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I made a TGraphicControl descendent which does fast overlayed drawings, keeping track of its own update regions, and clipping updates to those.
Problem is, if the form which the component is on gets hidden by some other window, it needs a full repaint when it is shown again. I don't have a clue how to intercept this situation on the component level.
There is the WinAPI GetUpdateRgn, but help on it says it's always empty as soon as BeginPaint is called, but if WM_PAINT reaches my TGraphicControl, BeginPaint has been called by its parent. Otherwise I could always just add what Windows thinks is the update region.
How would you solve this? Should I just derive from a TCustomControl instead?

## Answer 1

I had similar problems once and did an extra check on Canvas.ClipRect in the Paint procedure. When updating the component when only parts of the component needed to be redrawn, I used:

    { ... }
    var
      R: TRect;
    begin
      if FValue <> NewValue then
      begin
        FValue := NewValue;
        R := GetClientRect;
        InflateRect(R, - 1, - 1);
        InvalidateRect(Handle, @R, not (csOpaque in ControlStyle));
      end;
    end;

Now I could check for this rectangle in the Paint procedure. When the ClipRect was not the same as I expected it to be (eg caused by a form being moved which covered it partly or completely before the move) a full redraw was needed and performed.

    { ... }
    with Canvas do
      if (((ClipRect.Right - ClipRect.Left) <> (Width - 2)) and
          ((ClipRect.Bottom - ClipRect.Top) <> (Height - 2)))  then
        DoDrawAll := True
      else
        {only update what needs to be updated.}
    { ... }

When a full redraw was not required the rest of the paint routine knew what to update by other means and only updated these parts.

The component in which I used this was a descendent of TCustomPanel. I'm not sure if ClipRect is setup the same for a TGraphicControl but I guess so.

Tip by Pieter Zijlstra

## Answer 2

Thank you very much Pieter, it works! This is how I implemented it for the time being: In WM_WINDOWPOSCHANGED I assign a private field fTestRect like you do, but with

    fTestRect := BoundsRect;
    InflateRect(fTestRect,- 1,- 1);

Then I can override TControl.Repaint, it's natural for a user to call repaint on an animation step:

    { ... }
      if fUseRegions then
        if assigned(parent) then
          InvalidateRect(parent.handle, @fTestRect, False)
        else
          inherited
          else
            inherited;
    { ... }

fUseRegions is the flag the user can set to use those update regions for speedup. Then in Paint, I do as you do, and only really use the update regions if Canvas.ClipRect is the analogue of fTestRect. I can't see a slowdown. Of course, chances are that a window pops up on the control which has exactly the dimensions of fTestRect, but I think that chance is rather slim.

Tip by Renate Schaaf