Android输入子系统之启动过程分析

Android的输入子系统是在InputManagerService中启动的，而InputManagerService是在system_server中启动的。

我们先看下整个启动过程的序列图，然后根据序列图来一步步分析。

Step 1. SystemServer.startOtherServices

函数定义在frameworks/base/services/java/com/android/server/SystemServer.java中

//启动核心服务
private void startCoreServices() {
}

//启动其他的服务
private void startOtherServices() {
    //初始化一个InputManagerService对象
    inputManager = new InputManagerService(context);
    wm = WindowManagerService.main(context, inputManager,  mFactoryTestMode != FactoryTest.FACTORY_TEST_LOW_LEVEL,  !mFirstBoot, mOnlyCore);

    //将WindowManagerService加入到ServiceManager中
    ServiceManager.addService(Context.WINDOW_SERVICE, wm);
    //将InputManagerService加入到ServiceManager中
    ServiceManager.addService(Context.INPUT_SERVICE, inputManager);

    inputManager.setWindowManagerCallbacks(wm.getInputMonitor());
    inputManager.start();
}

在SystemServer中先构造了一个InputManagerService对象和一个WindowManagerService对象，
然后将InputManagerService对象传给WindowManagerService对象，
WindowManagerService中初始化了一个InputMonitor对象，
调用InputManagerService.setWindowManagerCallbacks 函数将 InputMonitor 传进去，后面native层回调时会调用到该InputMonitor对象。

Step 2. InputManagerService.init

InputManagerService.java的构造函数定义在
frameworks/base/services/core/java/com/android/server/input/InputManagerService.java

public class InputManagerService {
    public InputManagerService(Context context) {
        this.mContext = context;
        this.mHandler = new InputManagerHandler(DisplayThread.get().getLooper());
        //调用nativeInit来执行C++层的初始化操作
        mPtr = nativeInit(this, mContext, mHandler.getLooper().getQueue());

        LocalServices.addService(InputManagerInternal.class, new LocalService());
    }
}

构造函数调用nativeInit来执行C++层的初始化操作

Step 3. InputManagerService.nativeInit

该函数定义在frameworks/base/services/core/jni/com_android_server_input_InputManagerService.cpp

static jlong nativeInit(JNIEnv* env, jclass /* clazz */,
        jobject serviceObj, jobject contextObj, jobject messageQueueObj) {
    sp<MessageQueue> messageQueue = android_os_MessageQueue_getMessageQueue(env, messageQueueObj);
    //构造一个NativeInputManagera对象
    NativeInputManager* im = new NativeInputManager(contextObj, serviceObj,
            messageQueue->getLooper());
    im->incStrong(0);
    return reinterpret_cast<jlong>(im);
}

这个函数主要作用是创建一个NativeInputManager实例，并将其作为返回值保存在InputManagerService.java中的mPtr字段中。

Step 4. NativeInputManager.init

NativeInputManager 构造函数定义在 frameworks/base/services/core/jni/com_android_server_input_InputManagerService.cpp 文件中

NativeInputManager::NativeInputManager(jobject contextObj,
        jobject serviceObj, const sp<Looper>& looper) : mLooper(looper), mInteractive(true) 
{
    //构造一个EventHub对象,最原始的输入事件都是通过它收集并且粗加工然后给到InputReader对象
    sp<EventHub> eventHub = new EventHub();
    //构造一个InputManager对象
    mInputManager = new InputManager(eventHub, this, this);
}

NativeInputManager构造函数中创建了一个EventHub实例，
并且将这个实例作为参数来创建一个InputManager对象，这个对象会做一些初始化的操作。

Step 5. InputManager.init

InputManager的构造函数定义在
frameworks/native/services/inputflinger/InputManager.cpp

InputManager::InputManager(
        const sp<EventHubInterface>& eventHub,   
        const sp<InputReaderPolicyInterface>& readerPolicy,
        const sp<InputDispatcherPolicyInterface>& dispatcherPolicy) {
    mDispatcher = new InputDispatcher(dispatcherPolicy); 
    mReader = new InputReader(eventHub, readerPolicy, mDispatcher);
    initialize();
}

这里创建了InputDispatcher对象用于分发按键给当前focus的窗口的，同时创建了一个InputReader用于从EventHub中读取键盘事件。

Step 6. InputManager.initialize

void InputManager::initialize() {
    mReaderThread = new InputReaderThread(mReader);
    mDispatcherThread = new InputDispatcherThread(mDispatcher);
}

这里创建了一个InputReaderThread和InputDispatcherThread对象，
前面构造函数中创建的InputReader实际上是通过InputReaderThread来读取键盘事件，
而InputDispatcher实际通过InputDispatcherThread来分发键盘事件

Step 7. InputManagerService.start

InputManagerService.java中start函数定义在
frameworks/base/services/core/java/com/android/server/input/InputManagerService.java

public void start() {
    Slog.i(TAG, "Starting input manager");
    nativeStart(mPtr);
    ...
} 

该函数主要调用了nativeStart进入native层启动

Step 8. InputManagerService.nativeStart

该函数定义在
frameworks/base/services/core/jni/com_android_server_input_InputManagerService.cpp

static void nativeStart(JNIEnv* env, jclass /* clazz */, jlong ptr) {
    NativeInputManager* im = reinterpret_cast<NativeInputManager*>(ptr);

    status_t result = im->getInputManager()->start();
}

实际调用了InputManager的start函数

Step 9. InputManager.start

start函数定义在
frameworks/native/services/inputflinger/InputManager.cpp

status_t InputManager::start() {
    status_t result = mDispatcherThread->run("InputDispatcher", PRIORITY_URGENT_DISPLAY);

    result = mReaderThread->run("InputReader", PRIORITY_URGENT_DISPLAY);

    return OK; 
}

Step 10. InputDispatcherThread.threadLoop

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

status_t InputManager::start() {
    status_t result = mDispatcherThread->run("InputDispatcher", PRIORITY_URGENT_DISPLAY);
    result = mReaderThread->run("InputReader", PRIORITY_URGENT_DISPLAY);

    return OK; 
}

这里调用前面在Step 5中创建的实例InputDispatcher的dispatchOnce函数进行一次按键分发

Step 11. 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

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原文链接：https://blog.csdn.net/chenweiaiyanyan/article/details/72910734