目录：
1 MessageQueue next()
2 Vsync
3 Choreographer doFrame
4 input

系统是一个无限循环的模型， Android也不例外，进程被创建后就陷入了无限循环的状态

系统运行最重要的两个概念：输入，输出。

• 输入： 按键事件、触摸事件、鼠标事件、轨迹球事件
• 输出： Choreographer 如何指挥完成一帧的绘制

Android 中输入 输出 的往复循环都是在 looper 中消息机制驱动下完成的

looper 的循环中， messageQueue next 取消息进行处理， 处理输入事件， 进行输出， 完成和用户交互

主线程

应用生命周期内会不断 产生 message 到 messageQueue 中， 有： java层 也有 native层

其中最核心的方法就是 messageQueue 的 next 方法， 其中会先处理 java 层消息， 当 java 层没有消息时候， 会执行 nativePollOnce 来处理 native 的消息 以及监听 fd 各种事件

从硬件来看， 屏幕不会一直刷新， 屏幕的刷新只需要符合人眼的视觉停留机制

24Hz ， 连续刷新每一帧， 人眼就会认为画面是流畅的

所以我们只需要配合上这个频率， 在需要更新 UI 的时候执行绘制操作

如何以这个频率进行绘制每一帧： Android 的方案是 Vsync 信号驱动。

Vsync 信号的频率就是 24Hz ， 也就是每隔 16.6667 ms 发送一次 Vsync 信号提示系统合成一帧。

监听屏幕刷新来发送 Vsync 信号的能力，应用层 是做不到的， 系统是通过 jni 回调到 Choreographer 中的 Vsync 监听， 将这个重要信号从 native 传递到 java 层。

总体来说 输入事件获取 Vsync信号获取 都是先由 native 捕获事件 然后 jni 到 java 层实现业务逻辑

执行的是 messageQueue 中的关键方法： next

1 MessageQueue next()

next 主要的逻辑分为： java 部分 和 native 部分

java 上主要是取java层的 messageQueue msg 执行， 无 msg 就 idleHandler

java层 无 msg 会执行 native 的 pollOnce@Looper

next@MessageQueue

native message

native looper 中 fd 监听封装为 requestQueue， epoll_wait 将 fd 中的事件和对应 request 封装为 response 处理， 处理的时候会调用 fd 对应的 callback 的 handleEvent

    /**
     * Waits for events to be available, with optional timeout in milliseconds.
     * Invokes callbacks for all file descriptors on which an event occurred.
     *
     * If the timeout is zero, returns immediately without blocking.
     * If the timeout is negative, waits indefinitely until an event appears.
     *
     * Returns POLL_WAKE if the poll was awoken using wake() before
     * the timeout expired and no callbacks were invoked and no other file
     * descriptors were ready.
     *
     * Returns POLL_CALLBACK if one or more callbacks were invoked.
     *
     * Returns POLL_TIMEOUT if there was no data before the given
     * timeout expired.
     *
     * Returns POLL_ERROR if an error occurred.
     *
     * Returns a value >= 0 containing an identifier if its file descriptor has data
     * and it has no callback function (requiring the caller here to handle it).
     * In this (and only this) case outFd, outEvents and outData will contain the poll
     * events and data associated with the fd, otherwise they will be set to NULL.
     *
     * This method does not return until it has finished invoking the appropriate callbacks
     * for all file descriptors that were signalled.
     */
    int pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData);

native 层 pollOnce 主要做的事情是：

• epoll_wait 监听 fd 封装为 response
• 处理所有 native message
• 处理 response 回调 handleEvent ，handleEvent 很多回回调到 java 层

vsync 信号，输入事件， 都是通过这样的机制完成的。

2 vsync

epoll_wait 机制 拿到的 event ， 都在 response pollOnce pollInner 处理了

这里的 dispatchVsync 从 native 回到 java 层

native：

int DisplayEventDispatcher::handleEvent(int, int events, void*) {
    if (events & (Looper::EVENT_ERROR | Looper::EVENT_HANGUP)) {
        ALOGE("Display event receiver pipe was closed or an error occurred.  "
              "events=0x%x",
              events);
        return 0; // remove the callback
    }
......
        dispatchVsync(vsyncTimestamp, vsyncDisplayId, vsyncCount, vsyncEventData);
    }

java：

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

    private final class FrameDisplayEventReceiver extends DisplayEventReceiver
            implements Runnable {

        @Override
        public void onVsync(long timestampNanos, long physicalDisplayId, int frame) {
        
