请教用双缓冲区画动态直线

Question

我是这样写的，可是用了缓冲区后，就什么也不能画了
各位帮忙看看LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, 
LPARAM lParam)
{
 static HINSTANCE    hInst;
     static int          cxClient, cyClient, cxScreen, cyScreen;
     HDC                 hdc;
 static POINT        pt,lp;
 PAINTSTRUCT         ps ;
 HPEN                hpen;
 static int          bDown=0;
 
 static HBITMAP      hBitmap;
//  static SIZE  size;
 RECT rect;
 
 switch (message)
     {
 case WM_CREATE:
  GetClientRect(hwnd,&rect);
  hdc = GetDC(hwnd);
  hdcMem1 = CreateCompatibleDC(hdc);
  hBitMap1 = CreateCompatibleBitmap(hdc,rect.right-rect.left,rect.bottom-rect.top);
  SelectObject(hdcMem1,hBitMap1);
  hdcMem2 = CreateCompatibleDC(hdc);
  hBitMap2 = CreateCompatibleBitmap(hdc,hdc,rect.right-rect.left,rect.bottom-rect.top);
  SelectObject(hdcMem2,hBitMap2);
  hBitMap = CreateCompatibleBitmap(hdc,hdc,rect.right-rect.left,rect.bottom-rect.top);
  SelectObject(hdc,hBitMap);
  BitBlt(hdcMem1,0,0,0,0,hdc,0,0,SRCCOPY);
  BitBlt(hdcMem2,0,0,0,0,hdc,0,0,SRCCOPY);
  ReleaseDC(hwnd,hdc);
  return 0;  case WM_PAINT:
  hdc=BeginPaint(hwnd,&ps);
  BitBlt(hdc,0,0,0,0,hdcMem1,0,0,SRCCOPY);
  EndPaint(hwnd,&ps);
  return 0;
  
 case WM_LBUTTONDOWN:
  bDown = 1;
  pt.x = LOWORD(lParam);
  pt.y = HIWORD(lParam);
  
  return 0;
 
/*  case WM_SIZE:
  size.cx=LOWORD(lParam);
  size.cy=HIWORD(lParam);
  return 0;*/     case WM_LBUTTONUP:
  bDown = 0;   return 0;
 
 case WM_MOUSEMOVE:  // 鼠标移动
  if(bDown==1)
  {
  BitBlt(hdcMem1,0,0,0,0,hdcMem2,0,0,SRCCOPY);
  lp.x = LOWORD(lParam);
  lp.y = HIWORD(lParam);//   hdc = GetDC(hwnd);
  
  hpen=CreatePen(PS_SOLID, 5, RGB(255, 0, 0));
  SelectObject(hdcMem1,hpen);
  
  MoveToEx(hdcMem1,pt.x,pt.y,NULL);
  LineTo(hdcMem1,lp.x,lp.y);   
//   BitBlt(hdcMem2,0,0,size.cx,size.cy,hdcMem1,0,0,SRCCOPY);   InvalidateRect(hwnd, NULL, FALSE);
  
  }
//   InvalidateRect(hwnd, NULL, TRUE);
  return 0;
          
     case WM_DESTROY:
  
          PostQuitMessage (0) ;
          return 0 ;
     }
     return DefWindowProc (hwnd, message, wParam, lParam) ;
}

Answer 1

//我改了一下，有效果了，你看看是不是你想要的？
LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, 
  LPARAM lParam)
{
static HDC hdcMem1,hdcMem2;
static HBITMAP hBitMap,hBitMap1,hBitMap2;
static SIZE size;
// 上面的变量是我添加的，是不是原来的都是全局变量？
static HINSTANCE    hInst;
static int          cxClient, cyClient, cxScreen, cyScreen;
HDC                 hdc;
static POINT        pt,lp;
PAINTSTRUCT         ps ;
HPEN                hpen;
static int          bDown=0; static HBITMAP      hBitmap;
//  static SIZE  size;
RECT rect; switch (message)
{
case WM_CREATE:
GetClientRect(hwnd,&rect);
hdc = GetDC(hwnd);
hdcMem1 = CreateCompatibleDC(hdc);
hBitMap1 = CreateCompatibleBitmap(hdc,rect.right-rect.left,rect.bottom-rect.top);
SelectObject(hdcMem1,hBitMap1);
hdcMem2 = CreateCompatibleDC(hdc);
hBitMap2 = CreateCompatibleBitmap(hdc,rect.right-rect.left,rect.bottom-rect.top);
SelectObject(hdcMem2,hBitMap2);
hBitMap = CreateCompatibleBitmap(hdc,rect.right-rect.left,rect.bottom-rect.top);
SelectObject(hdc,hBitMap);
BitBlt(hdcMem1,0,0,0,0,hdc,0,0,SRCCOPY);
BitBlt(hdcMem2,0,0,0,0,hdc,0,0,SRCCOPY);
ReleaseDC(hwnd,hdc);
return 0; case WM_PAINT:
hdc=BeginPaint(hwnd,&ps);
                  // 这一句，参数中宽和高指定的都是0，我想错误的关键原因在这一句
// 现在cx,xy中存储的是客户区的宽和高
BitBlt(hdc,0,0,size.cx,size.cy,hdcMem1,0,0,SRCCOPY);
EndPaint(hwnd,&ps);
return 0; case WM_LBUTTONDOWN:
bDown = 1;
pt.x = LOWORD(lParam);
pt.y = HIWORD(lParam);
return 0;  case WM_SIZE:
// 这里的注释去掉了。
size.cx=LOWORD(lParam);
size.cy=HIWORD(lParam);
return 0; case WM_LBUTTONUP:
bDown = 0;
return 0; case WM_MOUSEMOVE:  // 鼠标移动
if(bDown==1)
{
BitBlt(hdcMem1,0,0,0,0,hdcMem2,0,0,SRCCOPY);
lp.x = LOWORD(lParam);
lp.y = HIWORD(lParam); //   hdc = GetDC(hwnd); hpen=CreatePen(PS_SOLID, 5, RGB(255, 0, 0));
SelectObject(hdcMem1,hpen); MoveToEx(hdcMem1,pt.x,pt.y,NULL);
LineTo(hdcMem1,lp.x,lp.y);
  BitBlt(hdcMem2,0,0,size.cx,size.cy,hdcMem1,0,0,SRCCOPY);
// 这里的注释也去掉了。
InvalidateRect(hwnd, NULL, FALSE); }
//   InvalidateRect(hwnd, NULL, TRUE);
return 0; case WM_DESTROY: PostQuitMessage (0) ;
return 0 ;
}
return DefWindowProc (hwnd, message, wParam, lParam) ;
}