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1、创建和终止线程
在MFC程序中创建一个线程,宜调用AfxBeginThread函数。该函数因参数不同而具有两种重载版本,分别对应工作者线程和用户接口(UI)线程。
工作者线程
CWinThread *AfxBeginThread( AFX_THREADPROC pfnThreadProc, //控制函数 LPVOID pParam, //传递给控制函数的参数 int nPriority = THREAD_PRIORITY_NORMAL, //线程的优先级 UINT nStackSize = 0, //线程的堆栈大小 DWORD dwCreateFlags = 0, //线程的创建标志 LPSECURITY_ATTRIBUTES lpSecurityAttrs = NULL //线程的安全属性 ); |
工作者线程编程较为简单,只需编写线程控制函数和启动线程即可。下面的代码给出了定义一个控制函数和启动它的过程:
//线程控制函数 UINT MfcThreadProc(LPVOID lpParam) { CExampleClass *lpObject = (CExampleClass*)lpParam; if (lpObject == NULL || !lpObject->IsKindof(RUNTIME_CLASS(CExampleClass))) return - 1; //输入参数非法 //线程成功启动 while (1) return 0; }
//在MFC程序中启动线程 AfxBeginThread(MfcThreadProc, lpObject); |
UI线程
创建用户界面线程时,必须首先从CWinThread 派生类,并使用 DECLARE_DYNCREATE 和 IMPLEMENT_DYNCREATE 宏声明此类。
下面给出了CWinThread类的原型(添加了关于其重要函数功能和是否需要被继承类重载的注释):
class CWinThread : public CCmdTarget { DECLARE_DYNAMIC(CWinThread)
public: // Constructors CWinThread(); BOOL CreateThread(DWORD dwCreateFlags = 0, UINT nStackSize = 0, LPSECURITY_ATTRIBUTES lpSecurityAttrs = NULL);
// Attributes CWnd* m_pMainWnd; // main window (usually same AfxGetApp()->m_pMainWnd) CWnd* m_pActiveWnd; // active main window (may not be m_pMainWnd) BOOL m_bAutoDelete; // enables 'delete this' after thread termination
// only valid while running HANDLE m_hThread; // this thread's HANDLE operator HANDLE() const; DWORD m_nThreadID; // this thread's ID
int GetThreadPriority(); BOOL SetThreadPriority(int nPriority);
// Operations DWORD SuspendThread(); DWORD ResumeThread(); BOOL PostThreadMessage(UINT message, WPARAM wParam, LPARAM lParam);
// Overridables //执行线程实例初始化,必须重写 virtual BOOL InitInstance();
// running and idle processing //控制线程的函数,包含消息泵,一般不重写 virtual int Run();
//消息调度到TranslateMessage和DispatchMessage之前对其进行筛选, //通常不重写 virtual BOOL PreTranslateMessage(MSG* pMsg);
virtual BOOL PumpMessage(); // low level message pump
//执行线程特定的闲置时间处理,通常不重写 virtual BOOL OnIdle(LONG lCount); // return TRUE if more idle processing virtual BOOL IsIdleMessage(MSG* pMsg); // checks for special messages
//线程终止时执行清除,通常需要重写 virtual int ExitInstance(); // default will 'delete this'
//截获由线程的消息和命令处理程序引发的未处理异常,通常不重写 virtual LRESULT ProcessWndProcException(CException* e, const MSG* pMsg);
// Advanced: handling messages sent to message filter hook virtual BOOL ProcessMessageFilter(int code, LPMSG lpMsg);
// Advanced: virtual access to m_pMainWnd virtual CWnd* GetMainWnd();
// Implementation public: virtual ~CWinThread(); #ifdef _DEBUG virtual void AssertValid() const; virtual void Dump(CDumpContext& dc) const; int m_nDisablePumpCount; // Diagnostic trap to detect illegal re-entrancy #endif void CommonConstruct(); virtual void Delete(); // 'delete this' only if m_bAutoDelete == TRUE
// message pump for Run MSG m_msgCur; // current message
