Create 消息队列创建.md

blogs/Android/Framework/Interview/线程通信相关/消息队列创建.md

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+---
+线程的消息队列是怎么创建的？
+---
+
+1. 可以在子线程创建 handler 嘛？
+2. 主线程的 Looper 和子线程的 Looper 有什么区别？
+3. Looper 的 MessageQueue 有什么关系？
+4. MessageQueue 是怎么创建的？
+
+NativeMessageQueue:
+
+```c++
+jlong MessageQueue_nativeInit(JNIEnv* env, jclass clazz){
+	NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
+	return reinterpret_cast<jlong>(nativeMessageQueue);
+}
+NativeMessageQueue::NativeMessageQueue(){
+    mLooper = Looper::getForThread();
+    if(mLooper == NULL){
+        mLooper = new Looper(false);
+        Looper::setForThread(mLooper);
+    }
+}
+sp<Looper> Looper::getForThread(){
+    int result = pthread_once(& gTLSOnce, initTLSKey);
+    return (Looper*)pthread_getspecific(gTLSKey);
+}
+```
+
+Native Looper:
+
+```c++
+Looper::Looper(bool allowNonCallbacks){
+	mWakeEventFd = eventfd(0, EFD_NONBLOCK);
+    rebuildEpollLocked();
+}
+void Looper::rebuildEpollLocked(){
+    mEpollFd = epoll_create(EPOLL_SIZE_HINT);
+    struct epoll_event eventItem;
+    memset(& eventItem, 0, sizeof(epoll_event));
+    eventItem.events = EPOLLIN;
+    eventItem.data.fd = mWakeEventFd;
+    epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);
+}
+void Looper::wake(){
+    uint64_t inc = 1;
+    ssize_t nWrite = write(mWakeEventFd, &inc, sizeof(uint64_t));
+}
+int Looper::pollOnce(int timeoutMillis, int* outFd, ...){
+    for(;;){
+        pollInner(timeoutMillis);
+    }
+}
+int Looper::pollInner(int timeoutMillis){
+    struct epoll_event eventItems[EPOLL_MAX_EVENTS];
+    int eventCount = epoll_wait(mEpollFd, eventItems, ...);
+    for(int i=0;i<eventCount;i++){
+        int fd = eventItems[i].data.fd;
+        uint32_t epollEvents = eventItems[i].events;
+        if(fd==mWakeEventFd&&(epollEvents&EPOLLIN)){
+            awoken();
+        }
+    }
+}
+void Looper::awoken(){
+    uint64_t counter;
+    read(mWakeEventFd, &counter, sizeof(uint64_t));
+}
+```