---
线程间消息传递机制
---

1. 消息循环过程是怎样的？
2. 消息是怎么发送的？
3. 消息是怎么处理的？

```java
// Looper#loop
public static void loop(){
    final Looper me = myLooper();
    final MessageQueue queue = me.mQueue;
    for(;;){
        Message msg = queue.next();
        if(msg == null){
            return;
        }
        msg.target.dispatchMessage(msg);
        msg.recycleUnchecked();
    }
}

public void dispatchMessage(Message msg){
    if(msg.callback != null){
        handleCallback(msg);
    }else{
        if(mCallback!=null){
            if(mCallback.handleMessage(msg)){
                return;
            }
        }
        handleMessage(msg);
    }
}

Message next(){
    int nextPollTimeoutMillis = 0;
    for(;;){
        nativePollOnce(ptr, nextPollTimeoutMillis);
        Message msg = mMessages;
        mMessages = msg.next;
        msg.next = null;
        msg.markInUse();
        return msg;
    }
}
```

```c++
void MessageQueue_nativePollOnce(JNIEnv* env, jobject obj, jlong ptr, jint timeoutMillis){
	NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}
void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis){
    mLooper->pollOnce(timeoutMillis);
}
int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, ...){
    int result = 0;
    for(;;){
        if(result!=0){
            return result;
        }
        result = pollInner(timeoutMillis);
    }
}
int Looper::pollInner(int timeoutMillis){
    struct epoll_event eventItems[EPOLL_MAX_EVENTS];
    int eventCount = epoll_wait(mEpollFd, eventItems, ..., timeoutMillis);
    for(int i=0;i<eventCount;i++){
        int fd = eventItems[i].data.fd;
        uint32_t epollEvents = eventItems[i].events;
        if(fd == mWakeEventFd){
            if(epollEvents & EPOLLIN){
                awoken();
            }
        }
    }
}
```

```java
Message next(){
    int nextPollTimeoutMillis = 0;
    for(;;){
        nativePollOnce(ptr, nextPollTimeoutMillis);
        Message msg = mMessage;
        mMessage = msg.next;
        msg.next = null;
        msg.markInUse();
        return msg;
    }
}
```

插入消息：

```java
boolean enqueueMessage(Message msg, long when) {
    nativeWake(mPrt);
    return true;
}
```

```c++
void MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr){
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    nativeMessageQueue->wake();
}
void Looper::wake(){
    uint64_t inc = 1;
    write(mWakeEventFd, &inc, sizeof(uint64_t));
}
```