大家讲起事件传递时，都会从Window RootViewImpl 讲起 然后再讲View层的一些处理逻辑，
但传递的事件到底是从哪里来的，当我们触摸屏幕的时候MotionEvent是如何被创建 并传送到目标窗口的，
这里面系统都做了什么呢？

猜想

事件一定有一个从捕获到分发的过程，为了保证实时性因此会有一个线程一直在轮询监听事件，一旦捕获到触摸事件，通过某种规则筛选出所需的窗口，然后通过某种通讯方式分发给所需窗口

一探究竟（基于Android7.1）

我们知道系统服务会在SystemServer中进行实例化操作，我们先将目光放在InputManagerService（以下简称IMS）上，InputManagerService是Android处理各种用户操作抽象的一个服务，是InputManager的真身，在SystemServer中实例化后与WindowManagerService（以下简称WMS）一起添加到ServiceManager中

private void startOtherServices() {inputManager = new InputManagerService(context);wm = WindowManagerService.main(context, inputManager,mFactoryTestMode != FactoryTest.FACTORY_TEST_LOW_LEVEL,!mFirstBoot, mOnlyCore);ServiceManager.addService(Context.WINDOW_SERVICE, wm);ServiceManager.addService(Context.INPUT_SERVICE, inputManager);}

由上面代码片段可以看出 IMS与WMS同时被添加进ServiceManager中，WMS有着IMS的引用
有此推断IMS负责触摸事件的捕获，WMS负责将事件派发到目标窗口上。
IMS构造方法中调用nativeInit创建NativeInputManager

NativeInputManager::NativeInputManager(jobject contextObj,jobject serviceObj, const sp<Looper>& looper) :mLooper(looper), mInteractive(true) {//......sp<EventHub> eventHub = new EventHub();mInputManager = new InputManager(eventHub, this, this);
}

这里EventHub 主要负责监听设备事件包括输入设备的拔插，触摸 按钮 鼠标事件等，通过EventHub的getEvents就可以监听并获取该事件，
主要是面向/dev/input 目录下的event设备节点，我们可以用cat 命令查看这些节点，当有输入设备有输入时对应节点会有相应的打印，或者通过getevent命令查看如下

此打印是我 按home键与返回键的打印（如果你此时触摸屏幕会看到跟多打印）
打印为16进制对应过来66 对应 102 ，9e 对应158
查看 system/usr/keylayout/ 对应关系如下

我们接下来看InputManager，InputManager 创建时伴随创建了 InputDispatcher 与 InputReader 并将
InputDispather 与EventHub传入了InputReader

通过上面可以看到 伴随创建完之后调用了initialize
并开启了两个线程 通过字面意思也可看出
InputReaderThread 负责读取 InputDispatcherThread 负责分发

先来看读取InputReaderThread

InputReaderThread::InputReaderThread(const sp<InputReaderInterface>& reader) :Thread(/*canCallJava*/ true), mReader(reader) {
}
bool InputReaderThread::threadLoop() {mReader->loopOnce();return true;
}
void InputReader::loopOnce() {//获取事件 并处理分发size_t count = mEventHub->getEvents(timeoutMillis, mEventBuffer, EVENT_BUFFER_SIZE);processEventsLocked(mEventBuffer, count);//派发 这里的 为在InputManger中穿件传入的 mDispatcher mQueuedListener->flush();
}

由上可知 InputManager -> initialize创建一个InputReaderThread ThreadLoop线程 不断的读取输入事件
然后通知 派发 读取流程图如下

接下来看如何分发 InputDispather

在InputManger创建时可知，InputReader传入的mQueuedListener其实就是InputDispatcher对象， 所以mQueuedListener->flush()就是通知InputDispatcher事件读取完毕，可以派发事件了

bool InputDispatcherThread::threadLoop() {mDispatcher->dispatchOnce();return true;
}
void InputDispatcher::dispatchOnce() {{ //被唤醒执行if (!haveCommandsLocked()) {dispatchOnceInnerLocked(&nextWakeupTime);}} //睡眠等待事件mLooper->pollOnce(timeoutMillis);
}

