执行效果为什么不同？

[World7th]

#include <windows.h>
LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM) ;
int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
PSTR szCmdLine, int iCmdShow)
{
static TCHAR szAppName[] = TEXT ("BlokOut2") ;
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 ("Program requires Windows NT!"),
szAppName, MB_ICONERROR) ;
return 0 ;
}

hwnd = CreateWindow (szAppName, TEXT ("Mouse Button & Capture Demo"),
WS_OVERLAPPEDWINDOW,
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 ;
}

void DrawBoxOutline (HWND hwnd, POINT ptBeg, POINT ptEnd)
{
HDC hdc ;
hdc = GetDC (hwnd) ;
SetROP2 (hdc, R2_NOT) ;
SelectObject (hdc, GetStockObject (NULL_BRUSH)) ;
Rectangle (hdc, ptBeg.x, ptBeg.y, ptEnd.x, ptEnd.y) ;

ReleaseDC (hwnd, hdc) ;
}

LRESULT CALLBACK WndProc ( HWND hwnd, UINT message, WPARAM wParam,LPARAM lParam)
{
static BOOL fBlocking, fValidBox ;
static POINT ptBeg, ptEnd, ptBoxBeg, ptBoxEnd ;
HDC hdc ;
PAINTSTRUCT ps ;

switch (message)
{
case WM_LBUTTONDOWN :
ptBeg.x = ptEnd.x = LOWORD (lParam) ;
ptBeg.y = ptEnd.y = HIWORD (lParam) ;

DrawBoxOutline (hwnd, ptBeg, ptEnd) ;

SetCapture (hwnd) ;
SetCursor (LoadCursor (NULL, IDC_CROSS)) ;

fBlocking = TRUE ;
return 0 ;

case WM_MOUSEMOVE :
if (fBlocking)
{
SetCursor (LoadCursor (NULL, IDC_CROSS)) ;

DrawBoxOutline (hwnd, ptBeg, ptEnd) ;

ptEnd.x = LOWORD (lParam) ;
ptEnd.y = HIWORD (lParam) ;

DrawBoxOutline (hwnd, ptBeg, ptEnd) ;
}
return 0 ;

case WM_LBUTTONUP :
if (fBlocking)
{
DrawBoxOutline (hwnd, ptBeg, ptEnd) ;

ptBoxBeg = ptBeg ;
ptBoxEnd.x = LOWORD (lParam) ;
ptBoxEnd.y = HIWORD (lParam) ;

ReleaseCapture () ;
SetCursor (LoadCursor (NULL, IDC_ARROW)) ;

fBlocking = FALSE ;
fValidBox = TRUE ;

InvalidateRect (hwnd, NULL, TRUE) ;
}
return 0 ;

case WM_CHAR :
if (fBlocking & wParam == '\x1B') // i.e., Escape
{
DrawBoxOutline (hwnd, ptBeg, ptEnd) ;
ReleaseCapture () ;
SetCursor (LoadCursor (NULL, IDC_ARROW)) ;

fBlocking = FALSE ;
}
return 0 ;

case WM_PAINT :
hdc = BeginPaint (hwnd, &ps) ;

if (fValidBox)
{
SelectObject (hdc, GetStockObject (BLACK_BRUSH)) ;
Rectangle (hdc, ptBoxBeg.x, ptBoxBeg.y,
ptBoxEnd.x, ptBoxEnd.y) ;
}

if (fBlocking)
{
SetROP2 (hdc, R2_NOT) ;
SelectObject (hdc, GetStockObject (NULL_BRUSH)) ;
Rectangle (hdc, ptBeg.x, ptBeg.y, ptEnd.x, ptEnd.y) ;
}

EndPaint (hwnd, &ps) ;
return 0 ;

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



在WM_MOUSEMOVE消息中，为什么必须出现两个DrawBoxOutline函数？为什么不能只含有第二个DrawBoxOutline函数。比如：

case WM_MOUSEMOVE :

if (fBlocking)
{
SetCursor (LoadCursor (NULL, IDC_CROSS)) ;

ptEnd.x = LOWORD (lParam) ;
ptEnd.y = HIWORD (lParam) ;

