开局一张图，先带大家看一下Activity、Window、View建立关系的流程
然后进入正题，下面代码中的两个个方法，一个能获取到，一个获取不到，从log的表现来看，前一个方法的log还出现的更晚，具体原因，咱们细细道来。
@Override
protected void onResume() {
super.onResume();
view.post(new Runnable() {
@Override
public void run() {
Log.e("Test", "onResume() mBtn post button width=" + mBtn.getWidth());
}
});
new Handler().post(new Runnable() {
@Override
public void run() {
Log.e("Test", "onResume() Handler button width=" + view.getWidth());
}
});
}
1、为什么在 onResume中， handler.post 获取控件的宽高是0
Activity启动流程做参考
在上图的最下方的
scheduleLaunchActivity() 接口方法之后的流程：handleLaunchActivity() -> handleResumeActivity() -> performResumeActivity() -> Activity#performResume() -> Instrumentation#callActivityOnResume() -> Activity#onResume()
由上面一段做引子，我们先来看ActivityThread.java类中handleResumeActivity函数
@Override
public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
String reason) {
...
// TODO Push resumeArgs into the activity for consideration
//执行 onResume
final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
if (r == null) {
// We didn't actually resume the activity, so skipping any follow-up actions.
return;
}
...
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (r.mPreserveWindow) {
a.mWindowAdded = true;
r.mPreserveWindow = false;
// Normally the ViewRoot sets up callbacks with the Activity
// in addView->ViewRootImpl#setView. If we are instead reusing
// the decor view we have to notify the view root that the
// callbacks may have changed.
ViewRootImpl impl = decor.getViewRootImpl();
if (impl != null) {
impl.notifyChildRebuilt();
}
}
if (a.mVisibleFromClient) {
if (!a.mWindowAdded) {
a.mWindowAdded = true;
wm.addView(decor, l); //建立联系
} else {
// The activity will get a callback for this {@link LayoutParams} change
// earlier. However, at that time the decor will not be set (this is set
// in this method), so no action will be taken. This call ensures the
// callback occurs with the decor set.
a.onWindowAttributesChanged(l);
}
}
// If the window has already been added, but during resume
// we started another activity, then don't yet make the
// window visible.
} else if (!willBeVisible) {
if (localLOGV) Slog.v(TAG, "Launch " + r + " mStartedActivity set");
r.hideForNow = true;
}
...
}
在handleResumeActivity方法中可见，performResumeActivity方法的执行先于wm.addView(decor, l)方法。
performResumeActivity方法代表着Activity.onResume的执行。
而wm.addView(decor, l)方法代表着window和view建立关系，然后一系列动作就开始了：
- 此时会新建
ViewRootImpl，并调用setView方法
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
//...
ViewRootImpl root;
View panelParentView = null;
synchronized (mLock) {
//...
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
}
// do this last because it fires off messages to start doing things
try {
//触发开发绘制
root.setView(view, wparams, panelParentView);
} catch (RuntimeException e) {
//...
throw e;
}
}
-
setView方法会调用requestLayout方法，触发界面刷新，requestLayout方法算是老熟人了。
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
synchronized (this) {
if (mView == null) {
mView = view;
//...
// Schedule the first layout -before- adding to the window
// manager, to make sure we do the relayout before receiving
// any other events from the system.
//触发界面刷新
requestLayout();
if ((mWindowAttributes.inputFeatures
& WindowManager.LayoutParams.INPUT_FEATURE_NO_INPUT_CHANNEL) == 0) {
mInputChannel = new InputChannel();
}
//...
try {
mOrigWindowType = mWindowAttributes.type;
mAttachInfo.mRecomputeGlobalAttributes = true;
collectViewAttributes();
res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
getHostVisibility(), mDisplay.getDisplayId(),
mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
mAttachInfo.mOutsets, mInputChannel);
} catch (RemoteException e) {
//...
throw new RuntimeException("Adding window failed", e);
} finally {
if (restore) {
attrs.restore();
}
}
//...
