《Windows程序设计》的一个问题

《Windows程序设计》里面有一个小节：建立更好的滚动，其中的一个例程的代码是：

#include "stdafx.h"

#include "SYSMETS1.h"



LRESULT CALLBACK WndProc(HWND hWnd,UINT msg, WPARAM wp,LPARAM lp);



int WINAPI WinMain(__in HINSTANCE hInstance, __in_opt HINSTANCE hPrevInstance, __in_opt LPSTR lpCmdLine, __in int nShowCmd)

{

	static TCHAR szAppName[] = _T("SysMets1");

	HWND hwnd;

	MSG msg;

    WNDCLASS wndclass;

	wndclass.style = CS_HREDRAW|CS_VREDRAW;

    wndclass.lpfnWndProc = WndProc;

	wndclass.cbClsExtra = 0;

	wndclass.cbWndExtra = 0;

	wndclass.hInstance = hInstance;

	// load the icon of application

	wndclass.hIcon = ::LoadIcon(NULL,IDI_APPLICATION);

	// load the cursor of application

	wndclass.hCursor = ::LoadCursor(NULL,IDC_ARROW);

	wndclass.hbrBackground = (HBRUSH)GetStockObject(GRAY_BRUSH);

	wndclass.lpszMenuName = NULL;

	wndclass.lpszClassName = szAppName;

	// register the window class 

	if (!::RegisterClass(&wndclass))

	{

		::MessageBox(NULL,TEXT("This program require Window NT"),szAppName,MB_ICONERROR);

		return 0;

	}

	// create really window

	hwnd = ::CreateWindow(szAppName,_T("Get System Metric No.1"),WS_OVERLAPPEDWINDOW,CW_USEDEFAULT,CW_USEDEFAULT,

		CW_USEDEFAULT,CW_USEDEFAULT,NULL,NULL,hInstance,NULL);

	::ShowWindow(hwnd,nShowCmd);

	::UpdateWindow(hwnd); // immedially post WM_PAINT message

	// when get the meaages,translte the message and dispatch message.

	while (::GetMessage(&msg,NULL,0,0))

	{

		TranslateMessage(&msg);

		DispatchMessage(&msg);

	}

	return msg.wParam;

}



LRESULT CALLBACK WndProc(HWND hWnd,UINT message, WPARAM wp,LPARAM lp)

{

	static int cxChar;

    static int cxCaps;

	static int cyChar;

	static int cyClient; // the height of client

	static int cxClient; // the width of client

	static int nPaintBeg;

	static int nPaintEnd;

	static int nMaxWidth;

	int nVertPos = 0;

	int nHorzPos = 0;

	SCROLLINFO si;

	HDC hdc = NULL;

	PAINTSTRUCT ps;

	TCHAR szBuffer[10];

	TEXTMETRIC tm;

	switch (message)

	{

	case WM_CREATE:

		{

			hdc = ::GetDC(hWnd);

			::GetTextMetrics(hdc,&tm); // get text size of system

			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,nVscrollPos,TRUE);

			// save the width of three columns

			nMaxWidth = 40*cxChar+22*cxCaps;

			return 0;

		}

	case WM_SIZE:

		{

//			cyClient = HIWORD(lp); // get the height of client

			// save the width and height of window when changed the size of window

			cxClient = LOWORD(lp);

			cyClient = HIWORD(lp);

            // set vertical scroll bar range and page size

			si.cbSize = sizeof(SCROLLBARINFO);

			si.fMask = SIF_RANGE|SIF_PAGE;

            si.nMin = 0;

            si.nMax = NUMLINES - 1;

			si.nPage = cyClient/cyChar;

            SetScrollInfo(hWnd,SB_VERT,&si,TRUE);

            // set horizontal scroll bar and page size

			si.cbSize = sizeof(SCROLLBARINFO);

			si.fMask = SIF_RANGE|SIF_PAGE;

			si.nMin = 0;

			si.nMax = 2 + nMaxWidth/cxChar;

			si.nPage = cxClient/cxChar;

			SetScrollInfo(hWnd,SB_HORZ,&si,TRUE);

			return 0;

		}

	case WM_VSCROLL:

		{

	// get all vertical scroll bar information

	si.cbSize = sizeof(SCROLLINFO);

	si.fMask = SIF_ALL;

	::GetScrollInfo(hWnd,SB_VERT,&si);

    // save the position for comparison later on

	nVertPos = si.nPos;

     switch (LOWORD(wp))

            {

			case SB_LINEUP:

				{

					si.nPos -=1;// the height decrease 1 unit

					break;

				}

			case SB_LINEDOWN:

				{

					si.nPos +=1;// the height increase 1 unit

					break;

				}

			case SB_PAGEUP:

				{

                    // back to prev page, the cyClient/cyChar is the number of row in one page

					si.nPos -= cyClient/cyChar; 

					break;

				}

			case SB_PAGEDOWN:

				{

					// back to next page

					si.nPos += cyClient/cyChar;

					break;

				}

			case SB_THUMBPOSITION:

				{

					si.nPos = HIWORD(wp);

					break;

				}

			default:

				break;

            }

            // set the position and then retrieve it.Due to adjustments

	        // by Windows it may not be the same as the value set.

	        si.fMask = SIF_POS;

			SetScrollInfo(hWnd,SB_VERT,&si,TRUE);

			GetScrollInfo(hWnd,SB_VERT,&si);

			//if the position has changed,scroll the window update it

			if (si.nPos!=nVertPos)

			{

				::ScrollWindow(hWnd,0,cyChar*(nVertPos-si.nPos),NULL,NULL);

