这里，我们考虑一个问题，Android中的应用是一个looper线程，没有任务时就阻塞着，其他线程通过handler调用等方式向主线程looper发送任务，
如果点击应用上的按钮，应用是怎么及时响应的呢，
是专门启动了一个额外的线程去监控input事件么？
这里直接给出答案，input事件传送给了系统IMS，IMS通过inputchannel把input传送给应用，而应用中的主线程looper监控了inputchannel的fd，
这里看一下主线程looper是怎么实现的监控inputchannel的fd。
在应用创建的时候，会调用ViewRootImpl的serView方法，
frameworks/base/core/java/android/view/ViewRootImpl.java
setView
mInputEventReceiver = new WindowInputEventReceiver(inputChannel,
这里传入了主线程的looper给WindowInputEventReceiver
这个构造方法，调用其父类的构造
public InputEventReceiver(InputChannel inputChannel, Looper looper) {
if (inputChannel == null) {
throw new IllegalArgumentException("inputChannel must not be null");
}
if (looper == null) {
throw new IllegalArgumentException("looper must not be null");
}
mMessageQueue = looper.getQueue();
mReceiverPtr = nativeInit(new WeakReference<InputEventReceiver>(this),
mInputChannel, mMessageQueue);
mCloseGuard.open("InputEventReceiver.dispose");
}
static jlong nativeInit(JNIEnv* env, jclass clazz, jobject receiverWeak,
jobject inputChannelObj, jobject messageQueueObj) {
std::shared_ptr<InputChannel> inputChannel =
android_view_InputChannel_getInputChannel(env, inputChannelObj);
if (inputChannel == nullptr) {
jniThrowRuntimeException(env, "InputChannel is not initialized.");
return 0;
}
sp<MessageQueue> messageQueue = android_os_MessageQueue_getMessageQueue(env, messageQueueObj);
if (messageQueue == nullptr) {
jniThrowRuntimeException(env, "MessageQueue is not initialized.");
return 0;
}
sp<NativeInputEventReceiver> receiver = new NativeInputEventReceiver(env,
receiverWeak, inputChannel, messageQueue);
status_t status = receiver->initialize();
其中，receiver->initialize();
调用到
status_t NativeInputEventReceiver::initialize() {
setFdEvents(ALOOPER_EVENT_INPUT);
return OK;
}
void NativeInputEventReceiver::setFdEvents(int events) {
auto&& fd = mInputConsumer.getChannel()->getFd();
if (events) {
mMessageQueue->getLooper()->addFd(fd.get(), 0, events, this, nullptr);
} else {
mMessageQueue->getLooper()->removeFd(fd.get());
}
}
这里，打不过就加入，把inputchannel的fd也加入到主线程looper的epool监控之中。