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> 2 {
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 = 0;
    BOOL bRet = pThis->ProcessWindowMessage(pThis->m_hWnd, uMsg, wParam, lParam, lRes, 0);
    // 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。