public: // constructor used by implementation of AfxBeginThread CWinThread(AFX_THREADPROC pfnThreadProc, LPVOID pParam);
// valid after construction LPVOID m_pThreadParams; // generic parameters passed to starting function AFX_THREADPROC m_pfnThreadProc;
// set after OLE is initialized void (AFXAPI* m_lpfnOleTermOrFreeLib)(BOOL, BOOL); COleMessageFilter* m_pMessageFilter;
protected: CPoint m_ptCursorLast; // last mouse position UINT m_nMsgLast; // last mouse message BOOL DispatchThreadMessageEx(MSG* msg); // helper void DispatchThreadMessage(MSG* msg); // obsolete };
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启动UI线程的AfxBeginThread函数的原型为:
CWinThread *AfxBeginThread( //从CWinThread派生的类的 RUNTIME_CLASS CRuntimeClass *pThreadClass, int nPriority = THREAD_PRIORITY_NORMAL, UINT nStackSize = 0, DWORD dwCreateFlags = 0, LPSECURITY_ATTRIBUTES lpSecurityAttrs = NULL ); |
我们可以方便地使用VC++ 6.0类向导定义一个继承自CWinThread的用户线程类。下面给出产生我们自定义的CWinThread子类CMyUIThread的方法。
打开VC++ 6.0类向导,在如下窗口中选择Base Class类为CWinThread,输入子类名为CMyUIThread,点击"OK"按钮后就产生了类CMyUIThread。
其源代码框架为:
///////////////////////////////////////////////////////////////////////////// // CMyUIThread thread
class CMyUIThread : public CWinThread { DECLARE_DYNCREATE(CMyUIThread) protected: CMyUIThread(); // protected constructor used by dynamic creation
// Attributes public:
// Operations public:
// Overrides // ClassWizard generated virtual function overrides //{{AFX_VIRTUAL(CMyUIThread) public: virtual BOOL InitInstance(); virtual int ExitInstance(); //}}AFX_VIRTUAL
// Implementation protected: virtual ~CMyUIThread();
// Generated message map functions //{{AFX_MSG(CMyUIThread) // NOTE - the ClassWizard will add and remove member functions here. //}}AFX_MSG
DECLARE_MESSAGE_MAP() };
///////////////////////////////////////////////////////////////////////////// // CMyUIThread
IMPLEMENT_DYNCREATE(CMyUIThread, CWinThread)
CMyUIThread::CMyUIThread() {}
CMyUIThread::~CMyUIThread() {}
BOOL CMyUIThread::InitInstance() { // TODO: perform and per-thread initialization here return TRUE; }
int CMyUIThread::ExitInstance() { // TODO: perform any per-thread cleanup here return CWinThread::ExitInstance(); }
BEGIN_MESSAGE_MAP(CMyUIThread, CWinThread) //{{AFX_MSG_MAP(CMyUIThread) // NOTE - the ClassWizard will add and remove mapping macros here. //}}AFX_MSG_MAP END_MESSAGE_MAP() |
使用下列代码就可以启动这个UI线程:
CMyUIThread *pThread; pThread = (CMyUIThread*) AfxBeginThread( RUNTIME_CLASS(CMyUIThread) ); |
另外,我们也可以不用AfxBeginThread 创建线程,而是分如下两步完成:
(1)调用线程类的构造函数创建一个线程对象;
(2)调用CWinThread::CreateThread函数来启动该线程。
在线程自身内调用AfxEndThread函数可以终止该线程:
void AfxEndThread( UINT nExitCode //the exit code of the thread ); |
对于UI线程而言,如果消息队列中放入了WM_QUIT消息,将结束线程。
关于UI线程和工作者线程的分配,最好的做法是:将所有与UI相关的操作放入主线程,其它的纯粹的运算工作交给独立的数个工作者线程。
候捷先生早些时间喜欢为MDI程序的每个窗口创建一个线程,他后来澄清了这个错误。因为如果为MDI程序的每个窗口都单独创建一个线程,在窗口进行切换的时候,将进行线程的上下文切换!