            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);
        }
    }

收到 Vsync 信号后， Choreographer 执行 doFrame

应用层重要的工作几乎都在 doFrame 中

3 Choreographer doFrame

首先看下 doFrame 执行了什么：

        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);
        }

UI 线程的核心工作就在这几个方法中：

上述执行 callback 的过程就对应了图片中 依次处理 input animation traversal 这几个关键过程

执行的周期是 16.6ms， 实际可能因为一些 delay 造成一些延迟、丢帧

4 input

input 事件的整体逻辑和 vsync 类似

native handleEvent ，在 NativeInputEventReceiver 中处理事件， 区分不同事件会通过 JNI

走到 java 层，WindowInputEventReceiver 然后进行分发消费

native :

int NativeInputEventReceiver::handleEvent(int receiveFd, int events, void* data) {
    // Allowed return values of this function as documented in LooperCallback::handleEvent
    constexpr int REMOVE_CALLBACK = 0;
    constexpr int KEEP_CALLBACK = 1;


    if (events & ALOOPER_EVENT_INPUT) {
        JNIEnv* env = AndroidRuntime::getJNIEnv();
        status_t status = consumeEvents(env, false /*consumeBatches*/, -1, nullptr);
        mMessageQueue->raiseAndClearException(env, "handleReceiveCallback");
        return status == OK || status == NO_MEMORY ? KEEP_CALLBACK : REMOVE_CALLBACK;
    }

status_t NativeInputEventReceiver::consumeEvents(JNIEnv* env,
        bool consumeBatches, nsecs_t frameTime, bool* outConsumedBatch) {
 
    for (;;) {
        uint32_t seq;
        InputEvent* inputEvent;

        status_t status = mInputConsumer.consume(&mInputEventFactory,
                consumeBatches, frameTime, &seq, &inputEvent);
        if (status != OK && status != WOULD_BLOCK) {
            ALOGE("channel '%s' ~ Failed to consume input event.  status=%s(%d)",
                  getInputChannelName().c_str(), statusToString(status).c_str(), status);
            return status;
        }

        if (status == WOULD_BLOCK) {
            if (!skipCallbacks && !mBatchedInputEventPending && mInputConsumer.hasPendingBatch()) {
                // There is a pending batch.  Come back later.
                if (!receiverObj.get()) {
                    receiverObj.reset(jniGetReferent(env, mReceiverWeakGlobal));
                    if (!receiverObj.get()) {
                        ALOGW("channel '%s' ~ Receiver object was finalized "
                              "without being disposed.",
                              getInputChannelName().c_str());
                        return DEAD_OBJECT;
                    }
                }

                mBatchedInputEventPending = true;
                if (kDebugDispatchCycle) {
                    ALOGD("channel '%s' ~ Dispatching batched input event pending notification.",
                          getInputChannelName().c_str());
                }

                env->CallVoidMethod(receiverObj.get(),
                                    gInputEventReceiverClassInfo.onBatchedInputEventPending,
                                    mInputConsumer.getPendingBatchSource());
                if (env->ExceptionCheck()) {
                    ALOGE("Exception dispatching batched input events.");
                    mBatchedInputEventPending = false; // try again later
                }
            }
            return OK;
        }
        assert(inputEvent);

        if (!skipCallbacks) {
            if (!receiverObj.get()) {
                receiverObj.reset(jniGetReferent(env, mReceiverWeakGlobal));
                if (!receiverObj.get()) {
                    ALOGW("channel '%s' ~ Receiver object was finalized "
                            "without being disposed.", getInputChannelName().c_str());
                    return DEAD_OBJECT;
                }
            }

            jobject inputEventObj;
            switch (inputEvent->getType()) {
            case AINPUT_EVENT_TYPE_KEY:
                if (kDebugDispatchCycle) {
                    ALOGD("channel '%s' ~ Received key event.", getInputChannelName().c_str());
                }
                inputEventObj = android_view_KeyEvent_fromNative(env,
                        static_cast<KeyEvent*>(inputEvent));
                break;

            case AINPUT_EVENT_TYPE_MOTION: {
                if (kDebugDispatchCycle) {
                    ALOGD("channel '%s' ~ Received motion event.", getInputChannelName().c_str());
                }
                MotionEvent* motionEvent = static_cast<MotionEvent*>(inputEvent);
                if ((motionEvent->getAction() & AMOTION_EVENT_ACTION_MOVE) && outConsumedBatch) {
                    *outConsumedBatch = true;
                }
                inputEventObj = android_view_MotionEvent_obtainAsCopy(env, motionEvent);
                break;
            }
            case AINPUT_EVENT_TYPE_FOCUS: {
                FocusEvent* focusEvent = static_cast<FocusEvent*>(inputEvent);
                if (kDebugDispatchCycle) {
                    ALOGD("channel '%s' ~ Received focus event: hasFocus=%s, inTouchMode=%s.",
                          getInputChannelName().c_str(), toString(focusEvent->getHasFocus()),
                          toString(focusEvent->getInTouchMode()));
                }
                env->CallVoidMethod(receiverObj.get(), gInputEventReceiverClassInfo.onFocusEvent,
                                    jboolean(focusEvent->getHasFocus()),
                                    jboolean(focusEvent->getInTouchMode()));
                finishInputEvent(seq, true /* handled */);
                continue;
            }
            case AINPUT_EVENT_TYPE_CAPTURE: {
                const CaptureEvent* captureEvent = static_cast<CaptureEvent*>(inputEvent);
                if (kDebugDispatchCycle) {
                    ALOGD("channel '%s' ~ Received capture event: pointerCaptureEnabled=%s",
                          getInputChannelName().c_str(),
                          toString(captureEvent->getPointerCaptureEnabled()));
                }
                env->CallVoidMethod(receiverObj.get(),
                                    gInputEventReceiverClassInfo.onPointerCaptureEvent,
                                    jboolean(captureEvent->getPointerCaptureEnabled()));
                finishInputEvent(seq, true /* handled */);
                continue;
            }
            case AINPUT_EVENT_TYPE_DRAG: {
                const DragEvent* dragEvent = static_cast<DragEvent*>(inputEvent);
                if (kDebugDispatchCycle) {
                    ALOGD("channel '%s' ~ Received drag event: isExiting=%s",
                          getInputChannelName().c_str(), toString(dragEvent->isExiting()));
                }
                env->CallVoidMethod(receiverObj.get(), gInputEventReceiverClassInfo.onDragEvent,
                                    jboolean(dragEvent->isExiting()), dragEvent->getX(),
                                    dragEvent->getY());
                finishInputEvent(seq, true /* handled */);
                continue;
            }

