How do I keep a consistent instance of a GDI+ canvas (TGPGraphics)? - delphi

Question

I'm making use of a GDI+ canvas in Delphi 10.1 Berlin, using the built-in units GDIPAPI and GDIPOBJ. I have a thread which is performing drawing, and while looking for ways to improve the performance of the thread, one major drawback is the fact that I'm currently forced to instantiate an instance of this canvas (TGPGraphics), perform the drawing, and destroy the canvas, all together at the same time, at the moment I wish to draw. Instead, I would like to maintain a single constant instance of TGPGraphics.
Problem
The problem is that when I attempt to create a single global instance of TGPGraphics and use it in the future, for some reason it ends up drawing nothing onto the canvas. It results in just an empty unpainted canvas. It only works when I create/destroy the canvas at the exact moment I need to actually draw anything. I seem to be forced to create the instance, perform the painting, then destroy it, before I'm allowed to read that bitmap image.
The same problem happens elsewhere, not just inside the thread and not just on a TBitmap. I faced this issue in the past, but I was able to get away with constantly creating/freeing it for that project. This one though, it's not acceptable.
Question
How can I retain a single instance of TGPGraphics instead of creating/destroying it each time I need to draw?
Example
Here's a minimal test application which demonstrates the issue. Turn the GLOBAL_CANVAS conditional on and off - be sure to do a Build when changing, don't immediately go to Run. It's just a blank form with no components, just code:
unit uMain;
interface
{$DEFINE GLOBAL_CANVAS}
{$DEFINE FLUSH_SYNC}
{ $DEFINE FLUSH_BEFORE}
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs,
GDIPAPI, GDIPOBJ;
type
TForm1 = class(TForm)
procedure FormCreate(Sender: TObject);
procedure FormPaint(Sender: TObject);
procedure FormDestroy(Sender: TObject);
procedure FormResize(Sender: TObject);
private
FCan: TGPGraphics;
FPen: TGPPen;
FBmp: TBitmap;
function CreateCanvas: TGPGraphics;
procedure DoFlush;
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.FormCreate(Sender: TObject);
begin
FBmp:= TBitmap.Create;
FBmp.Width:= ClientWidth;
FBmp.Height:= ClientHeight;
{$IFDEF GLOBAL_CANVAS}
FCan:= CreateCanvas;
{$ENDIF}
FPen:= TGPPen.Create(MakeColor(255, 0, 0));
FPen.SetWidth(4.0);
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
FPen.Free;
{$IFDEF GLOBAL_CANVAS}
FCan.Free;
{$ENDIF}
FBmp.Free;
end;
function TForm1.CreateCanvas: TGPGraphics;
begin
Result:= TGPGraphics.Create(FBmp.Canvas.Handle);
Result.SetInterpolationMode(InterpolationMode.InterpolationModeHighQuality);
Result.SetSmoothingMode(SmoothingMode.SmoothingModeHighQuality);
Result.SetCompositingQuality(CompositingQuality.CompositingQualityHighQuality);
end;
procedure TForm1.DoFlush;
begin
{$IFDEF FLUSH_SYNC}
FCan.Flush(FlushIntention.FlushIntentionSync);
{$ELSE}
FCan.Flush(FlushIntention.FlushIntentionFlush);
{$ENDIF}
end;
procedure TForm1.FormPaint(Sender: TObject);
begin
{$IFNDEF GLOBAL_CANVAS}
FCan:= CreateCanvas;
try
{$ENDIF}
{$IFDEF FLUSH_BEFORE}
DoFlush;
{$ENDIF}
FCan.DrawEllipse(FPen, 5, 5, 50, 50);
{$IFNDEF FLUSH_BEFORE}
DoFlush;
{$ENDIF}
{$IFNDEF GLOBAL_CANVAS}
finally
FCan.Free;
end;
{$ENDIF}
Caption:= 'Handle: ' + IntToStr(FBmp.Canvas.Handle);
Canvas.Draw(0, 0, FBmp);
end;
procedure TForm1.FormResize(Sender: TObject);
begin
FBmp.Width:= ClientWidth;
FBmp.Height:= ClientHeight;
end;
end.
The left is when I create/free the canvas at the time of painting, and the right is when I create/free the canvas at startup/shutdown.
EDIT
I just noticed something - when GLOBAL_CANVAS is enabled in the test above, and then you resize the form to complete minimal size, then make it bigger again, you can see little bits and pieces of the drawing, but not the entire thing.
EDIT 2
I tried the recommendation to use Flush, and I switched it from FlushIntentionFlush to FlushIntentionSync, and now I have about a 50/50 success rate. Sometimes I run the app and it shows, and other times, making no changes at all, I run the app and nothing draws. I tried many different combinations of using Flush, using both methods, before drawing and after drawing. The few times it does appear to work, I resize the form to hide it, and make it larger again, and I can only see a glitchy image...
EDIT 3
I discovered the cause of the problem: the canvas handle keeps getting recreated, so each time I go to draw, the canvas has a whole new handle. I get the same behavior with both TForm.Canvas.Handle and TBitmap.Canvas.Handle. I'm not sure what the appropriate solution is though. I can't find a way to pass the new handle into the canvas object. The reason why it sometimes draws a glitchy image is because sometimes it obtains the same handle, but most of the time, it's different.