MFC(5) GetMessage和PeekMessage的区别

关注点：

1.函数的处理和调用过程。

2.消息的发送方式、途径及对象。

PeekMessage与GetMessage的对比
相同点：
           PeekMessage函数与GetMessage函数都用于查看应用程序消息队列，有消息时将队列中 的消息派发出去。

不同点：
          无论应用程序消息队列是否有消息，PeekMessage函数都立即返回，程序得以继续执行后面的语句（无消息则执行其它指令，有消息时一般要将消息派发出去，再执行其它指令）。GetMessage函数只有在消息对立中有消息时返回，队列中无消息就会一直等，直至下一个消息出现时才返回。在等的这段时间，应用程序不能执行任何指令。

（从他们的不同点上来看，PeekMessage函数有点像“乞丐行乞”，有你就施舍点，没有也不强。GetMessage函数有点像“强盗打劫”，有你得给，没有我就等你什么时候有了再给，这段时间我什么都不干，我就等你。）

下面的程序用来在窗体内画随机生成的矩形，分别使用PeekMessage函数和GetMessage函数实现，读者可以种实际效果看出他们两者的区别。

 <span style="font-size:18px;">#include <windows.h>
 #include <stdlib.h>           // for the rand function

 LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM) ;
 void DrawRectangle (HWND) ;

 int cxClient, cyClient ;

 int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
                     PSTR szCmdLine, int iCmdShow)
 {
      static TCHAR szAppName[] = TEXT ("RandRect") ;
      HWND         hwnd ;
      MSG          msg ;
      WNDCLASS     wndclass ;

      wndclass.style         = CS_HREDRAW | CS_VREDRAW ;
      wndclass.lpfnWndProc   = WndProc ;
      wndclass.cbClsExtra    = 0 ;
      wndclass.cbWndExtra    = 0 ;
      wndclass.hInstance     = hInstance ;
      wndclass.hIcon         = LoadIcon (NULL, IDI_APPLICATION) ;
      wndclass.hCursor       = LoadCursor (NULL, IDC_ARROW) ;
      wndclass.hbrBackground = (HBRUSH) GetStockObject (WHITE_BRUSH) ;
      wndclass.lpszMenuName  = NULL ;
      wndclass.lpszClassName = szAppName ;

      if (!RegisterClass (&wndclass))
      {
           MessageBox (NULL, TEXT ("This program requires Windows NT!"),
                       szAppName, MB_ICONERROR) ;
           return 0 ;
      }

      hwnd = CreateWindow (szAppName, TEXT ("Random Rectangles"),
                           WS_OVERLAPPEDWINDOW,
                           CW_USEDEFAULT, CW_USEDEFAULT,
                           CW_USEDEFAULT, CW_USEDEFAULT,
                           NULL, NULL, hInstance, NULL) ;

      ShowWindow (hwnd, iCmdShow) ;
      UpdateWindow (hwnd) ;

      //用于替换的部分
     while (TRUE)
      {
           if (PeekMessage (&msg, NULL, 0, 0, PM_REMOVE))
           {
                if (msg.message == WM_QUIT)
                     break ;
                TranslateMessage (&msg) ;
                DispatchMessage (&msg) ;
           }
           else
                DrawRectangle (hwnd) ;
      }
     //用于替换的部分
      return msg.wParam ;
 }

 LRESULT CALLBACK WndProc (HWND hwnd, UINT iMsg, WPARAM wParam, LPARAM

 lParam)
 {
      switch (iMsg)
      {
      case WM_SIZE:
           cxClient = LOWORD (lParam) ;
           cyClient = HIWORD (lParam) ;
           return 0 ;

      case WM_DESTROY:
           PostQuitMessage (0) ;
           return 0 ;
      }
      return DefWindowProc (hwnd, iMsg, wParam, lParam) ;
 }

 void DrawRectangle (HWND hwnd)
 {
      HBRUSH hBrush ;
      HDC    hdc ;
      RECT   rect ;

      if (cxClient == 0 || cyClient == 0)
           return ;

      SetRect (&rect, rand () % cxClient, rand () % cyClient,
                      rand () % cxClient, rand () % cyClient) ;

      hBrush = CreateSolidBrush (
                     RGB (rand () % 256, rand () % 256, rand () % 256)) ;
      hdc = GetDC (hwnd) ;

      FillRect (hdc, &rect, hBrush) ;
      ReleaseDC (hwnd, hdc) ;
      DeleteObject (hBrush) ;
 }</span>