DrawBoxOutline (hwnd, ptBeg, ptEnd) ;
}

return 0 ;
大家把这段程序调试一下，解决我的问题，可以么？谢谢了

[galois_godel]

这样就可以了

#include <windows.h>
LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM) ;
int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
PSTR szCmdLine, int iCmdShow)
{
static TCHAR szAppName[] = TEXT ("BlokOut2") ;
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 ("Program requires Windows NT!"),
szAppName, MB_ICONERROR) ;
return 0 ;
}

hwnd = CreateWindow (szAppName, TEXT ("Mouse Button & Capture Demo"),
WS_OVERLAPPEDWINDOW,
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 ;
}

void DrawBoxOutline (HWND hwnd, POINT ptBeg, POINT ptEnd)
{
HDC hdc ;
hdc = GetDC (hwnd) ;
SetROP2 (hdc, R2_NOT) ;
SelectObject (hdc, GetStockObject (NULL_BRUSH)) ;
Rectangle (hdc, ptBeg.x, ptBeg.y, ptEnd.x, ptEnd.y) ;

ReleaseDC (hwnd, hdc) ;
}

LRESULT CALLBACK WndProc ( HWND hwnd, UINT message, WPARAM wParam,LPARAM lParam)
{
static BOOL fBlocking, fValidBox ;
static POINT ptBeg, ptEnd, ptBoxBeg, ptBoxEnd ;
HDC hdc ;
PAINTSTRUCT ps ;

switch (message)
{
case WM_LBUTTONDOWN :
ptBeg.x = ptEnd.x = LOWORD (lParam) ;
ptBeg.y = ptEnd.y = HIWORD (lParam) ;

DrawBoxOutline (hwnd, ptBeg, ptEnd) ;

SetCapture (hwnd) ;
SetCursor (LoadCursor (NULL, IDC_CROSS)) ;

fBlocking = TRUE ;
return 0 ;

case WM_MOUSEMOVE :
if (fBlocking)
{
SetCursor (LoadCursor (NULL, IDC_CROSS)) ;

// DrawBoxOutline (hwnd, ptBeg, ptEnd) ;

ptEnd.x = LOWORD (lParam) ;
ptEnd.y = HIWORD (lParam) ;

DrawBoxOutline (hwnd, ptBeg, ptEnd) ;
InvalidateRect (hwnd, NULL, TRUE) ;
}
return 0 ;

case WM_LBUTTONUP :
if (fBlocking)
{
DrawBoxOutline (hwnd, ptBeg, ptEnd) ;

ptBoxBeg = ptBeg ;
ptBoxEnd.x = LOWORD (lParam) ;
ptBoxEnd.y = HIWORD (lParam) ;

ReleaseCapture () ;
SetCursor (LoadCursor (NULL, IDC_ARROW)) ;

fBlocking = FALSE ;
fValidBox = TRUE ;

InvalidateRect (hwnd, NULL, TRUE) ;
}
return 0 ;

case WM_CHAR :
if (fBlocking & wParam == '\x1B') // i.e., Escape
{
DrawBoxOutline (hwnd, ptBeg, ptEnd) ;
ReleaseCapture () ;
SetCursor (LoadCursor (NULL, IDC_ARROW)) ;

fBlocking = FALSE ;
}
return 0 ;

case WM_PAINT :
hdc = BeginPaint (hwnd, &ps) ;

if (fValidBox)
{
SelectObject (hdc, GetStockObject (BLACK_BRUSH)) ;
Rectangle (hdc, ptBoxBeg.x, ptBoxBeg.y,
ptBoxEnd.x, ptBoxEnd.y) ;
}

if (fBlocking)
{
SetROP2 (hdc, R2_NOT) ;
SelectObject (hdc, GetStockObject (NULL_BRUSH)) ;
Rectangle (hdc, ptBeg.x, ptBeg.y, ptEnd.x, ptEnd.y) ;
}

EndPaint (hwnd, &ps) ;
return 0 ;

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

[galois_godel]

加句InvalidateRect (hwnd, NULL, TRUE) ;

重画下界面

[galois_godel]

你第一个DrawBoxOutline (hwnd, ptBeg, ptEnd) 起的作用只是把原先画的图掉