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1. 问题
在修改单线程MFC程序为多线程时,遇到了CWnd::AssertValid()函数执行出错问题。主要表现是在执行下面代码中绿色语句时出错
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#ifdef _DEBUG
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void CWnd::AssertValid() const
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{
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if (m_hWnd == NULL)
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return; // null (unattached) windows are valid
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// check for special wnd??? values
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ASSERT(HWND_TOP == NULL); // same as desktop
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if (m_hWnd == HWND_BOTTOM)
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ASSERT(this == &CWnd::wndBottom);
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else if (m_hWnd == HWND_TOPMOST)
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ASSERT(this == &CWnd::wndTopMost);
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else if (m_hWnd == HWND_NOTOPMOST)
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ASSERT(this == &CWnd::wndNoTopMost);
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else
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{
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// should be a normal window
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ASSERT(::IsWindow(m_hWnd));
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// should also be in the permanent or temporary handle map
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CHandleMap* pMap = afxMapHWND();
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ASSERT(pMap != NULL);
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CObject* p=NULL;
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if(pMap)
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{
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ASSERT( (p = pMap–>LookupPermanent(m_hWnd)) != NULL ||
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(p = pMap–>LookupTemporary(m_hWnd)) != NULL);
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}
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ASSERT((CWnd*)p == this); // must be us
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// Note: if either of the above asserts fire and you are
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// writing a multithreaded application, it is likely that
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// you have passed a C++ object from one thread to another
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// and have used that object in a way that was not intended.
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// (only simple inline wrapper functions should be used)
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//
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// In general, CWnd objects should be passed by HWND from
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// one thread to another. The receiving thread can wrap
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// the HWND with a CWnd object by using CWnd::FromHandle.
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//
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// It is dangerous to pass C++ objects from one thread to
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// another, unless the objects are designed to be used in
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// such a manner.
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}
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}
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#endif
2. 原因
通过在网上查询资料发现问题的原因:在B线程中,直接通过A线程的窗口实例指针调用窗口关联函数。在MFC中,窗口类的事件映射表是和线程相关联,故只有在本线程中才能通过窗口实例指针调用该窗口关联函数。下面这段代码是afxMapHWND函数的实现,其中绿色部分是线程相关的变量的结构体的获取,事件映射表就是这个结构体的成员变量
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CHandleMap* PASCAL afxMapHWND(BOOL bCreate)
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{
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AFX_MODULE_THREAD_STATE* pState = AfxGetModuleThreadState();
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if (pState–>m_pmapHWND == NULL && bCreate)
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{
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BOOL bEnable = AfxEnableMemoryTracking(FALSE);
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#ifndef _AFX_PORTABLE
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_PNH pnhOldHandler = AfxSetNewHandler(&AfxCriticalNewHandler);
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#endif
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pState–>m_pmapHWND = new CHandleMap(RUNTIME_CLASS(CWnd),
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ConstructDestruct<CWnd>::Construct, ConstructDestruct<CWnd>::Destruct,
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offsetof(CWnd, m_hWnd));
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#ifndef _AFX_PORTABLE
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AfxSetNewHandler(pnhOldHandler);
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#endif
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AfxEnableMemoryTracking(bEnable);
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}
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return pState–>m_pmapHWND;
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}
下面的一段代码是函数定义和函数调用。CMainFrame类的实例指针mainFrame是主线程(A线程)的窗口实例指针,该指针在工作线程(B线程)中被用来调用类的成员函数getHtmlView,该函数是窗口关联函数(获取窗口中的视窗)。当在工作线程中获取窗口视窗时,就会调用函数CWnd::AssertValid(),执行该函数时会调用上面的afxMapHWND函数。很显然上述代码中绿色中的结构体AFX_MODULE_THREAD_STATE获取的是当前线程(即工作线程)的结构体,而非主线程的结构体,故出现了标题1中描述的问题