以上就是分发线程的简化代码，dispatchOnceInnerLocked是具体的分发处理逻辑，简化后具体如下

void InputDispatcher::dispatchOnceInnerLocked(nsecs_t* nextWakeupTime) {case EventEntry::TYPE_CONFIGURATION_CHANGED: {ConfigurationChangedEntry* typedEntry =static_cast<ConfigurationChangedEntry*>(mPendingEvent);done = dispatchConfigurationChangedLocked(currentTime, typedEntry);dropReason = DROP_REASON_NOT_DROPPED; // configuration changes are never droppedbreak;}case EventEntry::TYPE_DEVICE_RESET: {DeviceResetEntry* typedEntry =static_cast<DeviceResetEntry*>(mPendingEvent);done = dispatchDeviceResetLocked(currentTime, typedEntry);dropReason = DROP_REASON_NOT_DROPPED; // device resets are never droppedbreak;}case EventEntry::TYPE_KEY: {KeyEntry* typedEntry = static_cast<KeyEntry*>(mPendingEvent);if (isAppSwitchDue) {if (isAppSwitchKeyEventLocked(typedEntry)) {resetPendingAppSwitchLocked(true);isAppSwitchDue = false;} else if (dropReason == DROP_REASON_NOT_DROPPED) {dropReason = DROP_REASON_APP_SWITCH;}}if (dropReason == DROP_REASON_NOT_DROPPED&& isStaleEventLocked(currentTime, typedEntry)) {dropReason = DROP_REASON_STALE;}if (dropReason == DROP_REASON_NOT_DROPPED && mNextUnblockedEvent) {dropReason = DROP_REASON_BLOCKED;}done = dispatchKeyLocked(currentTime, typedEntry, &dropReason, nextWakeupTime);break;}case EventEntry::TYPE_MOTION: {MotionEntry* typedEntry = static_cast<MotionEntry*>(mPendingEvent);if (dropReason == DROP_REASON_NOT_DROPPED && isAppSwitchDue) {dropReason = DROP_REASON_APP_SWITCH;}if (dropReason == DROP_REASON_NOT_DROPPED&& isStaleEventLocked(currentTime, typedEntry)) {dropReason = DROP_REASON_STALE;}if (dropReason == DROP_REASON_NOT_DROPPED && mNextUnblockedEvent) {dropReason = DROP_REASON_BLOCKED;}done = dispatchMotionLocked(currentTime, typedEntry,&dropReason, nextWakeupTime);break;}}
}

可以看到，真对不同的类型如 TYPE_MOTION、TYPE_KEY来进行不同的分发，本文探究的是触摸事件，
着重看 TYPE_MOTION 事件的分发 dispatchMotionLocked

bool InputDispatcher::dispatchMotionLocked(//找到 目标window injectionResult = findTouchedWindowTargetsLocked(currentTime,entry, inputTargets, nextWakeupTime, &conflictingPointerActions);//执行分发dispatchEventLocked(currentTime, entry, inputTargets);return true;
}
int32_t InputDispatcher::findTouchedWindowTargetsLocked(nsecs_t currentTime,const MotionEntry* entry, Vector<InputTarget>& inputTargets, nsecs_t* nextWakeupTime,bool* outConflictingPointerActions) {//遍历所有窗口size_t numWindows = mWindowHandles.size();for (size_t i = 0; i < numWindows; i++) {sp<InputWindowHandle> windowHandle = mWindowHandles.itemAt(i);const InputWindowInfo* windowInfo = windowHandle->getInfo();if (windowInfo->displayId != displayId) {continue; // wrong display}int32_t flags = windowInfo->layoutParamsFlags;if (windowInfo->visible) {if (! (flags & InputWindowInfo::FLAG_NOT_TOUCHABLE)) {isTouchModal = (flags & (InputWindowInfo::FLAG_NOT_FOCUSABLE| InputWindowInfo::FLAG_NOT_TOUCH_MODAL)) == 0;if (isTouchModal || windowInfo->touchableRegionContainsPoint(x, y)) {//找到 目标窗口newTouchedWindowHandle = windowHandle;break; // found touched window, exit window loop}}}void InputDispatcher::dispatchEventLocked(nsecs_t currentTime,EventEntry* eventEntry, const Vector<InputTarget>& inputTargets) {for (size_t i = 0; i < inputTargets.size(); i++) {ssize_t connectionIndex = getConnectionIndexLocked(inputTarget.inputChannel);sp<Connection> connection = mConnectionsByFd.valueAt(connectionIndex);prepareDispatchCycleLocked(currentTime, connection, eventEntry, &inputTarget);}}
}