//这里的 view 是 DecorView
view.assignParent(this);
//...
}
}
}
- requestLayout 会调用
scheduleTraversals方法去发送一个消息，然后执行doTraversal方法进而调用performTraversals方法
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
//准备刷新，参考 3
scheduleTraversals();
}
}
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
//设置同步障碍 Message
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
//屏幕刷新信号 VSYNC 监听回调把 mTraversalRunnable（执行doTraversal()）push 到主线程了，异步 Message 会优先得到执行 ，具体看下 Choreographer 的实现
//mTraversalRunnable，参考 TraversalRunnable
mChoreographer.postCallback(Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
final class TraversalRunnable implements Runnable {
@Override
public void run() {
//参考 doTraversal
doTraversal();
}
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
//移除同步障碍 Message
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
//参考 ，在上面移除同步障碍后，开始对控件树进行测量、布局、绘制
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
private void performTraversals() {
// cache mView since it is used so much below...
final View host = mView;
//...
Rect frame = mWinFrame;
if (mFirst) {
mFullRedrawNeeded = true;
mLayoutRequested = true;
//...
// We used to use the following condition to choose 32 bits drawing caches:
// PixelFormat.hasAlpha(lp.format) || lp.format == PixelFormat.RGBX_8888
// However, windows are now always 32 bits by default, so choose 32 bits
mAttachInfo.mUse32BitDrawingCache = true;
mAttachInfo.mHasWindowFocus = false;
mAttachInfo.mWindowVisibility = viewVisibility;
mAttachInfo.mRecomputeGlobalAttributes = false;
mLastConfiguration.setTo(host.getResources().getConfiguration());
mLastSystemUiVisibility = mAttachInfo.mSystemUiVisibility;
// Set the layout direction if it has not been set before (inherit is the default)
if (mViewLayoutDirectionInitial == View.LAYOUT_DIRECTION_INHERIT) {
host.setLayoutDirection(mLastConfiguration.getLayoutDirection());
}
host.dispatchAttachedToWindow(mAttachInfo, 0);
mAttachInfo.mTreeObserver.dispatchOnWindowAttachedChange(true);
dispatchApplyInsets(host);
//Log.i(mTag, "Screen on initialized: " + attachInfo.mKeepScreenOn);
} else {
desiredWindowWidth = frame.width();
desiredWindowHeight = frame.height();
if (desiredWindowWidth != mWidth || desiredWindowHeight != mHeight) {
if (DEBUG_ORIENTATION) Log.v(mTag, "View " + host + " resized to: " + frame);
mFullRedrawNeeded = true;
mLayoutRequested = true;
windowSizeMayChange = true;
}
}
//...
// Execute enqueued actions on every traversal in case a detached view enqueued an action
getRunQueue().executeActions(mAttachInfo.mHandler);
//...
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
performLayout(lp, mWidth, mHeight);
performDraw();
//...
}
所以 handler.post 消息回先执行导致获取 view 宽高失败，而new Handler.postDelay(runnable, 1000)再等待合适的时间后可能成功。
2.view.post 中为什么能获取控件宽高
1.我们来看View.java中的post方法
public boolean post(Runnable action) {
//mAttachInfo 是在 ViewRootImpl 的构造函数中初始化的
//而 ViewRootmpl 的初始化是在 addView() 中调用
//所以此处的 mAttachInfo 为空，所以不会执行该 if 语句
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.post(action);
}
// Postpone the runnable until we know on which thread it needs to run.
// Assume that the runnable will be successfully placed after attach.