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// 在B线程中执行下述代码
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CMainFrame *mainFrame = (CMainFrame *)AfxGetApp()–>GetMainWnd();
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utHtmlView *htmlView = (utHtmlView *)mainFrame–>getHtmlView();
// 函数定义
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CView *CMainFrame::getHtmlView( void )
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{
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if(!m_nMultiInterface)
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return GetActiveView();
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return (CView *)m_wndSplitter.GetPane(0, 1);
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}
3. 解决
1) 方法一
CWnd::AssertValid只会在Debug下有效,而上述问题只会在多线程程序中出现。简单的解决方法是在Release下运行程序,就可以避免上述问题
2)方法二
用 FromHandle 返回临时对象的指针,就可以调用各种类的方法。临时对象会在 OnIdle 中销毁。这里对 FromHandle 的实现原理从源码上进行解析
下面代码是 CWnd 的 FromHandle 方法,大致的意思为从 CHandleMap 中获取临时 CWnd 对象的指针
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CWnd* PASCAL CWnd::FromHandle(HWND hWnd)
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{
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CHandleMap* pMap = afxMapHWND(TRUE); //create map if not exist
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ASSERT(pMap != NULL);
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CWnd* pWnd = (CWnd*)pMap–>FromHandle(hWnd);
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#ifndef _AFX_NO_OCC_SUPPORT
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pWnd–>AttachControlSite(pMap);
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#endif
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ASSERT(pWnd == NULL || pWnd–>m_hWnd == hWnd);
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return pWnd;
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}
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// // 2 // CHandleMap* PASCAL afxMapHWND(BOOL bCreate) { AFX_MODULE_THREAD_STATE* pState = AfxGetModuleThreadState(); if (pState->m_pmapHWND == NULL && bCreate) { BOOL bEnable = AfxEnableMemoryTracking(FALSE); #ifndef _AFX_PORTABLE _PNH pnhOldHandler = AfxSetNewHandler(&AfxCriticalNewHandler); #endif pState->m_pmapHWND = new CHandleMap(RUNTIME_CLASS(CTempWnd), offsetof(CWnd, m_hWnd)); #ifndef _AFX_PORTABLE AfxSetNewHandler(pnhOldHandler); #endif AfxEnableMemoryTracking(bEnable); } return pState->m_pmapHWND; }再看#define offsetof(s,m) (size_t)&(((s *)0)->m)继续// // 3 // CObject* CHandleMap::FromHandle(HANDLE h) { ASSERT(m_pClass != NULL); ASSERT(m_nHandles == 1 || m_nHandles == 2); if (h == NULL) return NULL; CObject* pObject = LookupPermanent(h); if (pObject != NULL) return pObject; // return permanent one else if ((pObject = LookupTemporary(h)) != NULL) { HANDLE* ph = (HANDLE*)((BYTE*)pObject + m_nOffset); ASSERT(ph[0] == h || ph[0] == NULL); ph[0] = h; if (m_nHandles == 2) { ASSERT(ph[1] == h || ph[1] == NULL); ph[1] = h; } return pObject; // return current temporary one } // This handle wasn’t created by us, so we must create a temporary // C++ object to wrap it. We don’t want the user to see this memory // allocation, so we turn tracing off. BOOL bEnable = AfxEnableMemoryTracking(FALSE); #ifndef _AFX_PORTABLE _PNH pnhOldHandler = AfxSetNewHandler(&AfxCriticalNewHandler); #endif CObject* pTemp = NULL; TRY { pTemp = m_pClass->CreateObject(); if (pTemp == NULL) AfxThrowMemoryException(); m_temporaryMap.SetAt((LPVOID)h, pTemp); } CATCH_ALL(e) { #ifndef _AFX_PORTABLE AfxSetNewHandler(pnhOldHandler); #endif AfxEnableMemoryTracking(bEnable); THROW_LAST(); } END_CATCH_ALL #ifndef _AFX_PORTABLE AfxSetNewHandler(pnhOldHandler); #endif AfxEnableMemoryTracking(bEnable); // now set the handle in the object HANDLE* ph = (HANDLE*)((BYTE*)pTemp + m_nOffset); // after CObject ph[0] = h; if (m_nHandles == 2) ph[1] = h; return pTemp; }
对标题2中的程序进行修改如下,就可以避免出现上面的问题。首先获取到主窗口实例指针,然后根据主窗口实例指针的窗口句柄获取到临时主窗口实例指针,最后就可以通过临时实例指针操作该类中的各种方法了
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CWnd *mWnd = AfxGetApp()–>GetMainWnd();
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CMainFrame *mainFrame = (CMainFrame *)(mWnd–>FromHandle(mWnd–>GetSafeHwnd()));
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utHtmlView *htmlView = (utHtmlView *)mainFrame–>getHtmlView();
3)方法三
方法二虽然解决了上面遇到的问题,但是不能完全解决。比如在主窗口下还有子窗口,通过上述方法主窗口可以调用类的各种方法,但是通过类方法再去调用子窗口的窗口关联函数时还是会出现问题。比如在下面的代码中,在主窗口的方法中,调用子窗口的窗口方法还是会遇到类似问题
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CView *CMainFrame::getHtmlView( void )
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{
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if(!m_nMultiInterface)
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return GetActiveView();
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return (CView *)m_wndSplitter.GetPane(0, 1);
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}
要彻底解决这个问题,那么就需要通过多线程的线程间消息机制解决上述问题,MFC多线程消息机制
///
A. 声明与定义
(1) 消息声明
#define WM_MY_MESSAGE WM_USER + 100
(2) 类声明中添加以下声明
DECLARE_MESSAGE_MAP()
afx_msg LRESULT OnMyMessage(WPARAM wParam,LPARAM lParam);
(3) 类实现中添加下一代码
BEGIN_MESSAGE_MAP(MYCLASS, MYPARENTCLASS)
ON_MESSAGE(WM_MY_MESSAGE, OnMyMessage)
END_MESSAGE_MAP()
LRESULT MYCLASS::OnMyMessage(WPARAM wParam,LPARAM lParam)
{
return S_OK;
}
B. 工作线程代码实现
MYCLASS *myClass;
同步消息传递
HRESULT hr = myClass->SendMessage(WM_MY_MESSAGE, wParam, lParam);
异步消息传递
HRESULT hr = myClass->PostMessage(WM_MY_MESSAGE, wParam, lParam);
转载自:
http://blog.chinaunix.net/uid-24862988-id-3793502.html
转载于:https://blog.51cto.com/7044041/1891064
发布者:全栈程序员-用户IM,转载请注明出处:https://javaforall.cn/159090.html原文链接:https://javaforall.cn
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