delphi – 如何在TPanel上绘制

我认为这比尝试在应用程序代码中绘制TPanel更好的解决方案有以下几个原因：

> Canvas属性在TPanel中受到保护,因此您无法从类外部访问它.你可以用类型转换来解决这个问题,但这是作弊行为.
>“可恢复性”听起来更像是面板的一个功能,而不是应用程序的一个功能,因此将其放在面板控件的代码中,而不是应用程序的主代码中.

这是让你入门的东西：

type
  TSizablePanel = class(TPanel)
  private
    FDragOrigin: TPoint;
    FSizeRect: TRect;
  protected
    procedure Paint; override;
    procedure MouseDown(Button: TMouseButton; Shift: TShiftState;
      X,Y: Integer); override;
    procedure MouseMove(Shift: TShiftState; X,Y: Integer); override;
    procedure MouseUp(Button: TMouseButton; Shift: TShiftState;
      X,Y: Integer); override;
  end;

procedure TSizeablePanel.Paint;
begin
  inherited;
  // Draw a sizing grip on the Canvas property
  // There's a size-grip glyph in the Marlett font,// so try the Canvas.TextOut method in combination
  // with the Canvas.Font property.
end;

procedure TSizeablePanel.MouseDown;
begin
  if (Button = mbLeft) and (Shift = []) 
      and PtInRect(FSizeRect,Point(X,Y)) then begin
    FDragOrigin := Point(X,Y);
    // Need to capture mouse events even if the mouse
    // leaves the control. See also: ReleaseCapture.
    SetCapture(Handle);
  end else inherited;
end;