2.线程间通信
MFC中定义了继承自CSyncObject类的CCriticalSection 、CCEvent、CMutex、CSemaphore类封装和简化了WIN32 API所提供的临界区、事件、互斥和信号量。使用这些同步机制,必须包含"Afxmt.h"头文件。下图给出了类的继承关系:
作为CSyncObject类的继承类,我们仅仅使用基类CSyncObject的接口函数就可以方便、统一的操作CCriticalSection 、CCEvent、CMutex、CSemaphore类,下面是CSyncObject类的原型:
class CSyncObject : public CObject { DECLARE_DYNAMIC(CSyncObject)
// Constructor public: CSyncObject(LPCTSTR pstrName);
// Attributes public: operator HANDLE() const; HANDLE m_hObject;
// Operations virtual BOOL Lock(DWORD dwTimeout = INFINITE); virtual BOOL Unlock() = 0; virtual BOOL Unlock(LONG /* lCount */, LPLONG /* lpPrevCount="NULL" */) { return TRUE; }
// Implementation public: virtual ~CSyncObject(); #ifdef _DEBUG CString m_strName; virtual void AssertValid() const; virtual void Dump(CDumpContext& dc) const; #endif friend class CSingleLock; friend class CMultiLock; }; |
CSyncObject类最主要的两个函数是Lock和Unlock,若我们直接使用CSyncObject类及其派生类,我们需要非常小心地在Lock之后调用Unlock。
MFC提供的另两个类CSingleLock(等待一个对象)和CMultiLock(等待多个对象)为我们编写应用程序提供了更灵活的机制,下面以实际来阐述CSingleLock的用法:
class CThreadSafeWnd
~CThreadSafeWnd(){} void SetWindow(CWnd *pwnd) { m_pCWnd = pwnd; } void PaintBall(COLORREF color, CRect &rc); private: CWnd *m_pCWnd; CCriticalSection m_CSect; };
void CThreadSafeWnd::PaintBall(COLORREF color, CRect &rc) { CSingleLock csl(&m_CSect); //缺省的Timeout是INFINITE,只有m_Csect被激活,csl.Lock()才能返回 //true,这里一直等待 if (csl.Lock()) ; { // not necessary //AFX_MANAGE_STATE(AfxGetStaticModuleState( )); CDC *pdc = m_pCWnd->GetDC(); CBrush brush(color); CBrush *oldbrush = pdc->SelectObject(&brush); pdc->Ellipse(rc); pdc->SelectObject(oldbrush); GdiFlush(); // don't wait to update the display } } |
上述实例讲述了用CSingleLock对Windows GDI相关对象进行保护的方法,下面再给出一个其他方面的例子:
int array1[10], array2[10]; CMutexSection section; //创建一个CMutex类的对象
//赋值线程控制函数 UINT EvaluateThread(LPVOID param) { CSingleLock singlelock; singlelock(§ion);
//互斥区域 singlelock.Lock(); for (int i = 0; i < 10; i++) array1[i] = i; singlelock.Unlock(); } //拷贝线程控制函数 UINT CopyThread(LPVOID param) { CSingleLock singlelock; singlelock(§ion);
//互斥区域 singlelock.Lock(); for (int i = 0; i < 10; i++) array2[i] = array1[i]; singlelock.Unlock(); } }
AfxBeginThread(EvaluateThread, NULL); //启动赋值线程 AfxBeginThread(CopyThread, NULL); //启动拷贝线程 |
上面的例子中启动了两个线程EvaluateThread和CopyThread,线程EvaluateThread把10个数赋值给数组array1[],线程CopyThread将数组array1[]拷贝给数组array2[]。由于数组的拷贝和赋值都是整体行为,如果不以互斥形式执行代码段:
for (int i = 0; i < 10; i++) array1[i] = i; |
和
for (int i = 0; i < 10; i++) array2[i] = array1[i]; |
其结果是很难预料的!
