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Android输入子系统之InputDispatcher分发键盘消息过程分析

InputDispatcher分发键盘消息过程分析

在Android输入子系统之启动过程分析中，InputManagerService启动之后，会调用start函数，之后调用native层的nativeStart函数，然后调用InputManager::start函数，该函数会调用InputDispatcherThread的threadLoop函数，该函数会调用InputDispatcher的dispatchOnce函数进行一次按键分发，如果没有按键事件发生，则睡眠。

Step 1. InputDispatcher.dispatchOnce

这个函数定义在frameworks/native/services/inputflinger/InputDispatcher.cpp 中

void InputDispatcher::dispatchOnce() {
    nsecs_t nextWakeupTime = LONG_LONG_MAX;
    { // acquire lock
        AutoMutex _l(mLock);
        mDispatcherIsAliveCondition.broadcast();

        // Run a dispatch loop if there are no pending commands.
        // The dispatch loop might enqueue commands to run afterwards.
        if (!haveCommandsLocked()) {
            dispatchOnceInnerLocked(&nextWakeupTime);
        }

        // Run all pending commands if there are any.
        // If any commands were run then force the next poll to wake up immediately.
        if (runCommandsLockedInterruptible()) {
            nextWakeupTime = LONG_LONG_MIN;
        }
    } // release lock

    // Wait for callback or timeout or wake.  (make sure we round up, not down)
    nsecs_t currentTime = now();
    int timeoutMillis = toMillisecondTimeoutDelay(currentTime, nextWakeupTime);
    mLooper->pollOnce(timeoutMillis);
}

上述函数主要是调用dispatchOnceInnerLocked来进行一次按键分发，当没有按键消息时会走到mLooper->pollOnce(timeoutMillis);这个函数会进入睡眠状态，当有按键消息发生时该函数会返回，然后走到dispatchOnceInnerLocked函数。这里mLooper->pollOnce为何会睡眠涉及到Android的Handler机制，后续再研究

Step 2. InputDispatcher.dispatchOnceInnerLocked

函数定义在frameworks/native/services/inputflinger/InputDispatcher.cpp 中

void InputDispatcher::dispatchOnceInnerLocked(nsecs_t* nextWakeupTime) {
    nsecs_t currentTime = now();
    ...
    // Ready to start a new event.
    // If we don't already have a pending event, go grab one.
    if (! mPendingEvent) {
        //当InputReader往队列中插入了一个读取的键盘消息后，此处的mInboundQueue就不为空
        if (mInboundQueue.isEmpty()) {
            ...
        } else {
            // Inbound queue has at least one entry.
            mPendingEvent = mInboundQueue.dequeueAtHead();
            ...
        }
        ...
    }
    ...
    switch (mPendingEvent->type) {
    ...
    case EventEntry::TYPE_KEY: {

        KeyEntry* typedEntry = static_cast<KeyEntry*>(mPendingEvent);
        ...
        done = dispatchKeyLocked(currentTime, typedEntry, &dropReason, nextWakeupTime);
        break;
    }
    ...
    }

    if (done) {
        if (dropReason != DROP_REASON_NOT_DROPPED) {
            dropInboundEventLocked(mPendingEvent, dropReason);
        }
        mLastDropReason = dropReason;

        releasePendingEventLocked();
        *nextWakeupTime = LONG_LONG_MIN;  // force next poll to wake up immediately
    }
}

从前文InputReader读取键盘消息过程分析 InputReader读取到一个消息后会调用KeyboardInputMapper的processKey，该函数会调用InputDispatcher的notifyKey函数，然后InputDispatcher会调用enqueueInboundEventLocked函数，将EventEntry加入到mInboundQueue中，然后调用mLooper->wake函数会唤醒InputDispatcherThread线程，InputDispatcher中把队列的第一个事件取出来，因为这里是键盘事件，所以mPendingEvent->type是EventEntry::TYPE_KEY，然后调用dispatchKeyLocked函数