Android屏幕刷新机制（一）：屏幕刷新发生了什么

参考

文章完全参考自：Android 屏幕刷新机制

不过原文是从源码揭秘的角度从代码调用的层面层层深入到了底层，教着看源码摸原理。本文倒着来，直接说了原理，然后按着Android系统的步骤看代码是怎么走的。

开始

硬件——屏幕发送信号

Android 屏幕每16.6ms刷新一次【60fps，即1000ms/60f = 16.6ms】。也就是屏幕【Display】每隔16ms就会发出一次VSYNC信号。

image-20201031131730052.png

系统——接收屏幕传来的VSync信号

接收此信号系统类叫DisplayEventReceiver，是个抽象类。

/**
 * Provides a low-level mechanism for an application to receive display events
 * such as vertical sync.
 *
 * The display event receive is NOT thread safe.  Moreover, its methods must only
 * be called on the Looper thread to which it is attached.
 *
 * @hide
 */
public abstract class DisplayEventReceiver {
    ...
    /**
     * Called when a vertical sync pulse is received.
     * 在VSync脉冲到达时会被调用
     * The recipient should render a frame and then call {@link #scheduleVsync}
     * 接受者应该rende a frame(渲染好框架？)并调用 #scheduleVsync
     * to schedule the next vertical sync pulse.
     */
    @UnsupportedAppUsage
    public void onVsync(long timestampNanos, long physicalDisplayId, int frame) {
    }
    
    ... 
        
    /**
     * Schedules a single vertical sync pulse to be delivered when the next
     * display frame begins.
     * 在下一帧到来前Schedules(规划)一个 用于被接收的 VSync脉冲
     */
    @UnsupportedAppUsage
    public void scheduleVsync() {
        if (mReceiverPtr == 0) {
            Log.w(TAG, "Attempted to schedule a vertical sync pulse but the display event "
                    + "receiver has already been disposed.");
        } else {
            nativeScheduleVsync(mReceiverPtr);
        }
    }
    
    ...
    
    
}

其中两个方法：onVsync是接收到脉冲信息时的回调，注释告诉我们在这之前会先调用scheduleVsync；scheduleVsync相当于对即将到来的下一帧注册了一个监听，只有注册了，下一帧才会调用到onVsync方法中来。

具体实现这个抽象类的是FrameDisplayEventReceiver。

private final class FrameDisplayEventReceiver extends DisplayEventReceiver implements Runnable {
    @Override
    public void onVsync(long timestampNanos, long physicalDisplayId, int frame) {
        // Post the vsync event to the Handler.
        // The idea is to prevent incoming vsync events from completely starving
        // the message queue.  If there are no messages in the queue with timestamps
        // earlier than the frame time, then the vsync event will be processed immediately.
        // Otherwise, messages that predate the vsync event will be handled first.
        long now = System.nanoTime();
        if (timestampNanos > now) {
            Log.w(TAG, "Frame time is " + ((timestampNanos - now) * 0.000001f)
                  + " ms in the future!  Check that graphics HAL is generating vsync "
                  + "timestamps using the correct timebase.");
            timestampNanos = now;
        }

        if (mHavePendingVsync) {
            Log.w(TAG, "Already have a pending vsync event.  There should only be "
                  + "one at a time.");
        } else {
            mHavePendingVsync = true;
        }

        mTimestampNanos = timestampNanos;
        mFrame = frame;
        Message msg = Message.obtain(mHandler, this);
        msg.setAsynchronous(true);
        mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
    }
    
    @Override
    public void run() {
        mHavePendingVsync = false;
        doFrame(mTimestampNanos, mFrame);
    }
}

可以看到onVsync中接收到信号后创建了一个Message,将自身（this）传了进去【因为自身实现了Runnable接口】。再通过mHandler发了出去。mHandler是一个FrameHandler，所以要去看FrameHandler。

用Handler传递消息

private final class FrameHandler extends Handler {
    public FrameHandler(Looper looper) {
        super(looper);
    }

    @Override
    public void handleMessage(Message msg) {
        switch (msg.what) {
            case MSG_DO_FRAME://0
                doFrame(System.nanoTime(), 0);
                break;
            case MSG_DO_SCHEDULE_VSYNC://1
                doScheduleVsync();
                break;
            case MSG_DO_SCHEDULE_CALLBACK://2
                doScheduleCallback(msg.arg1);
                break;
        }
    }
}

显然，传入的msg未设置msg,所以msg为0。走doFrame(System.nanoTime(), 0);

