ActivityThread读书笔记2-onResume的调用时机

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之前一片文章，学习了ActivityThread是App的真正入口，他在内部维护了一个ApplicationThread对象，他和远程的ams进行通信。当创建Activity时，调用ams通过Binder机制调用ApplicationThread对象的方法，进而调用ActivityThread中的handleResumeActivity方法，最终调用Activity中的onResume()方法。

在以前的印象中onResume()这个生命周期表示当前的Activity对用户可见,这是不是表示onResume()之前页面就已经绘制出来了呢？Activity的UI实在什么时候开始绘制的呢？带着这些疑问来研究研究。。。

/**
 * <p>Keep in mind that onResume is not the best indicator that your activity
 * is visible to the user; a system window such as the keyguard may be in
 * front.  Use {@link #onWindowFocusChanged} to know for certain that your
 * activity is visible to the user (for example, to resume a game).
 * 只有当onWindowFocusChanged方法被调用的时候，才能确认当前的Activity对用户的可见的
 */
@CallSuper
protected void onResume() {
    if (DEBUG_LIFECYCLE) Slog.v(TAG, "onResume " + this);
    getApplication().dispatchActivityResumed(this);
    mActivityTransitionState.onResume();
    mCalled = true;
}

我去，颠覆三观，一直以为onResume的时候，Activity就对用户可见了。

为了研究这个问题，还得回到ActivityThread类里面来分析，看看handleResumeActivity方法

final void handleResumeActivity(IBinder token,
        boolean clearHide, boolean isForward, boolean reallyResume) {
    ...

    // 调用Activity的performResume()方法，即onResume()方法
    ActivityClientRecord r = performResumeActivity(token, clearHide);

    if (r != null) {
        final Activity a = r.activity;
        ...

        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 (a.mVisibleFromClient) {
                a.mWindowAdded = true;
                wm.addView(decor, 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;
        }

        // Get rid of anything left hanging around.
        cleanUpPendingRemoveWindows(r);

        // The window is now visible if it has been added, we are not
        // simply finishing, and we are not starting another activity.
        //window这个时候已经是可见的
        if (!r.activity.mFinished && willBeVisible
                && r.activity.mDecor != null && !r.hideForNow) {
            ...
            WindowManager.LayoutParams l = r.window.getAttributes();
            if ((l.softInputMode
                    & WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)
                    != forwardBit) {
                l.softInputMode = (l.softInputMode
                        & (~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION))
                        | forwardBit;
                if (r.activity.mVisibleFromClient) {
                    ViewManager wm = a.getWindowManager();
                    View decor = r.window.getDecorView();
                    wm.updateViewLayout(decor, l);//更新视图
                }
            }
            r.activity.mVisibleFromServer = true;
            mNumVisibleActivities++;
            if (r.activity.mVisibleFromClient) {
                r.activity.makeVisible();//调用Activity的makeVisible()方法
            }
        }

        ...

    } else {
        // If an exception was thrown when trying to resume, then
        // just end this activity.
        try {
            ActivityManagerNative.getDefault()
                .finishActivity(token, Activity.RESULT_CANCELED, null, false);
        } catch (RemoteException ex) {
        }
    }
}

在这个方法中，做了三件事：

• 调用performResumeActivity方法，这个方法会调用Activity的performResume()方法，即onResume()方法
• 绘制UI

    if (r.activity.mVisibleFromClient) {
         ViewManager wm = a.getWindowManager();
         view decor = r.window.getDecorView();
         wm.updateViewLayout(decor, l);//更新视图
    }

• 调用Activity的makeVisible()方法

    void makeVisible() {
            if (!mWindowAdded) {
                ViewManager wm = getWindowManager();
                wm.addView(mDecor, getWindow().getAttributes());
                mWindowAdded = true;
            }
            mDecor.setVisibility(View.VISIBLE);//这个时候Activity设置的视图才真正对用户可见
    }

从上面的分析来看，Activity的生命周期方法onResume()的调用，并不会意味着View已经对用户可见，只是表示系统将立刻绘制Activity设置的视图。