问题描述

vs中编写了个关于垂直滚动条的代码，其中有几处不理解，求解答

这是代码：
#include
#include"resource.h"
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
static char szAppName[] = "SysMets2";
HWND hwnd;
MSG msg;
WNDCLASS wndclass;
if (!hPrevInstance)
{
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;
RegisterClass(&wndclass);

}
hwnd = CreateWindow(szAppName, "Get System Metrics No.2",
    WS_OVERLAPPEDWINDOW |WS_VSCROLL,
    CW_USEDEFAULT, CW_USEDEFAULT,
    CW_USEDEFAULT, CW_USEDEFAULT,
    NULL, NULL, hInstance, NULL
    );

ShowWindow(hwnd, nCmdShow);
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 short cxChar, cxCaps, cyChar, cxClient, cyClient, nVscrollPos;
char szBuffer[10];
HDC hdc;
short i, y;
PAINTSTRUCT ps;
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, FALSE);
SetScrollPos(hwnd, SB_VERT, nVscrollPos, TRUE);
return 0;
case WM_SIZE:
cxClient = LOWORD(lParam);
cyClient = HIWORD(lParam);
return 0;
case WM_VSCROLL:
switch (wParam)
{
case SB_LINEUP:
nVscrollPos -= 1;
break;
case SB_LINEDOWN:
nVscrollPos += 1;
break;
case SB_PAGEDOWN:
nVscrollPos -= cyClient / cyChar;
break;
case SB_PAGEUP:
nVscrollPos -= cyClient / cyChar;
break;
default:
break;
}
nVscrollPos = max(0, min(nVscrollPos, NUMLINES));
if (nVscrollPos!=GetScrollPos(hwnd,SB_VERT))
{
SetScrollPos(hwnd, SB_VERT, nVscrollPos, TRUE);
InvalidateRect(hwnd, NULL, TRUE);
}
return 0;
case WM_PAINT:
hdc = BeginPaint(hwnd, &ps);
for (i=0; i<NUMLINES; i++)
{
y = cyChar*(1-nVscrollPos+i);
TextOut(hdc, cxChar, y, sysmetrics[i].szLabel, lstrlen(sysmetrics[i].szLabel));
TextOut(hdc, cxChar+18*cxCaps, y, sysmetrics[i].szDesc, lstrlen(sysmetrics[i].szDesc));
SetTextAlign(hdc, TA_RIGHT | TA_TOP);
TextOut(hdc, cxChar+18*cxCaps+40*cxChar, y,szBuffer,wsprintf(szBuffer,"%5d",GetSystemMetrics(sysmetrics[i].nIndex)));
SetTextAlign(hdc, TA_LEFT | TA_TOP);

    }
    EndPaint(hwnd, &ps);

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

}

其中 cxCaps = (tm.tmPitchAndFamily & 1 ? 3 : 2)*cxChar / 2;
还有case SB_THUMBPOSITION:
nVscrollPos = LOWORD(lParam);
break;
还有 if (nVscrollPos!=GetScrollPos(hwnd,SB_VERT))
{
SetScrollPos(hwnd, SB_VERT, nVscrollPos, TRUE);
InvalidateRect(hwnd, NULL, TRUE);
}
还有 nVscrollPos = max(0, min(nVscrollPos, NUMLINES));
这些到底是什么意思呢？ 有点长请见谅。