首次 View 的绘制流程是在什么时候触发的？

[Sogrey]

在ActivityThread.handleResumeActivity 里触发的。

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

    // ...
    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;
        // ...
        if (a.mVisibleFromClient) {
            if (!a.mWindowAdded) {
                a.mWindowAdded = true;
                wm.addView(decor, l);
            } else {
              // ...
            }
        }

        // 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;
    }
    // ...

}

ActivityThread.handleResumeActivity 里会调用 wm.addView 来添加 DecorView，wm 是 WindowManagerImpl

// WindowManagerImpl
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
    applyDefaultToken(params);
    mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}
// WindowManagerGlobal
public void addView(View view, ViewGroup.LayoutParams params,
        Display display, Window parentWindow) {
    // 这里的 view 就是 DecorView
    // ...
    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 {
            root.setView(view, wparams, panelParentView);
        } catch (RuntimeException e) {
        }
    }
}
// ViewRootImpl.setView
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
  requestLayout();
}

最终通过 WindowManagerImpl.addView -> WindowManagerGlobal.addView -> ViewRootImpl.setView -> ViewRootImpl.requestLayout 就触发了第一次 View 的绘制。