显示系统1--Choreograhper

1. Choreograhper简介

Choreograhper作为一个中间层，负责协调动画，输入，绘制的执行过程。将底层的如VSync信号做分发，按需传递给上层应用。

2. Choreograhper源码解析：

所有对Choreograhper的使用都是通过Choreograhper类的静态方法:

    public static Choreographer getInstance() {
        return sThreadInstance.get();
    }

sThreadInstance是ThreadLocal类型, 因而它的get() 方法是拿到本线程的Choreographer对象:

// Thread local storage for the choreographer.
    private static final ThreadLocal<Choreographer> sThreadInstance =
            new ThreadLocal<Choreographer>() {
        @Override
        protected Choreographer initialValue() {
            Looper looper = Looper.myLooper();
            if (looper == null) {
                throw new IllegalStateException("The current thread must have a looper!");
            }
            Choreographer choreographer = new Choreographer(looper, VSYNC_SOURCE_APP);
            if (looper == Looper.getMainLooper()) {
                mMainInstance = choreographer;
            }
            return choreographer;
        }
    };

构造函数:

private Choreographer(Looper looper, int vsyncSource) {
        mLooper = looper;
        mHandler = new FrameHandler(looper);
//创建接收VSync信号的FrameDisplayEventReceiver对象
        mDisplayEventReceiver = USE_VSYNC
                ? new FrameDisplayEventReceiver(looper, vsyncSource)
                : null;
        mLastFrameTimeNanos = Long.MIN_VALUE;
//计算刷新的时间间隔
        mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());
//创建三个CallbackQueue对象
        mCallbackQueues = new CallbackQueue[CALLBACK_LAST + 1];
        for (int i = 0; i <= CALLBACK_LAST; i++) {
            mCallbackQueues[i] = new CallbackQueue();
        }
        // b/68769804: For low FPS experiments.
        setFPSDivisor(SystemProperties.getInt(ThreadedRenderer.DEBUG_FPS_DIVISOR, 1));
    }

其中FrameDisplayEventReceiver是最重要的, 能够接收VSync信号, 继承自DisplayEventRececiver, 构造函数:

public DisplayEventReceiver(Looper looper, int vsyncSource) {
        if (looper == null) {
            throw new IllegalArgumentException("looper must not be null");
        }

        mMessageQueue = looper.getQueue();
        mReceiverPtr = nativeInit(new WeakReference<DisplayEventReceiver>(this), mMessageQueue,
                vsyncSource);

        mCloseGuard.open("dispose");
    }

nativeInit的代码:

static jlong nativeInit(JNIEnv* env, jclass clazz, jobject receiverWeak,
        jobject messageQueueObj, jint vsyncSource) {
    sp<MessageQueue> messageQueue = android_os_MessageQueue_getMessageQueue(env, messageQueueObj);
    if (messageQueue == NULL) {
        jniThrowRuntimeException(env, "MessageQueue is not initialized.");
        return 0;
    }

    sp<NativeDisplayEventReceiver> receiver = new NativeDisplayEventReceiver(env,
            receiverWeak, messageQueue, vsyncSource);
    status_t status = receiver->initialize();
    if (status) {
        String8 message;
        message.appendFormat("Failed to initialize display event receiver.  status=%d", status);
        jniThrowRuntimeException(env, message.string());
        return 0;
    }

    receiver->incStrong(gDisplayEventReceiverClassInfo.clazz); // retain a reference for the object
    return reinterpret_cast<jlong>(receiver.get());
}

这里新建了一个NativeDisplayEventReceiver, 然后NativeDisplayEventReceiver里又建立了一个DisplayEventReceiver对象, DisplayEventReceiver的构造函数:

DisplayEventReceiver::DisplayEventReceiver(ISurfaceComposer::VsyncSource vsyncSource) {
    sp<ISurfaceComposer> sf(ComposerService::getComposerService());
    if (sf != NULL) {
        mEventConnection = sf->createDisplayEventConnection(vsyncSource);
        if (mEventConnection != NULL) {
            mDataChannel = std::make_unique<gui::BitTube>();
            mEventConnection->stealReceiveChannel(mDataChannel.get());
        }
    }
}

DisplayEventReceiver里调用了SurfaceFlinger的createDisplayEventConnection来返回一个IDisplayEventConnection。createDisplayEventConnection正是调用了他的mEventThread变量的createEventConnection函数来得到IDisplayEventConnection的。得到IDisplayEventConnection对象后，再调用他的getDataChannel()函数就可以和SurfaceFlinger中的EventThread对象进行通信了。

