atlwin中不停发WM_PAINT消息原因分析

这里我们基于atlwin定义的窗口类SampleWindow继承自CWindowImpl（定义见atlwin.h）,并注册了WM_PAINT消息处理函数OnPaint，注意，我们在OnPaint函数中并没有直接处理WM_PAINT消息，而是直接返回了FALSE。

 class SampleWindow : public CWindowImpl<SampleWindow> {
 public:
     SampleWindow():

       DECLARE_WND_SUPERCLASS(_T("SampleWindow"), NULL)

       BEGIN_MSG_MAP(SampleWindow)

           MESSAGE_HANDLER(WM_PAINT, OnPaint)

       END_MSG_MAP()

         LRESULT OnPaint(UINT msg, WPARAM wparam, LPARAM lparam, BOOL &handled)
         {
             return FALSE;
         }

 }

参考MESSAGE_HANDLER的宏定义就会发现，在执行对应的消息响应函数之前，bHandled默认被赋值为了TRUE（所以通常情况下WM_PAINT消息需要在这里处理），但是参照上述代码，OnPaint函数并未对WM_PAINT消息处理，并且直接返回了FALSE，而MESSAGE_HANDLER默认是返回TRUE的。

 #define MESSAGE_HANDLER(msg, func) \
     if(uMsg == msg) \
     { \
         bHandled = TRUE; \
         lResult = func(uMsg, wParam, lParam, bHandled); \
         if(bHandled) \
             return TRUE; \
     }

在代码执行到WindowProc后，pThis->ProcessWindowMessage函数会依次执行上述注册的消息响应函数，在执行到WM_PAINT消息后，bRet为TRUE，导致代码不能够执行lRes = pThis->DefWindowProc(uMsg, wParam, lParam);

，而是执行了pThis->m_pCurrentMsg = pOldMsg;，消息处理过程函数WindowProc返回结果lRes为0，导致系统不停的发送WM_PAINT消息(导致消息不停发送的原因并不是WindowProc返回0，是WM_PAINT消息未得到处理)，造成死循环。

 template <class TBase, class TWinTraits>
 LRESULT CALLBACK CWindowImplBaseT< TBase, TWinTraits >::WindowProc(
     _In_ HWND hWnd,
     _In_ UINT uMsg,
     _In_ WPARAM wParam,
     _In_ LPARAM lParam)
 {
     CWindowImplBaseT< TBase, TWinTraits >* pThis = (CWindowImplBaseT< TBase, TWinTraits >*)hWnd;
     // set a ptr to this message and save the old value
     _ATL_MSG msg(pThis->m_hWnd, uMsg, wParam, lParam);
     const _ATL_MSG* pOldMsg = pThis->m_pCurrentMsg;
     pThis->m_pCurrentMsg = &msg;
     // pass to the message map to process
     LRESULT lRes = ;
     BOOL bRet = pThis->ProcessWindowMessage(pThis->m_hWnd, uMsg, wParam, lParam, lRes, );
     // restore saved value for the current message
     ATLASSERT(pThis->m_pCurrentMsg == &msg);

     // do the default processing if message was not handled
     if(!bRet)
     {
         if(uMsg != WM_NCDESTROY)
             lRes = pThis->DefWindowProc(uMsg, wParam, lParam);
         else
         {
             // unsubclass, if needed
             LONG_PTR pfnWndProc = ::GetWindowLongPtr(pThis->m_hWnd, GWLP_WNDPROC);
             lRes = pThis->DefWindowProc(uMsg, wParam, lParam);
             if(pThis->m_pfnSuperWindowProc != ::DefWindowProc && ::GetWindowLongPtr(pThis->m_hWnd, GWLP_WNDPROC) == pfnWndProc)
                 ::SetWindowLongPtr(pThis->m_hWnd, GWLP_WNDPROC, (LONG_PTR)pThis->m_pfnSuperWindowProc);
             // mark window as destryed
             pThis->m_dwState |= WINSTATE_DESTROYED;
         }
     }
     if((pThis->m_dwState & WINSTATE_DESTROYED) && pOldMsg== NULL)
     {
         // clear out window handle
         HWND hWndThis = pThis->m_hWnd;
         pThis->m_hWnd = NULL;
         pThis->m_dwState &= ~WINSTATE_DESTROYED;
         // clean up after window is destroyed
         pThis->m_pCurrentMsg = pOldMsg;
         pThis->OnFinalMessage(hWndThis);
     }else {
         pThis->m_pCurrentMsg = pOldMsg;
     }
     return lRes;
 }

所以，我们要么就不要注册消息对应的响应函数，注册了最好就在对应的响应函数中处理。如果非得注册而且不处理的话，就在对应的响应函数中显式的将bHandled设置为FALSE，让消息处理过程函数WindowProc执行默认的消息处理函数DefWindowProc。