Call TCanvas.TextExtent() when component Canvas handle is not allocated

[AlexTP]

In ATSynEdit component, I need to get TextExtent for a string. But even when TATSynEdit.Canvas handle is not yet allocated (HandleAllocated=False yet). I can use any other ready TCanvas, but how to get it?

- Maybe I can create temporary TCanvas and set its Handle to GetDC(0)? This is current approach and I have mem leaks with it (even with call ReleaseDC() after GetDC() ).
- Or maybe I can use some global Canvas, e.g. Application.MainForm.Canvas? Is it always ready to use?

[Blaazen]

You can create temporary TBitmap, I saw it somewhere in Lazarus, IIRC.

Code: Pascal  [Select][+][-]

1. var BMP: TBitmap;
2. begin
3.   BMP:=TBitmap.Create;
4.   //do the job with BMP.Canvas
5.   BMP.Free;

[KodeZwerg]

Or maybe...

Code: Pascal  [Select][+][-]

1. uses Types;
2.  
3. function GetTextExtend(const AString: string; const AFont: TFont): TSize;
4. var
5.   LCanvas: TCanvas;
6. begin
7.   LCanvas := TCanvas.Create;
8.   try
9.     LCanvas.Font := AFont;
10.     Result := LCanvas.TextExtent(AString);
11.   finally
12.     LCanvas.Free;
13.   end;
14. end;

[Blaazen]

@KodeZwerg: This does not work because TCanvas has no handle (at least on Qt).

[jamie]

TControlcanvas.

Create it, and then set a control to it.

[AlexTP]

I tried first with the Bitmap, seems working. I only fear that macOS HiDPI canvas will be not the same as Bitmap's canvas, so I may get not OK TextExtent. I will test it.

[lagprogramming]

In ATSynEdit component, I need to get TextExtent for a string. But even when TATSynEdit.Canvas handle is not yet allocated (HandleAllocated=False yet). I can use any other ready TCanvas, but how to get it?

- Maybe I can create temporary TCanvas and set its Handle to GetDC(0)? This is current approach and I have mem leaks with it (even with call ReleaseDC() after GetDC() ).
- Or maybe I can use some global Canvas, e.g. Application.MainForm.Canvas? Is it always ready to use?

You've mentioned "TextExtent", "GetDC" and "ReleaseDC", which reminded me of the forum subject at
Lazarus » Forum » Miscellaneous » Suggestions » LCL » lcl/include/canvas.inc function TCanvas.TextExtent ARect variable
https://forum.lazarus.freepascal.org/index.php/topic,62633.0.html

The last message in that topic might be of interest to you because it looks like there is a bug in LCL. TCanvas.TextExtent doesn't SaveDC and RestoreDC properly:
https://forum.lazarus.freepascal.org/index.php/topic,62633.msg474095.html#msg474095