Why Canvas is “hidden” in all VCL controls?

问题

I want to do a basic procedure that draws something (let's say a triangle, for simplicity) on any control's (button, panel, etc) canvas:

procedure DrawTriangle(Control: TCustomControl);

In this function I need to use Control.Width & Control.Height to know how big is the control. Turns out to be more difficult than imagined because Canvas is protected.

A solution would be to obtain the canvas of the control inside the procedure:

VAR
   ParentControl: TWinControl;
   canvas: TCanvas;
begin
 ParentControl:= Control.Parent;
 Canvas:= TCanvas.Create;
 TRY
  Canvas.Handle:= GetWindowDC(ParentControl.Handle);
  WITH Canvas DO
    xyz
 FINALLY
   FreeAndNil(canvas);
 END;
end;

But seems such a waste of CPU to create and destroy a canvas each time I want to paint something...

So, my questions are:

Why was canvas hidden (protected) by design?
How to solve this elegantly (one single parameter) and without wasting CPU?

Now I am overriding the Paint method, but this means duplication the painting code in several places. Of course, the DrawTriangle could receive more parameters (Canvas, Control Width/Height etc), .... but well... with an exposed Paint method, everything would have been so much more elegant.

回答1:

In a comment to the question it turns out that

it is enough for this solution to be restricted to TCustomControl descendants, and
it is "elegant" enough if the drawing procedure can obtain the canvas from the argument control with a simple function call.

If so, the following solution is possible:

//
// Infrastructure needed
//

type
  TCustomControlCracker = class(TCustomControl)
  end;

function CustomControlCanvas(AControl: TCustomControl): TCanvas;
begin
  Result := TCustomControlCracker(AControl).Canvas;
end;

//
// My reusable drawing functions
// (Can only be used in TCustomControl descendants)
//

procedure DrawFrog(AControl: TCustomControl);
var
  Canvas: TCanvas;
begin
  Canvas := CustomControlCanvas(AControl);
  Canvas.TextOut(10, 10, 'Frog');
end;

Notice that DrawFrog only takes a single parameter, the control itself. And it can then obtain the control's canvas using a simple function call with extremely little CPU overhead.

Full example:

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.ExtCtrls, Vcl.StdCtrls;

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

type
  TTestControl = class(TCustomControl)
  protected
    procedure Paint; override;
  end;

type
  TCustomControlCracker = class(TCustomControl)
  end;

function CustomControlCanvas(AControl: TCustomControl): TCanvas;
begin
  Result := TCustomControlCracker(AControl).Canvas;
end;

procedure DrawFrog(AControl: TCustomControl);
var
  Canvas: TCanvas;
begin
  Canvas := CustomControlCanvas(AControl);
  Canvas.TextOut(10, 10, 'Frog');
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  with TTestControl.Create(Self) do
  begin
    Parent := Self;
    Top := 100;
    Left := 100;
    Width := 400;
    Height := 200;
  end;
end;

{ TTestControl }

procedure TTestControl.Paint;
begin
  inherited;
  Canvas.Brush.Color := clSkyBlue;
  Canvas.FillRect(ClientRect);
  DrawFrog(Self); // use my reusable frog-drawing function
end;

end.

All this being said, however, I would personally still use the standard approach of passing a TCanvas (or even a HDC) instead of a control, together with some dimensions:

procedure DrawFrog(ACanvas: TCanvas; const ARect: TRect);

This will allow me to use it for other controls as well (not only TCustomControl descendants), as well as printer canvases etc.

回答2:

Why was canvas hidden by design?

No really hidden, but in protected section. To access it, you have to derive a new class from the one you are interested in and declare Canvas as public.

It is private because you are not supposed to access it at the application level.

You don't need to install your component if you use an interposer class in the source you need it.

You may also consider overriding the Paint method and put your drawing code there.

来源：https://stackoverflow.com/questions/65050463/why-canvas-is-hidden-in-all-vcl-controls

标签

Delphi

canvas

vcl

lazarus