Android Native层异步消息处理框架

 1 //=======================================//
 2 NuPlayerDriver::NuPlayerDriver(pid_t pid)
 3     : ......
 4      mLooper(new ALooper) 
 5       ......{
 6     mLooper->setName("NuPlayerDriver Looper");
 7     mLooper->start(
 8             false, /* runOnCallingThread */
 9             true,  /* canCallJava */
10             PRIORITY_AUDIO);
11 
12     mPlayer = new NuPlayer(pid);
13     mLooper->registerHandler(mPlayer) 
14 } 
15 
16 //=======================================//
17 sp<AMessage> msg = new AMessage(kWhatPerformSeek, this);
18 msg->setInt32("generation", ++mSeekGeneration);
19 msg->setInt64("timeUs", seekTimeUs);
20 status_t err = msg->postAndAwaitResponse(&response);
21 
22 //=======================================//
23 void NuPlayer::RTSPSource::onMessageReceived(const sp<AMessage> &msg) {
24     switch (what) {
25         case XXX:
26         case YYY:
27         ......
28         default:
29     }
30 }

 1 //AMessage类简析
 2 struct AMessage : public RefBase {
 3     //构造函数，在其中会对AMessage的两个重要数据成员mWhat和mTarget进行初始化设置。
 4     AMessage();
 5     AMessage(uint32_t what, const sp<const AHandler> &handler);
 6     void setWhat(uint32_t what);//设置mWhat
 7     uint32_t what() const;//返回mWhat
 8     void setTarget(const sp<const AHandler> &handler);//设置mTarget
 9     //一系列的set和find函数，用于在传递消息的过程中携带各种信息
10     void setInt32(const char *name, int32_t value);
11     void setInt64(const char *name, int64_t value);
12     //....
13     bool findInt32(const char *name, int32_t *value) const;
14     bool findInt64(const char *name, int64_t *value) const;
15     //投递消息到消息队列中
16     status_t post(int64_t delayUs = 0);
17     // Posts the message to its target and waits for a response (or error) before returning.
18     status_t postAndAwaitResponse(sp<AMessage> *response);
19 protected:
20     virtual ~AMessage();//析构函数
21 private:
22     friend struct ALooper; // deliver()
23     //两个重要的私有数据成员：
24     //mWhat指明这是一个什么消息,用于在onMessageReceived处理分支中进行匹配。
25     //mTarget用于后台线程在处理这个消息时判断发送给哪一个类处理。
26     uint32_t mWhat;
27     ALooper::handler_id mTarget;
28 
29     wp<AHandler> mHandler;
30     wp<ALooper> mLooper;

 1 AMessage::AMessage(uint32_t what, const sp<const AHandler> &handler)
 2     : mWhat(what),
 3       mNumItems(0) {
 4     setTarget(handler);
 5 }
 6 
 7 void AMessage::setTarget(const sp<const AHandler> &handler) {
 8     if (handler == NULL) {
 9         mTarget = 0;
10         mHandler.clear();
11         mLooper.clear();
12     } else {
13         mTarget = handler->id();
14         mHandler = handler->getHandler();
15         mLooper = handler->getLooper();
16     }
17 }

 1 //ALooper类简析
 2 struct ALooper : public RefBase {
 3     //定义的两个整型变量类型
 4     typedef int32_t event_id;//event id
 5     typedef int32_t handler_id;//handler id
 6     
 7     ALooper();//构造函数
 8     
 9     // Takes effect in a subsequent call to start().
10     void setName(const char *name); 
11 
12     handler_id registerHandler(const sp<AHandler> &handler);//注册handler
13     void unregisterHandler(handler_id handlerID);//注销handler
14     //启动后台处理线程处理事件
15     status_t start(
16             bool runOnCallingThread = false,
17             bool canCallJava = false,
18             int32_t priority = PRIORITY_DEFAULT
19             );
20 protected:
21     virtual ~ALooper();    
22 private:
23     friend struct AMessage;       // post()
24     //事件的结构体封装
25     struct Event {  
26         int64_t mWhenUs;
27         sp<AMessage> mMessage;
28     };
29     AString mName;
30     List<Event> mEventQueue;
31     struct LooperThread;
32     sp<LooperThread> mThread;//后台处理线程
33     // posts a message on this looper with the given timeout
34     void post(const sp<AMessage> &msg, int64_t delayUs);
35     bool loop();
36 }

 1 struct AHandler : public RefBase {
 2     AHandler()    //构造函数
 3         : mID(0),    //初始化mID为0
 4           mVerboseStats(false),
 5           mMessageCounter(0) {
 6     }
 7 
 8     ALooper::handler_id id() const {//这个函数用于返回内部变量mID的值，其初始值为0，但是可以通过setID()设置
 9         return mID;
10     }
11 
12 
13 protected:
14     virtual void onMessageReceived(const sp<AMessage> &msg) = 0;
15 
16 private:
17     friend struct AMessage;      // deliverMessage()
18     friend struct ALooperRoster; // setID()
19 
20     ALooper::handler_id mID;
21     wp<ALooper> mLooper;
22     //setID()在其友元类ALooperRoster的registerHandler()函数中调用。
23     inline void setID(ALooper::handler_id id, wp<ALooper> looper) {
24         mID = id;
25         mLooper = looper;
26     }
27 
28 };