activity view draw

Posted on Edited on Views: Views:

引言

activity 生命周期大家都比较清楚，onAttach->onCreate->onStart>onResume->onStop->onDestroy。 但是 activity 的 view 绘制到底发生在什么时候，它的创建、绘制、前台可交互等等的时期。

详解

我们在 activity onCreate 里面会设置 setContentView。实际上是创建了一个 view，它的目标是创建 DecorView ，进行初始化和布局的加载。

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mWindow = new PhoneWindow(this, window, activityConfigCallback);

    public void setContentView(@LayoutRes int layoutResID) {
       getWindow().setContentView(layoutResID);
       initWindowDecorActionBar();
   }

但在源码里面 ActivityThread 里可以看到是在 onResume 之后，把 DecorView 添加到 WindowManager，从这时候开始该 activity 开始绘制到屏幕上。由 WindowManager 移交给 ViewRootImpl 渲染绘制。绘制完成后即开始交互

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public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
           String reason) {
       ...
       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();
           ...
           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;
       }
       ...
}

总结

现在我们对 activity view 展示、初始化、绘制时机等等有了一个基础的了解。这有什么用呢？
比如如果我们想知道 activity 的绘制完成时间，那么很明显，除了重写 onWindowFocusChanged 方法外，最准确的是 DecorView 的绘制完成，可以监听其 onDraw 完成后的一次 vsync。