这几个步骤
}先画背景，然后 onDraw() 画自己，接着 dispatchDraw() 分发绘制子 View，最后画前景。来看看 FrameLayout 对dispatchDraw() 的实现，在它的父类 ViewGroup 中：
@Override
protected void dispatchDraw(Canvas canvas) {
// ...
final int childrenCount = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < childrenCount; i++) {
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
// Draw any disappearing views that have animations
if (mDisappearingChildren != null) {
for (int i = disappearingCount; i >= 0; i--) {
more |= drawChild(canvas, child, drawingTime);
}
}
// ...
}
rotected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
}最后又是一个递归调用过程。