java中的onresume_android onCreate onResume中获取 View 宽高为0分析

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1、问题测试

xmlns:tools=”http://schemas.android.com/tools”

android:layout_width=”match_parent”

android:layout_height=”match_parent”>

android:id=”@+id/btn”

android:layout_width=”100dp”

android:layout_height=”40dp” />

MainActivity.java

public class MainActivity extends AppCompatActivity {

private Button mBtn;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

mBtn = (Button) findViewById(R.id.btn);

Log.d(“TAG”, “onCreate() button width=” + mBtn.getWidth());

}

@Override

protected void onResume() {

super.onResume();

mBtn.post(new Runnable() {

@Override

public void run() {

Log.d(“TAG”, “onResume() mBtn post button width=” + mBtn.getWidth());

}

});

new Handler().post(new Runnable() {

@Override

public void run() {

Log.d(“TAG”, “onResume() Handler button width=” + mBtn.getWidth());

}

});

}

}

log 结果：

image.png

根据上面的结果回产生4个疑问：

1、setContentView后获取控件的宽高为什么是0；

2、在 onResume中 handler.post 中获取控件的宽高为什么是0；

3、在 onResume中的 view.post 中为什么能获取控件宽高；

4、在 onResume 中handler.post 在 View.post 后面为什么执行反而在前面；

针对以上4个疑问进行解答

1、setContentView后获取控件的宽高为什么为0；

这个很好理解， setContentView只是解析了 xml 文件并创建了对应的控件，并没有进行控件的测量等工作；

2、在 onResume中 handler.post 中获取控件的宽高为什么是0；

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

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);

} 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;

}

…………..

}

Activity.onResume是在performResumeActivity中调用，此时还没有执行wm.addView(decor, l)创建 ViewRootImpl ，同时在 ViewRootImpl中的doTraversal()是通过mChoreographer.postCallback( Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null)发送异步消息等待下一个刷新消息才执行。所以 handler.post 消息回先执行导致获取 view 宽高失败。

ViewRootImpl.java中

@UnsupportedAppUsage

void scheduleTraversals() {

if (!mTraversalScheduled) {

mTraversalScheduled = true;

mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();

mChoreographer.postCallback(

Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);

if (!mUnbufferedInputDispatch) {

scheduleConsumeBatchedInput();

}

notifyRendererOfFramePending();

pokeDrawLockIfNeeded();

}

}

3、在 onResume中的 view.post 中为什么能获取控件宽高；

View.java 中的 post()

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;

}

通过第2点可以知道ViewRootImpl 的创建是在onResume 之后,所以此时attachInfo == null,从而消息被保存到RunQueue中，而RunQueue在ViewRootImpl的performTraversals被中执行,所以可以获取到控件宽高。

private void performTraversals(){

…

// Execute enqueued actions on every traversal in case a detached view enqueued an action

getRunQueue().executeActions(mAttachInfo.mHandler);

…

}

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;

}

}

4、在 onResume 中handler.post 在 View.post 后面为什么执行反而在前面；

通过上面第2点和点3点分析可以知道View.post的在后面performTraversals中被执行，而handler.post在performTraversals之前就被执行

image.png

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