鼠标滚动

《windows程序设计》上第255页是一个关于鼠标滚轮的消息。

代码如下：

#include <Windows.h>
#include "SYSMETS.h"

LRESULT CALLBACK WndProc(HWND hwnd,UINT message,WPARAM wParam,LPARAM lParam);

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

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

	if (!RegisterClass(&wndclass))
	{
		MessageBox(hwnd,TEXT("ERROR"),TEXT("ERROR"),MB_OK);
		return 0;
	}

	hwnd=CreateWindow(szAppName,TEXT("First Program"),WS_OVERLAPPEDWINDOW|WS_HSCROLL|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,cyChar,cxCaps,cxClient,cyClient;
	static int iDeltaPerLine,iAccumDelta;
	//static int iDeltaPerLine=0;
	//int iAccumDelta;
	int iVertPos,i,x,y,iPaintBeg,iPaintEnd;
	TEXTMETRIC tm;
	SCROLLINFO si;
	HDC hdc;
	PAINTSTRUCT ps;
	ULONG ulScrollLines;
	TCHAR szBuffer[10];

	switch (message)
	{
	case WM_CREATE:
		hdc=GetDC(hwnd);

		GetTextMetrics(hdc,&tm);
		cxChar=tm.tmAveCharWidth;
		cyChar=tm.tmHeight+tm.tmExternalLeading;
		cxCaps=(tm.tmPitchAndFamily&1?3:2)*cxChar/2;

		ReleaseDC(hwnd,hdc);
	
	case WM_SETTINGCHANGE:
		SystemParametersInfo(SPI_GETWHEELSCROLLLINES,0,&ulScrollLines,0);

		if (ulScrollLines)
		{
			iDeltaPerLine=WHEEL_DELTA/ulScrollLines;
		}
		else
		{
			iDeltaPerLine=0;
		}
		return 0;

	case WM_SIZE:
		cxClient=LOWORD(lParam);
		cyClient=HIWORD(lParam);
		
		si.cbSize=sizeof(si);
		si.fMask=SIF_PAGE|SIF_RANGE;
		si.nMin=0;
		si.nMax=NUMLINES-1;
		si.nPage=cyClient/cyChar;
		SetScrollInfo(hwnd,SB_VERT,&si,TRUE);

		return 0;

	case WM_VSCROLL:
		si.cbSize=sizeof(si);
		si.fMask=SIF_ALL;
		GetScrollInfo(hwnd,SB_VERT,&si);
		iVertPos=si.nPos;

		switch (LOWORD(wParam))
		{
		case SB_TOP:
			si.nPos=si.nMin;
			break;

		case SB_BOTTOM:
			si.nPos=si.nMax;
			break;

		case SB_PAGEUP:
			si.nPos-=si.nPage;
			break;

		case SB_PAGEDOWN:
			si.nPos+=si.nPage;
			break;

		case SB_LINEUP:
			si.nPos-=1;
			break;

		case SB_LINEDOWN:
			si.nPos+=1;
			break;

		case SB_THUMBTRACK:
			si.nPos=si.nTrackPos;
			break;

		default:
			break;
		}

		SetScrollInfo(hwnd,SB_VERT,&si,TRUE);
		si.fMask=SIF_POS;
		GetScrollInfo(hwnd,SB_VERT,&si);

		if (si.nPos!=iVertPos)
		{
			ScrollWindow(hwnd,0,(iVertPos-si.nPos)*cyChar,NULL,NULL);
			UpdateWindow(hwnd);
		}
		return 0;

	case WM_KEYDOWN:
		switch (wParam)
		{
		case VK_PRIOR:
			SendMessage(hwnd,WM_VSCROLL,SB_PAGEUP,lParam);
			break;

		case VK_NEXT:
			SendMessage(hwnd,WM_VSCROLL,SB_PAGEDOWN,lParam);
			break;

		case VK_UP:
			SendMessage(hwnd,WM_VSCROLL,SB_LINEUP,lParam);
			break;

		case VK_DOWN:
			SendMessage(hwnd,WM_VSCROLL,SB_LINEDOWN,lParam);
			break;

		default:
			break;
		}
		return 0;

	case WM_MOUSEWHEEL:
		if (iDeltaPerLine==0)
		{
			break;
		}

		iAccumDelta+=(short)HIWORD(wParam); //HIWORD(wParam) is a delta value (120 or -120)

		while (iAccumDelta>=iDeltaPerLine) //iDeltaPerLine is 40,then Rolled up 3 lines
		{
			SendMessage(hwnd,WM_VSCROLL,SB_LINEUP,0);
			iAccumDelta-=iDeltaPerLine;
		}

		while (iAccumDelta<=-iDeltaPerLine)  //Rolled down 3 lines
		{
			SendMessage(hwnd,WM_VSCROLL,SB_LINEDOWN,0);
			iAccumDelta+=iDeltaPerLine;
		}

		return 0;

	case WM_PAINT:
		si.cbSize=sizeof(si);
		GetScrollInfo(hwnd,SB_VERT,&si);
		iVertPos=si.nPos;

		hdc=BeginPaint(hwnd,&ps);
		iPaintBeg=max(0,iVertPos+ps.rcPaint.top/cyChar);
		iPaintEnd=min(NUMLINES-1,iVertPos+ps.rcPaint.bottom/cyChar);

		for (i=iPaintBeg;i<=iPaintEnd;i++)
		{
			x=0;
			y=(i-iVertPos)*cyChar;

			TextOut(hdc,x,y,sysmetrics[i].szLabel,lstrlen(sysmetrics[i].szLabel));
			TextOut(hdc,x+22*cxCaps,y,sysmetrics[i].szDesc,lstrlen(sysmetrics[i].szDesc));
			SetTextAlign(hdc,TA_RIGHT|TA_TOP);
			TextOut(hdc,x+66*cxCaps,y,szBuffer,wsprintf(szBuffer,TEXT("%d"),GetSystemMetrics(sysmetrics[i].Index)));
            SetTextAlign(hdc,TA_TOP|TA_LEFT);
		}

		EndPaint(hwnd,&ps);
		return 0;

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

这段代码有一个地方极易出错！

就是在WM_CREATE消息后千万不能加return 0；

不然WM_SETTINGCHANGE消息就不会触发，那么iDeltaPerLine也就为0，那么WM_MOUSEWHEEL消息得不到执行，于是产生不了预期的效果！！