除了可使用CCriticalSection、CEvent、CMutex、CSemaphore作为线程间同步通信的方式以外,我们还可以利用PostThreadMessage函数在线程间发送消息:
BOOL PostThreadMessage(DWORD idThread, // thread identifier UINT Msg, // message to post WPARAM wParam, // first message parameter LPARAM lParam // second message parameter
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3.线程与消息队列
在WIN32中,每一个线程都对应着一个消息队列。由于一个线程可以产生数个窗口,所以并不是每个窗口都对应着一个消息队列。下列几句话应该作为"定理"被记住:
"定理" 一
所有产生给某个窗口的消息,都先由创建这个窗口的线程处理;
"定理" 二
Windows屏幕上的每一个控件都是一个窗口,有对应的窗口函数。
消息的发送通常有两种方式,一是SendMessage,一是PostMessage,其原型分别为:
LRESULT SendMessage(HWND hWnd, // handle of destination window UINT Msg, // message to send WPARAM wParam, // first message parameter LPARAM lParam // second message parameter ); BOOL PostMessage(HWND hWnd, // handle of destination window UINT Msg, // message to post WPARAM wParam, // first message parameter LPARAM lParam // second message parameter ); |
两个函数原型中的四个参数的意义相同,但是SendMessage和PostMessage的行为有差异。SendMessage必须等待消息被处理后才返回,而PostMessage仅仅将消息放入消息队列。SendMessage的目标窗口如果属于另一个线程,则会发生线程上下文切换,等待另一线程处理完成消息。为了防止另一线程当掉,导致SendMessage永远不能返回,我们可以调用SendMessageTimeout函数:
LRESULT SendMessageTimeout( HWND hWnd, // handle of destination window UINT Msg, // message to send WPARAM wParam, // first message parameter LPARAM lParam, // second message parameter UINT fuFlags, // how to send the message UINT uTimeout, // time-out duration LPDWORD lpdwResult // return value for synchronous call ); |
4. MFC线程、消息队列与MFC程序的"生死因果"
分析MFC程序的主线程启动及消息队列处理的过程将有助于我们进一步理解UI线程与消息队列的关系,为此我们需要简单地叙述一下MFC程序的"生死因果"(侯捷:《深入浅出MFC》)。
使用VC++ 6.0的向导完成一个最简单的单文档架构MFC应用程序MFCThread:
(1) 输入MFC EXE工程名MFCThread;
(2) 选择单文档架构,不支持Document/View结构;
(3) ActiveX、3D container等其他选项都选择无。
我们来分析这个工程。下面是产生的核心源代码:
MFCThread.h 文件
class CMFCThreadApp : public CWinApp { public: CMFCThreadApp();
// Overrides // ClassWizard generated virtual function overrides //{{AFX_VIRTUAL(CMFCThreadApp) public: virtual BOOL InitInstance(); //}}AFX_VIRTUAL
// Implementation
public: //{{AFX_MSG(CMFCThreadApp) afx_msg void OnAppAbout(); // NOTE - the ClassWizard will add and remove member functions here. // DO NOT EDIT what you see in these blocks of generated code ! //}}AFX_MSG DECLARE_MESSAGE_MAP() }; |
MFCThread.cpp文件
CMFCThreadApp theApp;
///////////////////////////////////////////////////////////////////////////// // CMFCThreadApp initialization
BOOL CMFCThreadApp::InitInstance() { … CMainFrame* pFrame = new CMainFrame; m_pMainWnd = pFrame;
// create and load the frame with its resources pFrame->LoadFrame(IDR_MAINFRAME,WS_OVERLAPPEDWINDOW | FWS_ADDTOTITLE, NULL,NULL); // The one and only window has been initialized, so show and update it. pFrame->ShowWindow(SW_SHOW); pFrame->UpdateWindow();
return TRUE; } |
MainFrm.h文件
#include "ChildView.h"
class CMainFrame : public CFrameWnd { public: CMainFrame(); protected: DECLARE_DYNAMIC(CMainFrame)
// Attributes public:
// Operations public: // Overrides // ClassWizard generated virtual function overrides //{{AFX_VIRTUAL(CMainFrame) virtual BOOL PreCreateWindow(CREATESTRUCT& cs); virtual BOOL OnCmdMsg(UINT nID, int nCode, void* pExtra, AFX_CMDHANDLERINFO* pHandlerInfo); //}}AFX_VIRTUAL
// Implementation public: virtual ~CMainFrame(); #ifdef _DEBUG virtual void AssertValid() const; virtual void Dump(CDumpContext& dc) const; #endif CChildView m_wndView;
