webrtc中的MethodCall0程式碼功能分析

MethodCall0實現了這樣一個功能：

將一個類物件的成員函式(無引數)放在某個執行緒上下文執行，無返回(void)。

涉及到webRTC的類有：

Message, MessageHandler, Thread, ReturnType

呼叫例子：下例是將proxy物件的Release_s方法放在work_thread執行上下文執行。

VideoSourceProxy *proxy;

talk_base::Thread* work_thread;

...

MethodCall0<VideoSourceProxy, void> call(proxy, &VideoSourceProxy::Release_s);  //宣告一個MethodCall0物件call

call.Marshal(work_thread); // 將VideoSourceProxy的成員Release_s放線上程thread的上下文執行，無返回結果(void)
 

MethodCall0的實現如下：

template <typename R>  
class ReturnType {  
 public:  
  template<typename C, typename M>  
  void Invoke(C* c, M m) { r_ = (c->*m)(); }  
  template<typename C, typename M, typename T1>  
  void Invoke(C* c, M m, T1 a1) { r_ = (c->*m)(a1); }  
  template<typename C
 
, typename M, typename T1, typename T2>  
  void Invoke(C* c, M m, T1 a1, T2 a2) { r_ = (c->*m)(a1, a2); }  
  template<typename C, typename M, typename T1, typename T2, typename T3>  
  void Invoke(C* c, M m, T1 a1, T2 a2, T3 a3) { r_ = (c->*m)(a1, a2, a3); }  

  R value() { return r_; }  

 private:  
  R
 
 r_;  
};  

template <typename C, typename R>  
class MethodCall0 : public talk_base::Message,  
                    public talk_base::MessageHandler {  
 public:  
  typedef R (C::*Method)();  
  MethodCall0(C* c, Method m) : c_(c), m_(m) {}  

  R Marshal(talk_base::Thread* t) {  
    t->Send(this, 0);  
    return r_.value();  
  }  

 private:  
  void OnMessage(talk_base::Message*) {  r_.Invoke(c_, m_);}  

  C* c_;  
  Method m_;  
  ReturnType<R> r_;  
};  

struct Message {  
  Message() {  
    memset(this, 0, sizeof(*this));  
  }  
  inline bool Match(MessageHandler* handler, uint32 id) const {  
    return (handler == NULL || handler == phandler)  
           && (id == MQID_ANY || id == message_id);  
  }  
  MessageHandler *phandler;  
  uint32 message_id;  
  MessageData *pdata;  
  uint32 ts_sensitive;  
};  

// Messages get dispatched to a MessageHandler  
class MessageHandler {  
 public:  
  virtual void OnMessage(Message* msg) = 0;  

 protected:  
  MessageHandler() {}  
  virtual ~MessageHandler();  

 private:  
  DISALLOW_COPY_AND_ASSIGN(MessageHandler);  
};

從原始碼中可以看出，MethodCall0 最終呼叫了webrtc中Thread的send()方法來執行指定的函式，send()方法是指讓函式在Thread執行緒中同步執行，webrtc中大量使用次方法來保證執行緒安全。