拿到通道后，当通道有信号到达时，会调用NativeDisplayEventReceiver对象的handleEvent函数，取到数据后调用dispatchVsync（）函数：

void NativeDisplayEventReceiver::dispatchVsync(nsecs_t timestamp, int32_t id, uint32_t count) {
    JNIEnv* env = AndroidRuntime::getJNIEnv();

    ScopedLocalRef<jobject> receiverObj(env, jniGetReferent(env, mReceiverWeakGlobal));
    if (receiverObj.get()) {
        ALOGV("receiver %p ~ Invoking vsync handler.", this);
        env->CallVoidMethod(receiverObj.get(),
                gDisplayEventReceiverClassInfo.dispatchVsync, timestamp, id, count);
        ALOGV("receiver %p ~ Returned from vsync handler.", this);
    }

    mMessageQueue->raiseAndClearException(env, "dispatchVsync");
}

dispatchVsync会调用java层的dispatchVsync，这样信号就传到了java层。
java层的dispatchVsync：

// Called from native code.
    @SuppressWarnings("unused")
private void dispatchVsync(long timestampNanos, int builtInDisplayId, int frame) {
        onVsync(timestampNanos, builtInDisplayId, frame);
    }

在Choreography里的DisplayEventReceiver是FrameDisplayEventReceiver， onVsync函数：

@Override
        public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
            // Ignore vsync from secondary display.
            // This can be problematic because the call to scheduleVsync() is a one-shot.
            // We need to ensure that we will still receive the vsync from the primary
            // display which is the one we really care about.  Ideally we should schedule
            // vsync for a particular display.
            // At this time Surface Flinger won't send us vsyncs for secondary displays
            // but that could change in the future so let's log a message to help us remember
            // that we need to fix this.
            if (builtInDisplayId != SurfaceControl.BUILT_IN_DISPLAY_ID_MAIN) {
                Log.d(TAG, "Received vsync from secondary display, but we don't support "
                        + "this case yet.  Choreographer needs a way to explicitly request "
                        + "vsync for a specific display to ensure it doesn't lose track "
                        + "of its scheduled vsync.");
                scheduleVsync();
                return;
            }

            // 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);
        }

onVsync发出一个消息后就返回了，并不影响VSync信号的继续传递，对消息的处理在run方法里。

@Override
        public void run() {
            mHavePendingVsync = false;
            doFrame(mTimestampNanos, mFrame);
        }

附FrameHandler的定义：

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:
                    doFrame(System.nanoTime(), 0);
                    break;
                case MSG_DO_SCHEDULE_VSYNC:
                    doScheduleVsync();
                    break;
                case MSG_DO_SCHEDULE_CALLBACK:
                    doScheduleCallback(msg.arg1);
                    break;
            }
        }
    }

doFramef函数：

void doFrame(long frameTimeNanos, int frame) {
    final long startNanos;
    synchronized (mLock) {
        if (!mFrameScheduled) {
            return; // mFrameScheduled=false，则直接返回。
        }

        long intendedFrameTimeNanos = frameTimeNanos; //原本计划的绘帧时间点
        startNanos = System.nanoTime();
        final long jitterNanos = startNanos - frameTimeNanos;
        if (jitterNanos >= mFrameIntervalNanos) {
            
            final long skippedFrames = jitterNanos / mFrameIntervalNanos;
            //当掉帧个数超过30，则输出相应log
            if (skippedFrames >= SKIPPED_FRAME_WARNING_LIMIT) {
                Log.i(TAG, "Skipped " + skippedFrames + " frames! "
                        + "The application may be doing too much work on its main thread.");
            }
            final long lastFrameOffset = jitterNanos % mFrameIntervalNanos;
            frameTimeNanos = startNanos - lastFrameOffset; //对齐帧的时间间隔
        }

        if (frameTimeNanos < mLastFrameTimeNanos) {
            scheduleVsyncLocked();
            return;
        }

        mFrameInfo.setVsync(intendedFrameTimeNanos, frameTimeNanos);
        mFrameScheduled = false;
        mLastFrameTimeNanos = frameTimeNanos;
    }

    try {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame");

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

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

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

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

最终有4个回调方法，依次为如下：
INPUT：输入事件
ANIMATION：动画
TRAVERSAL：窗口刷新
COMMIT

参考文章：
深入解析Android5.0系统刘超著
https://developer.android.com/reference/android/view/Choreographer
http://gityuan.com/2017/02/25/choreographer/
https://www.cnblogs.com/dasusu/p/8311324.html