// Generated message map functions protected: //{{AFX_MSG(CMainFrame) afx_msg void OnSetFocus(CWnd *pOldWnd); // NOTE - the ClassWizard will add and remove member functions here. // DO NOT EDIT what you see in these blocks of generated code! //}}AFX_MSG DECLARE_MESSAGE_MAP() }; |
MainFrm.cpp文件
IMPLEMENT_DYNAMIC(CMainFrame, CFrameWnd)
BEGIN_MESSAGE_MAP(CMainFrame, CFrameWnd) //{{AFX_MSG_MAP(CMainFrame) // NOTE - the ClassWizard will add and remove mapping macros here. // DO NOT EDIT what you see in these blocks of generated code ! ON_WM_SETFOCUS() //}}AFX_MSG_MAP END_MESSAGE_MAP()
///////////////////////////////////////////////////////////////////////////// // CMainFrame construction/destruction
CMainFrame::CMainFrame() { // TODO: add member initialization code here }
CMainFrame::~CMainFrame() {}
BOOL CMainFrame::PreCreateWindow(CREATESTRUCT& cs) { if( !CFrameWnd::PreCreateWindow(cs) ) return FALSE; // TODO: Modify the Window class or styles here by modifying // the CREATESTRUCT cs
cs.dwExStyle &= ~WS_EX_CLIENTEDGE; cs.lpszClass = AfxRegisterWndClass(0); return TRUE; } |
ChildView.h文件
// CChildView window
class CChildView : public CWnd { // Construction public: CChildView();
// Attributes public: // Operations public: // Overrides // ClassWizard generated virtual function overrides //{{AFX_VIRTUAL(CChildView) protected: virtual BOOL PreCreateWindow(CREATESTRUCT& cs); //}}AFX_VIRTUAL
// Implementation public: virtual ~CChildView();
// Generated message map functions protected: //{{AFX_MSG(CChildView) afx_msg void OnPaint(); //}}AFX_MSG DECLARE_MESSAGE_MAP() };
ChildView.cpp文件 // CChildView
CChildView::CChildView() {}
CChildView::~CChildView() {}
BEGIN_MESSAGE_MAP(CChildView,CWnd ) //}AFX_MSG_MAP END_MESSAGE_MAP()
///////////////////////////////////////////////////////////////////////////// // CChildView message handlers
BOOL CChildView::PreCreateWindow(CREATESTRUCT& cs) { if (!CWnd::PreCreateWindow(cs)) return FALSE;
cs.dwExStyle |= WS_EX_CLIENTEDGE; cs.style &= ~WS_BORDER; cs.lpszClass = AfxRegisterWndClass(CS_HREDRAW|CS_VREDRAW|CS_DBLCLKS,::LoadCursor(NULL, IDC_ARROW), HBRUSH(COLOR_WINDOW+1),NULL);
return TRUE; }
void CChildView::OnPaint() { CPaintDC dc(this); // device context for painting
// TODO: Add your message handler code here // Do not call CWnd::OnPaint() for painting messages } |
文件MFCThread.h和MFCThread.cpp定义和实现的类CMFCThreadApp继承自CWinApp类,而CWinApp类又继承自CWinThread类(CWinThread类又继承自CCmdTarget类),所以CMFCThread本质上是一个MFC线程类,下图给出了相关的类层次结构:
我们提取CWinApp类原型的一部分:
class CWinApp : public CWinThread { DECLARE_DYNAMIC(CWinApp) public: // Constructor CWinApp(LPCTSTR lpszAppName = NULL);// default app name // Attributes // Startup args (do not change) HINSTANCE m_hInstance; HINSTANCE m_hPrevInstance; LPTSTR m_lpCmdLine; int m_nCmdShow; // Running args (can be changed in InitInstance) LPCTSTR m_pszAppName; // human readable name LPCTSTR m_pszExeName; // executable name (no spaces) LPCTSTR m_pszHelpFilePath; // default based on module path LPCTSTR m_pszProfileName; // default based on app name
// Overridables virtual BOOL InitApplication(); virtual BOOL InitInstance(); virtual int ExitInstance(); // return app exit code virtual int Run(); virtual BOOL OnIdle(LONG lCount); // return TRUE if more idle processing virtual LRESULT ProcessWndProcException(CException* e,const MSG* pMsg);
