Thread: Drawing a rectangle in a control

I'm trying to draw a rectangle, like in Spy++

Code:

case WM_MOUSEMOVE:
		{
			if (bSearching) {
				POINT pMouse = {0};
				HWND hwnd;
				char szMouse[64];

				GetCursorPos(&pMouse);
				hwnd = WindowFromPoint(pMouse);
				wsprintf(szMouse, "Cursor: %i x %i", pMouse.x, pMouse.y);
				SendMessage(hStatus, SB_SETTEXT, 0, (LPARAM)szMouse);
				if (IsWindow(hwnd)) {
					RECT rc;
					HDC hdc;
					HGDIOBJ hPrevPen;
					HGDIOBJ hPrevBrush;
					HPEN hPen;

					hdc = GetWindowDC(hwnd);
					GetWindowRect(hwnd, &rc);

					hPen = CreatePen(PS_SOLID, 2, RGB(0,0,0));
					hPrevPen = SelectObject(hdc, hPen);
					hPrevBrush = SelectObject (hdc, GetStockObject(HOLLOW_BRUSH));
					Rectangle(hdc, 0, 0, rc.right-rc.left, rc.bottom-rc.top);
					SelectObject(hdc, hPrevBrush);
					SelectObject(hdc, hPrevPen);
					DeleteObject(hPen);
					ReleaseDC(hwnd, hdc);
				}
			}
			return 0;
		}

Ok, when I point to a control, the rectangle is corretly draw...but when I left that control, the rectangle stays.... How to erase that rectangle?

One possibility is to redraw the rectangle after changing the raster op code for the device context to effectively erase the first rectangle. This approach is used in msdn: How To Use Win32 API to Draw a Dragging Rectangle on Screen DC

ok...I'm still lost :\

@Ken Fitlike:

I see the code but, still confused at how implement it

The important part is the SetROP2 function which defines how drawing is done for the device context in question. Note that the possible raster operation codes are similar to those listed for Bitblt so if you have familiarity with that api function then it shouldn't be too obscure.

Essentially what you do is draw the rectangle once normally and this is what shows up. When you want to erase that visible rectangle, you set the raster operation code for the same device context to the inverse xor of what you just drew (R2_NOTXORPEN code) and redraw the rectangle which effectively removes the original.

So you'd need to introduce the SetROP2(and store its return value) call when the mouse pointer leaves the control - once to draw the inverse of the original rectangle (ie the erasing action) and once more to restore the raster operation code to its original value when you're done erasing the rectangle.

Code:

/*pseudocode*/
/*erase rectangle*/
int oldcode=SetROP2(hdc,R2_NOTXORPEN);
/*now redraw rectangle which erases original*/
Rectangle(hdc,original_left,original_top,original_right,original_bottom);
/*restore device context*/
SetROP2(hdc,oldcode);

ah..ok...I think I got it...now I have another issue:

Code:

case WM_MOUSEMOVE:
		{
			if (bnowDraw) {
				HDC hdc;
				hdc = GetDC(hWnd);
				MoveToEx(hdc, pCursorPos.x, pCursorPos.y, NULL);
				pCursorPos = MAKEPOINTS(lParam);
				LineTo(hdc, pCursorPos.x, pCursorPos.y);
				ReleaseDC(hWnd, hdc);
			}
			return 0;
		}

The line is correctly, but when I minimized the window...the line is gone...any ideas why or how can I draw it without been erase?

Which window? Your own or another application's? If it's your own then you should move drawing code to the WM_PAINT handler and just InvalidateRect if/as necessary. If it's another application's window then maybe you should periodically check the activation state of the current target window and redraw it as required; a timer (see SetTimer, KillTimer, WM_TIMER) may be an option here.