            default:
                assert(false); // InputConsumer should prevent this from ever happening
                inputEventObj = nullptr;
            }

            if (inputEventObj) {
                if (kDebugDispatchCycle) {
                    ALOGD("channel '%s' ~ Dispatching input event.", getInputChannelName().c_str());
                }
                env->CallVoidMethod(receiverObj.get(),
                        gInputEventReceiverClassInfo.dispatchInputEvent, seq, inputEventObj);
                if (env->ExceptionCheck()) {
                    ALOGE("Exception dispatching input event.");
                    skipCallbacks = true;
                }
                env->DeleteLocalRef(inputEventObj);
            } else {
                ALOGW("channel '%s' ~ Failed to obtain event object.",
                        getInputChannelName().c_str());
                skipCallbacks = true;
            }
        }
    }
}

java:

    final class WindowInputEventReceiver extends InputEventReceiver {
        public WindowInputEventReceiver(InputChannel inputChannel, Looper looper) {
            super(inputChannel, looper);
        }

        @Override
        public void onInputEvent(InputEvent event) {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "processInputEventForCompatibility");
            List<InputEvent> processedEvents;
            try {
                processedEvents =
                    mInputCompatProcessor.processInputEventForCompatibility(event);
            } finally {
                Trace.traceEnd(Trace.TRACE_TAG_VIEW);
            }
            if (processedEvents != null) {
                if (processedEvents.isEmpty()) {
                    // InputEvent consumed by mInputCompatProcessor
                    finishInputEvent(event, true);
                } else {
                    for (int i = 0; i < processedEvents.size(); i++) {
                        enqueueInputEvent(
                                processedEvents.get(i), this,
                                QueuedInputEvent.FLAG_MODIFIED_FOR_COMPATIBILITY, true);
                    }
                }
            } else {
                enqueueInputEvent(event, this, 0, true);
            }
        }

        @Override
        public void onBatchedInputEventPending(int source) {
            // mStopped: There will be no more choreographer callbacks if we are stopped,
            // so we must consume all input immediately to prevent ANR
            final boolean unbuffered = mUnbufferedInputDispatch
                    || (source & mUnbufferedInputSource) != SOURCE_CLASS_NONE
                    || mStopped;
            if (unbuffered) {
                if (mConsumeBatchedInputScheduled) {
                    unscheduleConsumeBatchedInput();
                }
                // Consume event immediately if unbuffered input dispatch has been requested.
                consumeBatchedInputEvents(-1);
                return;
            }
            scheduleConsumeBatchedInput();
        }

        @Override
        public void onFocusEvent(boolean hasFocus, boolean inTouchMode) {
            windowFocusChanged(hasFocus, inTouchMode);
        }

        @Override
        public void dispose() {
            unscheduleConsumeBatchedInput();
            super.dispose();
        }
    }

input事件的处理流程：

输入event deliverInputEvent

deliver的 input 事件会来到 InputStage

InputStage 是一个责任链， 会分发消费这些 InputEvent

下面以滑动一下 recyclerView 为例子， 整体逻辑如下：

native post input callback

vsync 信号到来， 执行 doFrame，执行到 input 阶段

do input callback

touchEvent 消费， recyclerView layout 一些 ViewHolder

onbindViewHolder

scroll 中 fill 结束，会执行 一个 recyclerView viewProperty 变化， 触发了invalidate

invalidate 会走硬件加速， 一直到达 ViewRootImpl ， 从而将 Traversal 的 callback post choreographer执行到 traversal 阶段就会执行

滚动 schduleTraversals

ViewRootImpl 执行 performTraversal ， 会根据目前是否需要重新layout ， 然后执行layout， draw 等流程

Traversal

整个 input 到 traversal 结束，硬件绘制后， sync 任务到 GPU ， 然后合成一帧。

交给 SurfaceFlinger 来显示。

SurfaceFlinger 是系统进程， 每一个应用进程是一个 client 端， 通过 IPC 机制，client 将图像显示工作交给 SurfaceFlinger

launch 一个 app：