public: virtual ~CWinApp(); protected: DECLARE_MESSAGE_MAP() }; |
SDK程序的WinMain 所完成的工作现在由CWinApp 的三个函数完成:
virtual BOOL InitApplication(); virtual BOOL InitInstance(); virtual int Run(); |
"CMFCThreadApp theApp;"语句定义的全局变量theApp是整个程式的application object,每一个MFC 应用程序都有一个。当我们执行MFCThread程序的时候,这个全局变量被构造。theApp 配置完成后,WinMain开始执行。但是程序中并没有WinMain的代码,它在哪里呢?原来MFC早已准备好并由Linker直接加到应用程序代码中的,其原型为(存在于VC++6.0安装目录下提供的APPMODUL.CPP文件中):
extern "C" int WINAPI _tWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPTSTR lpCmdLine, int nCmdShow) { // call shared/exported WinMain return AfxWinMain(hInstance, hPrevInstance, lpCmdLine, nCmdShow); } |
其中调用的AfxWinMain如下(存在于VC++6.0安装目录下提供的WINMAIN.CPP文件中):
int AFXAPI AfxWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPTSTR lpCmdLine, int nCmdShow) { ASSERT(hPrevInstance == NULL);
int nReturnCode = -1; CWinThread* pThread = AfxGetThread(); CWinApp* pApp = AfxGetApp();
// AFX internal initialization if (!AfxWinInit(hInstance, hPrevInstance, lpCmdLine, nCmdShow)) goto InitFailure;
// App global initializations (rare) if (pApp != NULL && !pApp->InitApplication()) goto InitFailure;
// Perform specific initializations if (!pThread->InitInstance()) { if (pThread->m_pMainWnd != NULL) { TRACE0("Warning: Destroying non-NULL m_pMainWndn"); pThread->m_pMainWnd->DestroyWindow(); } nReturnCode = pThread->ExitInstance(); goto InitFailure; } nReturnCode = pThread->Run();
InitFailure: #ifdef _DEBUG // Check for missing AfxLockTempMap calls if (AfxGetModuleThreadState()->m_nTempMapLock != 0) { TRACE1("Warning: Temp map lock count non-zero (%ld).n", AfxGetModuleThreadState()->m_nTempMapLock); } AfxLockTempMaps(); AfxUnlockTempMaps(-1); #endif
AfxWinTerm(); return nReturnCode; } |
我们提取主干,实际上,这个函数做的事情主要是:
CWinThread* pThread = AfxGetThread(); CWinApp* pApp = AfxGetApp(); AfxWinInit(hInstance, hPrevInstance, lpCmdLine, nCmdShow) pApp->InitApplication() pThread->InitInstance() pThread->Run(); |
其中,InitApplication 是注册窗口类别的场所;InitInstance是产生窗口并显示窗口的场所;Run是提取并分派消息的场所。这样,MFC就同WIN32 SDK程序对应起来了。CWinThread::Run是程序生命的"活水源头"(侯捷:《深入浅出MFC》,函数存在于VC++ 6.0安装目录下提供的THRDCORE.CPP文件中):
// main running routine until thread exits int CWinThread::Run() { ASSERT_VALID(this);
// for tracking the idle time state BOOL bIdle = TRUE; LONG lIdleCount = 0;
// acquire and dispatch messages until a WM_QUIT message is received. for (;;) { // phase1: check to see if we can do idle work while (bIdle && !::PeekMessage(&m_msgCur, NULL, NULL, NULL, PM_NOREMOVE)) { // call OnIdle while in bIdle state if (!OnIdle(lIdleCount++)) bIdle = FALSE; // assume "no idle" state }
// phase2: pump messages while available do { // pump message, but quit on WM_QUIT if (!PumpMessage()) return ExitInstance();
// reset "no idle" state after pumping "normal" message if (IsIdleMessage(&m_msgCur)) { bIdle = TRUE; lIdleCount = 0; }
} while (::PeekMessage(&m_msgCur, NULL, NULL, NULL, PM_NOREMOVE)); } ASSERT(FALSE); // not reachable } |
其中的PumpMessage函数又对应于:
///////////////////////////////////////////////////////////////////////////// // CWinThread implementation helpers
BOOL CWinThread::PumpMessage() { ASSERT_VALID(this);
if (!::GetMessage(&m_msgCur, NULL, NULL, NULL)) { return FALSE; }
