如何制作“拖动窗口时显示其内容”的窗口

用SetCapture捕捉鼠标，GetCursorPos取鼠标位置，SetWindowPos设置窗口位置啊
    POINT piXY;
    GetCursorPos((LPPOINT)&piXY);
    piXY.x-=rWindRect.left;
    piXY.y-=rWindRect.top;
    SetCursor(LoadCursor(NULL,IDC_SIZEALL));
    SetCapture(hWnd);
    while(GetCapture()==hWnd){
     if(!GetMessage(&msg,NULL,0,0)){
      PostQuitMessage(msg.wParam);
      break;}
     switch(msg.message){
      case WM_LBUTTONUP:
       ReleaseCapture();
       SetCursor(LoadCursor(NULL,IDC_ARROW));
       break;
      case WM_MOUSEMOVE:
       GetCursorPos((LPPOINT)&point);
       uiWindowPosX=point.x-piXY.x;
       uiWindowPosY=point.y-piXY.y;
       SetWindowPos(hWnd,NULL,point.x-piXY.x,point.y-piXY.y,228,22,SWP_SHOWWINDOW);
       break;
      default:
       DispatchMessage(&msg);
       break;}}