delphi – 在TFrame表面上绘制最安全,最正确的方法是什么？

正如我在论坛中看到的,常见的方法是覆盖PaintWindow方法.

我在一个干净的项目上试过这个：

type
  TMyFrame = class(TFrame)
  private
    FCanvas: TCanvas;
  protected
    procedure PaintWindow(DC: HDC); override;
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy(); override;
  end;

implementation

{$R *.dfm}

constructor TMyFrame.Create(AOwner: TComponent);
begin
  inherited;
  FCanvas := TCanvas.Create();
end;

destructor TMyFrame.Destroy();
begin
  FCanvas.Free();
  inherited;
end;

procedure TMyFrame.PaintWindow(DC: HDC);
begin
  inherited;
  FCanvas.Handle := DC;
  FCanvas.Pen.Width := 3;
  FCanvas.Pen.Color := clRed;
  FCanvas.MoveTo(0,0);
  FCanvas.LineTo(ClientWidth,ClientHeight);
  FCanvas.Pen.Color := clGreen;
  FCanvas.MoveTo(ClientWidth,0);
  FCanvas.LineTo(0,ClientHeight);
end;

但是,在将我的框架放在主窗体上后,调试器从未进入此方法,直到我在框架的属性中启用了DoubleBuffered. ParentBackground的任何值都不会影响结果.

覆盖WM_PAINT处理程序也解决了问题：

type
  TMyFrame = class(TFrame)
  protected
    procedure WMPaint(var Message: TWMPaint); message WM_PAINT;
  ...

procedure TMyFrame.WMPaint(var Message: TWMPaint);
begin
  inherited;
  FCanvas.Handle := GetDC(Handle);
  FCanvas.Pen.Width := 3;
  FCanvas.Pen.Color := clRed;
  FCanvas.MoveTo(0,ClientHeight);
  ReleaseDC(Handle,FCanvas.Handle);
end;

无论将哪个值分配给DoubleBuffered或ParentBackground,此代码始终绘制交叉线.

但是当我尝试使用BeginPaint / EndPaint而不是GetDC / ReleaseDC时,问题又返回了：

procedure TMyFrame.WMPaint(var Message: TWMPaint);
var
  PS: PAINTSTRUCT;
begin
  inherited;
  FCanvas.Handle := BeginPaint(Handle,PS);
  FCanvas.Pen.Width := 3;
  FCanvas.Pen.Color := clRed;
  FCanvas.MoveTo(0,ClientHeight);
  EndPaint(Handle,PS);
end;

FCanvas.Handle不为零,但结果是空白帧.在这种情况下,设置DoubleBuffered或ParentBackground不会改变任何东西.

也许我说他们错了？

现在我使用带有GetDC / ReleaseDC的WM_PAINT处理程序,因为我不想在这个帧上启用DoubleBuffered.另外我担心其他程序员在将我的框架放入他们的项目之后会意外地禁用DoubleBuffered并且会像我一样头疼.

但也许有更安全和正确的方法在框架表面上绘画？