				::UpdateWindow(hWnd);

			}

			return 0;

		}

	case WM_HSCROLL:

		{

			// get all the vertical scroll bar information

			si.cbSize = sizeof(si);

			si.fMask = SIF_ALL;

			// save the position for comparison later on

			::GetScrollInfo(hWnd,SB_HORZ,&si);

			nHorzPos = si.nPos;

			switch (LOWORD(wp))

			{

			case SB_LINELEFT:

				{

					si.nPos -=1;

					break;

				}

			case SB_LINERIGHT:

				{

					si.nPos +=1;

					break;

				}

			case SB_PAGELEFT:

				{

					si.nPos -= si.nPage;

					break;

				}

			case SB_PAGERIGHT:

				{

					si.nPos += si.nPage;

					break;

				}

			case SB_THUMBPOSITION:

				{

					si.nPos = si.nTrackPos;

					break;

				}

			default:

                break;

			}

			// set the position and then retrieve it.due to adjustments

			// by windows it may not be the same as the value set

			si.fMask = SIF_POS;

			::SetScrollInfo(hWnd,SB_HORZ,&si,TRUE);

			::GetScrollInfo(hWnd,SB_HORZ,&si);

			// if the postion has changed ,scroll the window

			if (si.nPos!=nHorzPos)

			{

				// why (nHorzPos-si.nPos)

				::ScrollWindow(hWnd,cxChar*(nHorzPos-si.nPos),0,NULL,NULL);

			}

			return 0;

		}

	case WM_PAINT:

		{

			hdc = ::BeginPaint(hWnd,&ps);

            // get vertical scroll bar position

			si.cbSize = sizeof(si);

            si.fMask = SIF_POS;

			::GetScrollInfo(hWnd,SB_VERT,&si);

			nVertPos = si.nPos;

			// get horizontal scroll bar position

            GetScrollInfo(hWnd,SB_HORZ,&si);

            nHorzPos = si.nPos;

			// find painting limits 

            int nPaintBeg = max(0,nVertPos+ps.rcPaint.top/cyChar); // the begin row

            int nPaintEnd = min(NUMLINES-1,nVertPos+ps.rcPaint.bottom/cyChar); // the end row



			for (int i =nPaintBeg;i<=nPaintEnd;i++)

			{

                // calculate the y position of draw region, when y position less 0,skip

				int x = cxChar*(1-nHorzPos); // why is 1-nHorzPos

				int y = cyChar*(i-nVertPos);



				::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+22*cxCaps+40*cxChar,y,szBuffer,wsprintf(szBuffer,_T("%5d"),::GetSystemMetrics(sysmetrics[i].Index)));

				::SetTextAlign(hdc,TA_LEFT|TA_TOP);

			}

			::EndPaint(hWnd,&ps);

			return 0;

		}

	case WM_DESTROY:

		// close the application

		::PostQuitMessage(0);

		return 0;

	default:

		break;

	}

	// default process

	return ::DefWindowProc(hWnd,message,wp,lp);

}

为什么这里是

if (si.nPos!=nHorzPos)

			{

				// why (nHorzPos-si.nPos)

				::ScrollWindow(hWnd,cxChar*(nHorzPos-si.nPos),0,NULL,NULL);

			}

我查了MSDN：
BOOL ScrollWindow( HWND hWnd,
int XAmount,
int YAmount,
const RECT *lpRect,
const RECT *lpClipRect
);
Parameters

hWnd
[in] Handle to the window where the client area is to be scrolled.
XAmount
[in] Specifies the amount, in device units, of horizontal scrolling. If the window being scrolled has the CS_OWNDC or CS_CLASSDC style, then this parameter uses logical units rather than device units. This parameter must be a negative value to scroll the content of the window to the left.
YAmount
[in] Specifies the amount, in device units, of vertical scrolling. If the window being scrolled has the CS_OWNDC or CS_CLASSDC style, then this parameter uses logical units rather than device units. This parameter must be a negative value to scroll the content of the window up.
lpRect
[in] Pointer to the RECT structure specifying the portion of the client area to be scrolled. If this parameter is NULL, the entire client area is scrolled.
lpClipRect
[in] Pointer to the RECT structure containing the coordinates of the clipping rectangle. Only device bits within the clipping rectangle are affected. Bits scrolled from the outside of the rectangle to the inside are painted; bits scrolled from the inside of the rectangle to the outside are not painted.
Return Value

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero. To get extended error information, call GetLastError.

这里nHorzPos指旧的滚动条位置，si.nPos为新的滚动条位置，那么::ScrollWindow(hWnd,cxChar*(nHorzPos-si.nPos),0,NULL,NULL);为什么不是::ScrollWindow(hWnd,cxChar*(si.nPos-nHorzPos),0,NULL,NULL);，而是::ScrollWindow(hWnd,cxChar*(nHorzPos-si.nPos),0,NULL,NULL);呢？MSDN上说XAmount
[in] Specifies the amount, in device units, of horizontal scrolling. If the window being scrolled has the CS_OWNDC or CS_CLASSDC style, then this parameter uses logical units rather than device units. This parameter must be a negative value to scroll the content of the window to the left.

就是向左移动必须是一个负值。向左移动nHorzPos必定大于si.nPos，cxChar为单位字符的偏移量，是一个正值，nHorzPos大于si.nPos，nHorzPos-si.nPos也必定是一个正值，这岂不是和MSDN冲突。但是我把代码改为::ScrollWindow(hWnd,cxChar*(si.nPos-nHorzPos),0,NULL,NULL);却是不对的。