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Drag & Drop in a CScrollView

Posted on 1998-07-04
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Last Modified: 2013-11-20
I'm implementing my own drag and drop of bitmaps in a CScrollView window. Right now I redraw the entire view for each movement, but this is not a good solution since I'm going to have quite a lot of graphics in this window. What is the most efficient way to do this? Can I somehow read a section of the view into memory, draw my bitmap there, and the next time redraw only that section and so on.
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Question by:zingo
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eugenem earned 100 total points
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First thing you need is clipping. This means you need to repaint only what you need, not the whole view. For example, when you drag a bitmap, you take previous rectangle of the bitmap and current rectangle of bitmap and compute the smallest rectangle that contains both of these. Then you set this rectangle as a clipping rectangle and draw your view. It will automatically prevent any output to any place of your DC outside this rectangle.
To set clipping rectangle you can use
void CWnd::InvalidateRect( LPCRECT lpRect, BOOL bErase = TRUE );
If you want to get this rectangle from OnPaint, use
CPaintDC dc( this );
CRect rectUpdate;
dc.GetClipBox(&rectUpdate);

If you want some more serious clipping you can use CDC::SelectClipRgn. You should also look for CRgn.

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