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copied!<p>Detecting creation is easy. Just override <a href="http://docwiki.embarcadero.com/VCL/en/Graphics.TCanvas.CreateHandle" rel="nofollow noreferrer"><code>CreateHandle</code></a> in a descendant <code>TControlCanvas</code> and put yours in place of the default one <a href="https://stackoverflow.com/questions/1615984/delphi-gdi-when-is-a-device-context-created-destroyed/1618742#1618742">as Jeroen's answer demonstrates</a>. Detecting destruction is harder.</p> <p>One way to avoid the issue is to check whether the TGpGraphics handle is equal to the paint-box's handle, so, rather than detect the moment when the device context is freed, you simply check before you need to know.</p> <pre><code>if not Assigned(FGraphics) or (FGraphics.GetHDC <> PaintBox1.Canvas.Handle) then begin FGraphics.Free; FGraphics := TGpGraphics.Create(PaintBox1.Canvas.Handle); end; </code></pre> <p>This probably isn't reliable, though; handle values are liable to be re-used, so although the HDC value might be the same between two checks, there's no guarantee that it still refers to the same OS device-context object.</p> <hr> <p>The <code>TCanvas</code> base class never clears its own <code>Handle</code> property, so anything that invalidates the canvas must occur externally. <a href="http://docwiki.embarcadero.com/VCL/en/Controls.TControlCanvas" rel="nofollow noreferrer"><code>TControlCanvas</code></a> clears its <code>Handle</code> property when its <code>Control</code> property gets re-assigned, but that usually only happens when the control is created since <code>TControlCanvas</code> instances are rarely shared. However, <code>TControlCanvas</code> instances work from a pool of device-context handles kept in <code>CanvasList</code>. Whenever one of them needs a DC (in <code>TControlCanvas.CreateHandle</code>), it calls <code>FreeDeviceContext</code> to make room in the canvas cache for the handle it's about to create. That function calls the (non-virtual) <code>FreeHandle</code> method. The cache size is 4 (see <code>CanvasListCacheSize</code>), so if you have several descendants of <code>TCustomControl</code> or <code>TGraphicControl</code> in your program, chances are high that you'll get cache misses whenever more than four of them need to be repainted at once.</p> <p><a href="http://docwiki.embarcadero.com/VCL/en/Controls.TControlCanvas.FreeHandle" rel="nofollow noreferrer"><code>TControlCanvas.FreeHandle</code></a> is not virtual, and it doesn't call any virtual methods. Although you could make a descendant of that class and give it virtual methods, the rest of the VCL is going to continue calling the non-virtual methods, oblivious to any of your additions.</p> <hr> <p>Instead of trying to detect when a device context is released, you might be better off using a different TGpGraphics constructor. Use <a href="http://msdn.microsoft.com/en-us/library/ms536162.aspx" rel="nofollow noreferrer">the one that takes a window handle</a> instead of a DC handle, for instance. Window-handle destruction is much easier to detect. For a one-off solution, assign your own method to the <a href="http://docwiki.embarcadero.com/VCL/en/Controls.TControl.WindowProc" rel="nofollow noreferrer"><code>TPaintBox.WindowProc</code></a> property and watch for <a href="http://msdn.microsoft.com/en-us/library/ms632620.aspx" rel="nofollow noreferrer"><code>wm_Destroy</code></a> messages. If you're doing this often, then make a descendant class and override <a href="http://docwiki.embarcadero.com/VCL/en/Controls.TWinControl.DestroyWnd" rel="nofollow noreferrer"><code>DestroyWnd</code></a>.</p>

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