由于显示的字符可能会不全，我们很容易想到的一个解决办法是使用滚动条。
先看一下代码，再进行分析：
/*---------------------------------------------------- SYSMETS2.C -- System Metrics Display Program No. 2 (c) Charles Petzold, 1998 ----------------------------------------------------*/ #define WINVER 0x0500 #include <windows.h> #include "sysmets.h" LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM) ; int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine, int iCmdShow) { static TCHAR szAppName[] = TEXT ("SysMets2") ; 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 ("Get System Metrics No. 2"), WS_OVERLAPPEDWINDOW | WS_VSCROLL , CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, NULL, NULL, hInstance, NULL) ; ShowWindow (hwnd, iCmdShow) ; UpdateWindow (hwnd) ; while (GetMessage (&msg, NULL, 0, 0)) { TranslateMessage (&msg) ; DispatchMessage (&msg) ; } return msg.wParam ; } LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam) { static int cxChar, cxCaps, cyChar, cyClient, iVscrollPos ; HDC hdc ; int i, y ; PAINTSTRUCT ps ; TCHAR szBuffer[10] ; TEXTMETRIC tm ; switch (message) { case WM_CREATE: hdc = GetDC (hwnd) ; GetTextMetrics (hdc, &tm) ; cxChar = tm.tmAveCharWidth ; cxCaps = (tm.tmPitchAndFamily & 1 ? 3 : 2) * cxChar / 2 ; cyChar = tm.tmHeight + tm.tmExternalLeading ; ReleaseDC (hwnd, hdc) ; SetScrollRange (hwnd, SB_VERT, 0, NUMLINES - 1, FALSE) ; SetScrollPos (hwnd, SB_VERT, iVscrollPos, TRUE) ; return 0 ; case WM_SIZE: cyClient = HIWORD (lParam) ; return 0 ; case WM_VSCROLL: switch (LOWORD (wParam)) { case SB_LINEUP: iVscrollPos -= 1 ; break ; case SB_LINEDOWN: iVscrollPos += 1 ; break ; case SB_PAGEUP: iVscrollPos -= cyClient / cyChar ; break ; case SB_PAGEDOWN: iVscrollPos += cyClient / cyChar ; break ; case SB_THUMBPOSITION: iVscrollPos = HIWORD (wParam) ; break ; default : break ; } iVscrollPos = max (0, min (iVscrollPos, NUMLINES - 1)) ; if (iVscrollPos != GetScrollPos (hwnd, SB_VERT)) { SetScrollPos (hwnd, SB_VERT, iVscrollPos, TRUE) ; InvalidateRect (hwnd, NULL, TRUE) ; } return 0 ; case WM_PAINT: hdc = BeginPaint (hwnd, &ps) ; for (i = 0 ; i < NUMLINES ; i++) { y = cyChar * (i - iVscrollPos) ; TextOut (hdc, 0, y, sysmetrics[i].szLabel, lstrlen (sysmetrics[i].szLabel)) ; TextOut (hdc, 22 * cxCaps, y, sysmetrics[i].szDesc, lstrlen (sysmetrics[i].szDesc)) ; SetTextAlign (hdc, TA_RIGHT | TA_TOP) ; TextOut (hdc, 22 * cxCaps + 40 * cxChar, y, szBuffer, wsprintf (szBuffer, TEXT ("%5d"), GetSystemMetrics (sysmetrics[i].iIndex))) ; SetTextAlign (hdc, TA_LEFT | TA_TOP) ; } EndPaint (hwnd, &ps) ; return 0 ; case WM_DESTROY: PostQuitMessage (0) ; return 0 ; } return DefWindowProc (hwnd, message, wParam, lParam) ; }
我们在CreateWindow函数的第三个参数加上 WS_VSCROLL 即垂直滚动条。
同时在WM_CREATE中加入：
SetScrollRange (hwnd, SB_VERT, 0, NUMLINES - 1, FALSE) ; SetScrollPos (hwnd, SB_VERT, iVscrollPos, TRUE) ;
SetScrollRange设置滚动条范围，NUMLINES是在头文件中定义的行数，
滚动条的每个位置对应一行文字。
iVscrollPos变量记录这滑块的当前位置。
在鼠标拖动滑块时，会产生WM_VSCROLL消息，对iVscrollPos变量进行赋值操作。
还有更改的一处是Y坐标： y = cyChar * (i - iVscrollPos) ; 这保证了滑动之后，小于当前位置的字符不会输出到屏幕（因为他们Y坐标是负的）
同时在滑动滚动条的时候：
if (iVscrollPos != GetScrollPos (hwnd, SB_VERT)) { SetScrollPos (hwnd, SB_VERT, iVscrollPos, TRUE) ; InvalidateRect (hwnd, NULL, TRUE) ; }
当滚动条位置改变之后我们会用InvalidateRect函数使窗口无效，会生成WM_PAINT消息，重绘窗口。