Create 线程间消息传递机制.md

blogs/Android/Framework/Interview/线程通信相关/线程间消息传递机制.md

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+---
+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));
+}
+```
+