 <span style="font-size:18px;">#include <windows.h>
 #include <stdlib.h>           // for the rand function

 LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM) ;
 void DrawRectangle (HWND) ;

 int cxClient, cyClient ;

 int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
                     PSTR szCmdLine, int iCmdShow)
 {
      static TCHAR szAppName[] = TEXT ("RandRect") ;
      HWND         hwnd ;
      MSG          msg ;
      WNDCLASS     wndclass ;

      wndclass.style         = CS_HREDRAW | CS_VREDRAW ;
      wndclass.lpfnWndProc   = WndProc ;
      wndclass.cbClsExtra    = 0 ;
      wndclass.cbWndExtra    = 0 ;
      wndclass.hInstance     = hInstance ;
      wndclass.hIcon         = LoadIcon (NULL, IDI_APPLICATION) ;
      wndclass.hCursor       = LoadCursor (NULL, IDC_ARROW) ;
      wndclass.hbrBackground = (HBRUSH) GetStockObject (WHITE_BRUSH) ;
      wndclass.lpszMenuName  = NULL ;
      wndclass.lpszClassName = szAppName ;

      if (!RegisterClass (&wndclass))
      {
           MessageBox (NULL, TEXT ("This program requires Windows NT!"),
                       szAppName, MB_ICONERROR) ;
           return 0 ;
      }

      hwnd = CreateWindow (szAppName, TEXT ("Random Rectangles"),
                           WS_OVERLAPPEDWINDOW,
                           CW_USEDEFAULT, CW_USEDEFAULT,
                           CW_USEDEFAULT, CW_USEDEFAULT,
                           NULL, NULL, hInstance, NULL) ;

      ShowWindow (hwnd, iCmdShow) ;
      UpdateWindow (hwnd) ;

      //用于替换的部分
     while (TRUE)
      {
           if (PeekMessage (&msg, NULL, 0, 0, PM_REMOVE))
           {
                if (msg.message == WM_QUIT)
                     break ;
                TranslateMessage (&msg) ;
                DispatchMessage (&msg) ;
           }
           else
                DrawRectangle (hwnd) ;
      }
     //用于替换的部分
      return msg.wParam ;
 }

 LRESULT CALLBACK WndProc (HWND hwnd, UINT iMsg, WPARAM wParam, LPARAM

 lParam)
 {
      switch (iMsg)
      {
      case WM_SIZE:
           cxClient = LOWORD (lParam) ;
           cyClient = HIWORD (lParam) ;
           return 0 ;

      case WM_DESTROY:
           PostQuitMessage (0) ;
           return 0 ;
      }
      return DefWindowProc (hwnd, iMsg, wParam, lParam) ;
 }

 void DrawRectangle (HWND hwnd)
 {
      HBRUSH hBrush ;
      HDC    hdc ;
      RECT   rect ;

      if (cxClient == 0 || cyClient == 0)
           return ;

      SetRect (&rect, rand () % cxClient, rand () % cyClient,
                      rand () % cxClient, rand () % cyClient) ;

      hBrush = CreateSolidBrush (
                     RGB (rand () % 256, rand () % 256, rand () % 256)) ;
      hdc = GetDC (hwnd) ;

      FillRect (hdc, &rect, hBrush) ;
      ReleaseDC (hwnd, hdc) ;
      DeleteObject (hBrush) ;
 }</span>

以上程序用PeekMessage函数实现，在应用程序消息队列没有消息时，随机生成的矩形将坚持不懈地画下去。当有消息产生时，PeekMessage函数获取并派发消息出去，然后继续画随机生成的矩形。

下面部分代码用于替换WinMain函数中“用于替换的部分”的代码。

 <span style="font-size:18px;">while (TRUE)
      {
           if (GetMessage (&msg, NULL, 0, 0))
           {
                if (msg.message == WM_QUIT)
                     break ;
                TranslateMessage (&msg) ;
                DispatchMessage (&msg) ;
            DrawRectangle (hwnd) ;
           }
           else
                 break;
      }</span>

 <span style="font-size:18px;">while (TRUE)
      {
           if (GetMessage (&msg, NULL, 0, 0))
           {
                if (msg.message == WM_QUIT)
                     break ;
                TranslateMessage (&msg) ;
                DispatchMessage (&msg) ;
            DrawRectangle (hwnd) ;
           }
           else
                 break;
      }</span>

替换后，应用程序产生一个消息（鼠标移动、改变窗体大小等），在窗体内画一个随机生成的矩形，无消息产生时，窗体无变化。