涉及到的源代码及路径：
frameworks/base/core/java/android/view/WindowManagerImpl.java
frameworks/base/core/java/android/view/WindowManagerGlobal.java
frameworks/base/core/java/android/view/ViewRootImpl.java
frameworks/base/services/core/java/com/android/server/wm/Session.java
frameworks/base/services/core/java/com/android/server/wm/WindowManagerService.java
frameworks/base/services/core/java/com/android/server/wm/WindowState.java
frameworks/base/core/java/android/view/InputChannel.java
frameworks/base/services/core/jni/com_android_server_input_InputWindowHandle.cpp
frameworks/base/services/core/jni/com_android_server_input_InputApplicationHandle.cpp
frameworks/native/libs/input/InputTransport.cpp
我们知道，应用层的事件是从 Activity 开始分发给 View 树的，那么事件产生的源头又是 哪里呢？这次我们就来分析一下事件的起源。
事件分发采用责任链模式，其处理基类为 ViewRootImpl#InputStage：
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abstract class InputStage {
// 指向下一个处理对象
private final InputStage mNext;
public InputStage(InputStage next) { mNext = next;}
// 将事件传递给下一个处理对象
public final void deliver(QueuedInputEvent q) { }
protected void finish(QueuedInputEvent q, boolean handled) { }
protected void forward(QueuedInputEvent q) { }
protected void apply(QueuedInputEvent q, int result) { }
protected int onProcess(QueuedInputEvent q) { }
protected void onDeliverToNext(QueuedInputEvent q) { }
protected boolean shouldDropInputEvent(QueuedInputEvent q) { }
}
事件处理优先级自上而下依次是： SyntheticInputStage：负责处理未经处理的新输入事件 ViewPostImeInputStage：负责将 post-ime 类型输入事件传递到 view 层级中 NativePostImeInputStage：负责将 post-ime 类型输入事件传递到 native Activity EarlyPostImeInputStage：负责对 post-ime 类型输入事件进行早期处理 ImeInputStage：负责将输入事件传递给 IME（不支持 pointer 事件） ViewPreImeInputStage：负责将 pre-ime 类型输入事件传递到 view 层级中（不支持 pointer 事件） NativePreImeInputStage：负责将 pre-ime 类型输入事件传递到 native Activity（不支持 pointer 事件）
SystemService#main() -> run() -> startOtherServices() 方法中启动 InputManagerService （简称为 IMS）和 WindowManagerService（简称为 WMS），首先先实例化 IMS，然后将 IMS 对象作为参数传递给 WMS，最后将依次启动 WMS 和 IMS
WindowManagerService
WMS 初始化：
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InputChannel inputChannel = mInputManager.monitorInput(TAG_WM);
mPointerEventDispatcher = new PointerEventDispatcher(inputChannel);
打开输入 channel，并使用此 channel 实例化 PointerEventDispatcher 用于接收输入事件
InputEventReceiver#dispatchInputEvent() 此方法由 native 层调用，事件由 native 层 传递到 Java 层
WMS#addWindow() 方法
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public void addWindow() {
boolean openInputChannels = outInputChannel != null
&& (attrs.inputFeatures & INPUT_FEATURE_NO_INPUT_CHANNEL) == 0;
if (openInputChannels) {
// 打开输入管道
win.openInputChannel(outInputChannel);
}
}
WMS#openInputChannel() 方法
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void openInputChannel() {
// 打开一对管道
InputChannel[] inputChannels = InputChannel.openInputChannelPair(getName());
// 输入管道
mInputChannel = inputChannels[0];
mClientChannel = inputChannels[1];
mInputWindowHandle.inputChannel = inputChannels[0];
if (outInputChannel != null) {
mClientChannel.transferTo(outInputChannel);
mClientChannel.dispose();
mClientChannel = null;
} else {
// If the window died visible, we setup a dummy input channel, so that taps
// can still detected by input monitor channel, and we can relaunch the app.
// Create dummy event receiver that simply reports all events as handled.
mDeadWindowEventReceiver = new DeadWindowEventReceiver(mClientChannel);
}
// 向 IMS 注册管道
mService.mInputManager.registerInputChannel(mInputChannel, mInputWindowHandle);
}
ViewRootImpl
setView() 方法中会初始化 InputChannel
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public void setView() {
if ((mWindowAttributes.inputFeatures & WindowManager.LayoutParams
.INPUT_FEATURE_NO_INPUT_CHANNEL) == 0) {
mInputChannel = new InputChannel();
}
// 下面的方法实际上是调用了 WMS#addWindow() 方法
res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
getHostVisibility(), mDisplay.getDisplayId(), mWinFrame,
mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
mAttachInfo.mOutsets, mAttachInfo.mDisplayCutout, mInputChannel
);
}