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PODelphi/GDI+: When is a Device Context created/destroyed?
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  1. PODelphi/GDI+: When is a Device Context created/destroyed?
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    copied!<p>Normally using GDI+ in Delphi you can use a <strong>TPaintBox</strong>, and paint during the <strong>OnPaint</strong> event:</p> <pre><code>procedure TForm1.PaintBox1Paint(Sender: TObject); var g: TGPGraphics; begin g := TGPGraphics.Create(PaintBox1.Canvas.Handle); try g.DrawImage(FSomeImage, 0, 0); finally g.Free; end; end; </code></pre> <p>The problem with this paradigm is that creating a destroying a <strong>Graphics</strong> object each time is wasteful and poorly performing. Additionally, there are a few <a href="http://msdn.microsoft.com/en-us/library/ms534425.aspx" rel="nofollow noreferrer">constructs availabe in GDI+</a> you can <em>only</em> use when you have a persistent <strong>Graphics</strong> object.</p> <p>The problem, of course, is <em>when</em> can i create that <strong>Graphics</strong> object? i need to know when the handle becomes available, and then when it is no longer valid. i need this information so i can create and destroy my <strong>Graphics</strong> object.</p> <hr> <h2>Solution Attempt Nº1</h2> <p>i can solve the creation problem by creating it when it is really needed - on the first time the <em>paint cycle</em> is called:</p> <pre><code>procedure TForm1.PaintBox1Paint(Sender: TObject); begin if FGraphics = nil then FGraphics := TGPGraphics.Create(PaintBox1.Canvas.Handle); FGraphics.DrawImage(FSomeImage, 0, 0); end; </code></pre> <p>But i have to know when the device context is no longer valid, so i can destroy my <strong>FGraphcis</strong> object, so that it is re-created the next time it's needed. If for some reason the <strong>TPaintBox</strong>'s device context gets recreated, i would be drawing on an invalid device context the next time <strong>OnPaint</strong> is called.</p> <p>What is the intended mechanism in Delphi for me to know when the <strong>device context handle</strong> of a <strong>TPaintBox</strong> is created, destroyed, or re-created?</p>
 

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