走错了，因为Handler 在处理消息时会先查看 Message 是否有 callback，有则优先交由 Message 的 callback 处理消息，没有的话再去看看Handler 有没有 callback，如果也没有才会交由 handleMessage() 这个方法执行。

所以这里会直接执行doFrame（）方法。

void doFrame(long frameTimeNanos, int frame) {
    final long startNanos;
    synchronized (mLock) {
        ...
        mFrameInfo.setVsync(intendedFrameTimeNanos, frameTimeNanos);
        mFrameScheduled = false;
        mLastFrameTimeNanos = frameTimeNanos;
    }

    try {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame");
        AnimationUtils.lockAnimationClock(frameTimeNanos / TimeUtils.NANOS_PER_MS);

        mFrameInfo.markInputHandlingStart();
        doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);

        mFrameInfo.markAnimationsStart();
        doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
        doCallbacks(Choreographer.CALLBACK_INSETS_ANIMATION, frameTimeNanos);

        mFrameInfo.markPerformTraversalsStart();
        doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);

        doCallbacks(Choreographer.CALLBACK_COMMIT, frameTimeNanos);
    } finally {
        AnimationUtils.unlockAnimationClock();
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }

}

doFrame代码有点长，里面有一段try catch 包裹的代码，不停的在doCallbacks(int,long)。源码往下滑两行就发现doCallbacks就在下面。

Handler收到消息走doFrame，又走doCallbacks

void doCallbacks(int callbackType, long frameTimeNanos) {
    CallbackRecord callbacks;
    synchronized (mLock) {
        // We use "now" to determine when callbacks become due because it's possible
        // for earlier processing phases in a frame to post callbacks that should run
        // in a following phase, such as an input event that causes an animation to start.
        final long now = System.nanoTime();
        //① 这句是关键，取出callbacks
        callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(
                now / TimeUtils.NANOS_PER_MS);
        if (callbacks == null) {
            return;
        }
        mCallbacksRunning = true;
        ...
    }
    try {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, CALLBACK_TRACE_TITLES[callbackType]);
        for (CallbackRecord c = callbacks; c != null; c = c.next) {
            if (DEBUG_FRAMES) {
                Log.d(TAG, "RunCallback: type=" + callbackType
                        + ", action=" + c.action + ", token=" + c.token
                        + ", latencyMillis=" + (SystemClock.uptimeMillis() - c.dueTime));
            }
            //② 循环调用callback.run
            c.run(frameTimeNanos);
        }
    } finally {
        synchronized (mLock) {
            mCallbacksRunning = false;
            do {
                final CallbackRecord next = callbacks.next;
                recycleCallbackLocked(callbacks);
                callbacks = next;
            } while (callbacks != null);
        }
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

① mCallbackQueues

从mCallbackQueues的第callbackType个元素中，提取一个callback【extractDueCallbacksLocked()】。

mCallbackQueues是一个长度为5的队列，存的是元素CallbackQueue。

mCallbackQueues = new CallbackQueue[CALLBACK_LAST + 1];
for (int i = 0; i <= CALLBACK_LAST; i++) {
    mCallbackQueues[i] = new CallbackQueue();
}

CallbackQueue听名字又是一个队列。没错，就是队列中存放着队列。CallbackQueue中存放的元素是CallbackRecord，链表的形式。所以大概是这个样子：

image-20201101151630877.png

②c.run(frameTimeNanos);

取出来是一串链子(callbacks)，所以一个for循环，循环调用run方法。

private static final class CallbackRecord {
    public CallbackRecord next;
    public long dueTime;
    public Object action; // Runnable or FrameCallback
    public Object token;

    @UnsupportedAppUsage
    public void run(long frameTimeNanos) {
        if (token == FRAME_CALLBACK_TOKEN) {
            ((FrameCallback)action).doFrame(frameTimeNanos);
        } else {
            ((Runnable)action).run();
        }
    }
}

嗯？token是啥？action是啥？接下来怎么走？？不知道。因为这个CallbackRecord又不是我们加进来的，至少在上面分析的代码中没看到add操作。

总结：屏幕刷新发生了什么

屏幕每16.6ms向系统层面发送一个VSync信号，系统在FrameDisplayEventReceiver的onVsync()里通过Handler发送了个Message（把FrameDisplayEventReceiver自己作为runnable传了进去）。Handler中收到message后就调用doFrame中的doCallbacks方法，从一个**链表组成的列表（CallbackQueue）**中取链子（CallbackRecord），然后调用每个链子（CallbackRecord`）的run方法。