[W8App] Make Native Runtime Component (2): Delegate and Event

除了async / await等待非同步的RunTime Component完成工作外，Native Runtime Component能否透過callback (或 message)來取得運算更新？W8App已不再用傳統的callback / message loop來進行元件之間的溝通，取而代之的是delegate / event機制，我們必需做三件事使呼叫端能得知元件的事件觸發。

1. 元件必需提供delegate handler：
#1 delegate的宣告必需在class的宣告之前
#9 class中必需有delegate hander event，讓元件可發送事件，呼叫端可聆聽事件
#5 class中必需有dispatcher用來記錄呼叫端執行緒的dispatcher

   1: public delegate void ProcessedHandler(int retry);

   2: public ref class myTRDLL sealed

   3: {

   4: private:

   5:   Windows::UI::Core::CoreDispatcher^ m_dispatcher;

   6:  

   7:   // DO SOMETHING

   8: public:

   9:   event ProcessedHandler^ ProcessedEvent;

  10: };

2. 呼叫端（C# app）必需 += 聆聽這個事件

   1: _myRTDLL.ProcessedEvent += OnRTDLLProcessedEvent;

3. 元件觸發事件
#4 在產生非同步的task前記錄呼叫端AP執行緒的dispatcher
#16 觸發ProcessedEvent事件
#14 確保AP是在main thread接收到event。如果忘了做這個動作，會收到0x8001010E的例外事件。

   1: IAsyncOperation<int>^ myRTDLL::RunProcessAsync(int maxretry)

   2: {

   3:   auto window = Windows::UI::Core::CoreWindow::GetForCurrentThread();

   4:   m_dispatcher = window->Dispatcher;

   5:  

   6:   return create_async([this,maxretry]() {

   7:     int retry = 0;

   8:  

   9:     for( ; retry<maxretry ; retry++)

  10:     {

  11:       if(RunProcess(retry))  break;

  12:     }

  13:  

  14:     m_dispatcher->RunAsync( CoreDispatcherPriority::Normal, ref new DispatchedHandler([this, retry]() 

  15:     {

  16:       this->ProcessedEvent(retry);

  17:     }, Platform::CallbackContext::Any));

  18:  

  19:     return (retry < maxretry) ? retry : -1;

  20:   });

  21: }

（當初沒做dispatch這個動作花了好多時間debug…）