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IMS分为Java层与Native层两个部分，其启动过程是从Java部分的初始化开始，进而完成Native部分的初始化。

□创建新的IMS对象。

□调用IMS对象的start（）函数完成启动

同其他系统服务一样，IMS在SystemServer中的ServerThread线程中启动。

    private void startOtherServices(@NonNull TimingsTraceAndSlog t) {t.traceBegin("startOtherServices");

            inputManager = new InputManagerService(context);inputManager.setWindowManagerCallbacks(wm.getInputManagerCallback());inputManager.start();

创建新的IMS对象

线程 mLooper = DisplayThread.get().getLooper()

构造NativeInputManagerService

    /** Point of injection for test dependencies. */@VisibleForTestingstatic class Injector {private final Context mContext;private final Looper mLooper;Injector(Context context, Looper looper) {mContext = context;mLooper = looper;}Context getContext() {return mContext;}Looper getLooper() {return mLooper;}NativeInputManagerService getNativeService(InputManagerService service) {return new NativeInputManagerService.NativeImpl(service, mLooper.getQueue());}void registerLocalService(InputManagerInternal localService) {LocalServices.addService(InputManagerInternal.class, localService);}}public InputManagerService(Context context) {this(new Injector(context, DisplayThread.get().getLooper()));}

    class NativeImpl implements NativeInputManagerService {/** Pointer to native input manager service object, used by native code. */@SuppressWarnings({"unused", "FieldCanBeLocal"})private final long mPtr;NativeImpl(InputManagerService service, MessageQueue messageQueue) {mPtr = init(service, messageQueue);}

nativeInit（）函数创建了一个类型为NativeInputManager的对象，它是Java层与Native层互相通信的桥梁。

com_android_server_input_InputManagerService.cpp
static jlong nativeInit(JNIEnv* env, jclass /* clazz */, jobject serviceObj,jobject messageQueueObj) {sp<MessageQueue> messageQueue = android_os_MessageQueue_getMessageQueue(env, messageQueueObj);if (messageQueue == nullptr) {jniThrowRuntimeException(env, "MessageQueue is not initialized.");return 0;}static std::once_flag nativeInitialize;NativeInputManager* im = nullptr;std::call_once(nativeInitialize, [&]() {// Create the NativeInputManager, which should not be destroyed or deallocated for the// lifetime of the process.im = new NativeInputManager(serviceObj, messageQueue->getLooper());});LOG_ALWAYS_FATAL_IF(im == nullptr, "NativeInputManager was already initialized.");return reinterpret_cast<jlong>(im);
}

看下这个类的声明可以发现，它实现了InputReaderPolicyInterface与InputDispatcher-PolicyInterface两个接口。这说明上一节曾经介绍过的两个重要的输入系统参与者InputReaderPolicy和InputDispatcherPolicy是由NativeInputManager实现的，然而它仅仅为两个策略提供接口实现而已，并不是策略的实际实现者。NativeInputManager通过JNI回调Java层的IMS，由它完成决策。本节暂不讨论其实现细节，读者只要先记住两个策略参与者的接口实现位于NativeInputManager即可。

4  class NativeInputManager : public virtual InputReaderPolicyInterface,
265                             public virtual InputDispatcherPolicyInterface,
266                             public virtual PointerControllerPolicyInterface {

NativeInputManager构造如下：

1、创建一个全局引用，并通过mServiceObj指向上层的InputManagerService对象

2、创建并注册服务InputManager。

原来，InputManager才是底层输入系统的服务，而NativeInputManagerService通过mServiceObj保存了上层InputManagerService引用，并且上层InputManagerService通过mPtr指向底层的NativeInputManager。因此，我们可以判定NativeInputManagerService就是一个连接上层与底层的桥梁。

NativeInputManager::NativeInputManager(jobject serviceObj, const sp<Looper>& looper): mLooper(looper), mInteractive(true) {JNIEnv* env = jniEnv();mServiceObj = env->NewGlobalRef(serviceObj);InputManager* im = new InputManager(this, *this);mInputManager = im;defaultServiceManager()->addService(String16("inputflinger"), im);
}

InputManager.cpp
InputManager::InputManager(const sp<InputReaderPolicyInterface>& readerPolicy,InputDispatcherPolicyInterface& dispatcherPolicy) {mDispatcher = createInputDispatcher(dispatcherPolicy);  4、创建InputDispatcher对象，使用InputReaderPolicyInterfacemProcessor = std::make_unique<InputProcessor>(*mDispatcher);mBlocker = std::make_unique<UnwantedInteractionBlocker>(*mProcessor);mReader = createInputReader(readerPolicy, *mBlocker);  4、创建InputReader对象，使用InputReaderPolicyInterface和InputListenerInterface
}

可以看到InputManager主要做以下几件事：

• 构造InputDispatcher对象；（用于后续事件分发处理）
• 构造InputReader对象；（用于事件输入监听）
• 调用InputDispatcher和InputReader的start()方法；

InputManager构造函数所使用的两个接口，分别由InputDispatcher和InputReader所使用。因此InputManager向上通信的能力是由子模块InputDispatcher和InputReader实现的。

InputManager创建了，InputReader、InputDispatcher。

InputReader负责从EventHub中获取事件，然后把事件加工后，发送给InputClassifier。

最后InputDispatcher会对事件进行分发。

frameworks/native/services/inputflinger/dispatcher/InputDispatcherFactory.cppstd::unique_ptr<InputDispatcherInterface> createInputDispatcher(InputDispatcherPolicyInterface& policy) {return std::make_unique<android::inputdispatcher::InputDispatcher>(policy);
}

std::unique_ptr<InputReaderInterface> createInputReader(const sp<InputReaderPolicyInterface>& policy, InputListenerInterface& listener) {return std::make_unique<InputReader>(std::make_unique<EventHub>(), policy, listener);
}

调用IMS对象的start（）函数完成启动

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

static void nativeStart(JNIEnv* env, jobject nativeImplObj) {NativeInputManager* im = getNativeInputManager(env, nativeImplObj);status_t result = im->getInputManager()->start();if (result) {jniThrowRuntimeException(env, "Input manager could not be started.");}
}

status_t InputManager::start() {status_t result = mDispatcher->start();if (result) {ALOGE("Could not start InputDispatcher thread due to error %d.", result);return result;}result = mReader->start();if (result) {ALOGE("Could not start InputReader due to error %d.", result);mDispatcher->stop();return result;}return OK;
}

启动mDispatcher和mReader

status_t InputReader::start() {if (mThread) {return ALREADY_EXISTS;}mThread = std::make_unique<InputThread>("InputReader", [this]() { loopOnce(); }, [this]() { mEventHub->wake(); });return OK;
}

status_t InputDispatcher::start() {if (mThread) {return ALREADY_EXISTS;}mThread = std::make_unique<InputThread>("InputDispatcher", [this]() { dispatchOnce(); }, [this]() { mLooper->wake(); });return OK;
}