prepareDispatchCycleLocked()调用 enqueueDispatchEntriesLocked()然后调用
startDispatchCycleLocked

void InputDispatcher::startDispatchCycleLocked(nsecs_t currentTime,const sp<Connection>& connection) {
//......case EventEntry::TYPE_MOTION: {// Publish the motion event.status = connection->inputPublisher.publishMotionEvent(dispatchEntry->seq,motionEntry->deviceId, motionEntry->source,dispatchEntry->resolvedAction, motionEntry->actionButton,dispatchEntry->resolvedFlags, motionEntry->edgeFlags,motionEntry->metaState, motionEntry->buttonState,xOffset, yOffset, motionEntry->xPrecision, motionEntry->yPrecision,motionEntry->downTime, motionEntry->eventTime,motionEntry->pointerCount, motionEntry->pointerProperties,usingCoords);
//......}
}

mWindowHandles为所有窗口，findTouchedWindowTargetsLocked的就是从mWindowHandles中找到目标窗口，规则太复杂，有兴趣可以自行分析。mWindowHandles的值是在InputDispatcher::setInputWindows中传入的，
setInputWindows 由WMS进行调用，WMS中的InputMonitor会间接调用InputDispatcher::setInputWindows，这个时机主要是跟窗口增删等逻辑相关
获取事件与查找窗口已经安排完毕，但到此逻辑都是在SystemServer进程中，SystemServer进程是如何将事件发给应用进程，InputChannel是什么时候创建的，其实应用在setView()的是会调用IWindowSession的addToDisplay()函数。
IWindowSession的具体实现在\services\core\java\com\android\server\wm\Session.java中
它包含了InputChannel的建立。它本身是个binder调用，服务端是WMS，最终调用的是WMS的addWindow方法。
InputChannel通过InputChannel.openInputChannelPair分别窗建一对InputChannel，然后将Server端的InputChannel注册到InputDispatcher中，将Client端的InputChannel返回给客户端应用。InputChannel数组的一对InputChannel，一个注册给了InputDispatcher，另一个交给应用程序ViewRootImpl。

public int addWindow(Session session, IWindow client, int seq,LayoutParams attrs, int viewVisibility, int displayId,Rect outContentInsets, Rect outStableInsets, Rect outOutsets,InputChannel outInputChannel) {.....WindowState win = new WindowState(this, session, client, token, attachedWindow, appOp[0], seq, attrs, viewVisibility, displayContent);...if (outInputChannel != null && (attrs.inputFeatures& LayoutParams.INPUT_FEATURE_NO_INPUT_CHANNEL) == 0) {String name = win.makeInputChannelName();//创建InputChannel[] inputChannels = InputChannel.openInputChannelPair(name);//服务端InputChannelwin.setInputChannel(inputChannels[0]);//应用客户端InputChannelinputChannels[1].transferTo(outInputChannel);    mInputManager.registerInputChannel(win.mInputChannel, win.mInputWindowHandle);}.....
}//InputTransport.cpp中status_t InputChannel::openInputChannelPair(const String8& name,sp<InputChannel>& outServerChannel, sp<InputChannel>& outClientChannel) {int sockets[2];//setsockopt设置socketint bufferSize = SOCKET_BUFFER_SIZE;setsockopt(sockets[0], SOL_SOCKET, SO_SNDBUF, &bufferSize, sizeof(bufferSize));setsockopt(sockets[0], SOL_SOCKET, SO_RCVBUF, &bufferSize, sizeof(bufferSize));setsockopt(sockets[1], SOL_SOCKET, SO_SNDBUF, &bufferSize, sizeof(bufferSize));setsockopt(sockets[1], SOL_SOCKET, SO_RCVBUF, &bufferSize, sizeof(bufferSize));//创建服务端InputChannelString8 serverChannelName = name;serverChannelName.append(" (server)");outServerChannel = new InputChannel(serverChannelName, sockets[0]);//创建应用客户端InputChannelString8 clientChannelName = name;clientChannelName.append(" (client)");outClientChannel = new InputChannel(clientChannelName, sockets[1]);return OK;
}

调用链如下

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