Allow user to draw a drag rectangle in CStatic C++ MFC App

Question

I have a MFC application where I have a Picture Control in the dialog. Eventually, I want to allow a user to draw a resizeable rectangle via mouse drag in the picture control over an image that I loaded.

I defined my own picture control class as a sub class of CStatic and am working with the mouse down, mouse up, and mouse move events but I can't seem to figure out how to allow the user to draw a rectangle. Any guidance on this would be appreciated.

Most of the examples I've looked at show me how to draw a rectangle in a CView:CWnd, but I'm not too familiar with MFC yet so I'm a bit lost. Thanks.

Answer 1

The usual technique for drawing a drag rect on top of the window contents is illustrated here:

http://msdn.microsoft.com/en-us/library/windows/desktop/dd145184(v=vs.85).aspx

That is Win32 API coding instead of MFC coding but the differences are minimal. The basic idea is that by drawing with SetROP2(hdc, R2_NOTXORPEN); you invert the existing pixels, then drawing the same rect again re-inverts those pixels back to the original image.

Answer 2

When the user clicks the mouse button you need to record the mouse coordinates so you know where the rectangle starts. You should also set some type of flag to indicate that the user is dragging the mouse. When the user moves the mouse get the current mouse position and use DrawDragRect or similar function to draw the rectangle. When the user releases the mouse button clear the previously mentioned "flag" and you're done with that part of the process.

You will also need to handle other events such as the control and/or parent window losing focus so that you can cancel the drag/draw operation. Since you did not include any code in your question it's hard to say what else you will need to do but those are the basics.