//保存消息到 RunQueue 中，等到在 performTraversals() 方法中被执行
getRunQueue().post(action);
return true;
}
2.getRunQueue返回HandlerActionQueue
private HandlerActionQueue getRunQueue() {
if (mRunQueue == null) {
mRunQueue = new HandlerActionQueue();
}
return mRunQueue;
}
3.把View#post()的消息放到了HandlerAction数组中
public class HandlerActionQueue {
private HandlerAction[] mActions;
private int mCount;
public void post(Runnable action) {
postDelayed(action, 0);
}
public void postDelayed(Runnable action, long delayMillis) {
final HandlerAction handlerAction = new HandlerAction(action, delayMillis);
synchronized (this) {
if (mActions == null) {
mActions = new HandlerAction[4];
}
mActions = GrowingArrayUtils.append(mActions, mCount, handlerAction);
mCount++;
}
}
...
}
4.在View#dispatchAttachedToWindow()方法中，才给mAttachInfo赋了值，也是在这个方法里执行了executeActions方法，相当于是mAttachInfo不为空了，开始把HandlerAction保存下来的消息发出去吧
void dispatchAttachedToWindow(AttachInfo info, int visibility) {
mAttachInfo = info;
if (mOverlay != null) {
mOverlay.getOverlayView().dispatchAttachedToWindow(info, visibility);
}
mWindowAttachCount++;
// We will need to evaluate the drawable state at least once.
mPrivateFlags |= PFLAG_DRAWABLE_STATE_DIRTY;
if (mFloatingTreeObserver != null) {
info.mTreeObserver.merge(mFloatingTreeObserver);
mFloatingTreeObserver = null;
}
registerPendingFrameMetricsObservers();
if ((mPrivateFlags&PFLAG_SCROLL_CONTAINER) != 0) {
mAttachInfo.mScrollContainers.add(this);
mPrivateFlags |= PFLAG_SCROLL_CONTAINER_ADDED;
}
// Transfer all pending runnables.
if (mRunQueue != null) {
mRunQueue.executeActions(info.mHandler);
mRunQueue = null;
}
...
}
5.HandlerActionQueue#executeActions()把消息从HandlerAction数组中转到handler中，正儿八经开始发送消息
public void executeActions(Handler handler) {
synchronized (this) {
final HandlerAction[] actions = mActions;
for (int i = 0, count = mCount; i < count; i++) {
final HandlerAction handlerAction = actions[i];
handler.postDelayed(handlerAction.action, handlerAction.delay);
}
mActions = null;
mCount = 0;
}
}
按理说这样已经能够基本证明为什么view.post能够获取高度，大白话说就是
因为attachInfo为空，View把post的runnable保存了下来，等待attachInfo不为空的时候发消息去执行runnable
但是为了严谨，还要继续做一下验证，例如
6.步骤4中的info.handler是ViewRootImpl的handler
public ViewRootImpl(Context context, Display display) {
mContext = context;
mWindowSession = WindowManagerGlobal.getWindowSession();
mDisplay = display;
mBasePackageName = context.getBasePackageName();
mThread = Thread.currentThread();
mLocation = new WindowLeaked(null);
mLocation.fillInStackTrace();
mWidth = -1;
mHeight = -1;
mDirty = new Rect();
mTempRect = new Rect();
mVisRect = new Rect();
mWinFrame = new Rect();
mWindow = new W(this);
mTargetSdkVersion = context.getApplicationInfo().targetSdkVersion;
mViewVisibility = View.GONE;
mTransparentRegion = new Region();
mPreviousTransparentRegion = new Region();
mFirst = true; // true for the first time the view is added
mAdded = false;
mAttachInfo = new View.AttachInfo(mWindowSession, mWindow, display, this, mHandler, this,
context);
...
}
...
final ViewRootHandler mHandler = new ViewRootHandler();
...
7.布局测量的handler是ViewRootImpl的，和步骤4中View#post()的handler是同一个
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
//指的是这个handler
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
8.ViewRootImpl#performTraversals()调用View#dispatchAttachedToWindow()，说明 界面绘制 已经开始了
步骤4中，View#post()的消息从HandlerAction数组中转到ViewRootImpl#mHandler中，说明在ViewRootImpl#mHandler中，View#post()的消息是在 界面绘制 消息之后的，所以View#post()这个消息是能获得宽高的。
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