// process this message if(m_msgCur.message != WM_KICKIDLE && !PreTranslateMessage(&m_msgCur)) return TRUE; } |
因此,忽略IDLE状态,整个RUN的执行提取主干就是:
do while (::PeekMessage(...)); |
由此,我们建立了MFC消息获取和派生机制与WIN32 SDK程序之间的对应关系。下面继续分析MFC消息的"绕行"过程。
在MFC中,只要是CWnd 衍生类别,就可以拦下任何Windows消息。与窗口无关的MFC类别(例如CDocument 和CWinApp)如果也想处理消息,必须衍生自CCmdTarget,并且只可能收到WM_COMMAND消息。所有能进行MESSAGE_MAP的类都继承自CCmdTarget,如:
MFC中MESSAGE_MAP的定义依赖于以下三个宏:
DECLARE_MESSAGE_MAP()
BEGIN_MESSAGE_MAP( theClass, //Specifies the name of the class whose message map this is baseClass //Specifies the name of the base class of theClass )
END_MESSAGE_MAP() |
我们程序中涉及到的有:MFCThread.h、MainFrm.h、ChildView.h文件
DECLARE_MESSAGE_MAP() MFCThread.cpp文件 BEGIN_MESSAGE_MAP(CMFCThreadApp, CWinApp) //{{AFX_MSG_MAP(CMFCThreadApp) ON_COMMAND(ID_APP_ABOUT, OnAppAbout) // NOTE - the ClassWizard will add and remove mapping macros here. // DO NOT EDIT what you see in these blocks of generated code! //}}AFX_MSG_MAP END_MESSAGE_MAP() MainFrm.cpp文件 BEGIN_MESSAGE_MAP(CMainFrame, CFrameWnd) //{{AFX_MSG_MAP(CMainFrame) // NOTE - the ClassWizard will add and remove mapping macros here. // DO NOT EDIT what you see in these blocks of generated code ! ON_WM_SETFOCUS() //}}AFX_MSG_MAP END_MESSAGE_MAP() ChildView.cpp文件 BEGIN_MESSAGE_MAP(CChildView,CWnd ) //}AFX_MSG_MAP END_MESSAGE_MAP() |
由这些宏,MFC建立了一个消息映射表(消息流动网),按照消息流动网匹配对应的消息处理函数,完成整个消息的"绕行"。
看到这里相信你有这样的疑问:程序定义了CWinApp类的theApp全局变量,可是从来没有调用AfxBeginThread或theApp.CreateThread启动线程呀,theApp对应的线程是怎么启动的?
答:MFC在这里用了很高明的一招。实际上,程序开始运行,第一个线程是由
操作系统(OS)启动的,在CWinApp的构造函数里,MFC将theApp"对应"向了这个线程,具体的实现是这样的:
CWinApp::CWinApp(LPCTSTR lpszAppName) { if (lpszAppName != NULL) m_pszAppName = _tcsdup(lpszAppName); else m_pszAppName = NULL;
// initialize CWinThread state AFX_MODULE_STATE *pModuleState = _AFX_CMDTARGET_GETSTATE(); AFX_MODULE_THREAD_STATE *pThreadState = pModuleState->m_thread; ASSERT(AfxGetThread() == NULL); pThreadState->m_pCurrentWinThread = this; ASSERT(AfxGetThread() == this); m_hThread = ::GetCurrentThread(); m_nThreadID = ::GetCurrentThreadId();
// initialize CWinApp state ASSERT(afxCurrentWinApp == NULL); // only one CWinApp object please pModuleState->m_pCurrentWinApp = this; ASSERT(AfxGetApp() == this);
// in non-running state until WinMain m_hInstance = NULL; m_pszHelpFilePath = NULL; m_pszProfileName = NULL; m_pszRegistryKey = NULL; m_pszExeName = NULL; m_pRecentFileList = NULL; m_pDocManager = NULL; m_atomApp = m_atomSystemTopic = NULL; //微软懒鬼?或者他认为 //这样连等含义更明确? m_lpCmdLine = NULL; m_pCmdInfo = NULL;
// initialize wait cursor state m_nWaitCursorCount = 0; m_hcurWaitCursorRestore = NULL;
// initialize current printer state m_hDevMode = NULL; m_hDevNames = NULL; m_nNumPreviewPages = 0; // not specified (defaults to 1)
// initialize DAO state m_lpfnDaoTerm = NULL; // will be set if AfxDaoInit called
// other initialization m_bHelpMode = FALSE; m_nSafetyPoolSize = 512; // default size } |
很显然,theApp成员变量都被赋予OS启动的这个当前线程相关的值,如代码:
m_hThread = ::GetCurrentThread();//theApp的线程句柄等于当前线程句柄 m_nThreadID = ::GetCurrentThreadId();//theApp的线程ID等于当前线程ID |
所以CWinApp类几乎只是为MFC程序的第一个线程量身定制的,它不需要也不能被AfxBeginThread或theApp.CreateThread"再次"启动。这就是CWinApp类和theApp全局变量的内涵!如果你要再增加一个UI线程,不要继承类CWinApp,而应继承类CWinThread。而参考第1节,由于我们一般以主线程(在MFC程序里实际上就是OS启动的第一个线程)处理所有窗口的消息,所以我们几乎没有再启动UI线程的需求!