delphi – 根据需要在运行时更改组件类

发布时间：2020-12-15 10:08:22 所属栏目：大数据来源：网络整理

导读：我的问题类似于这里的想法： Replacing a component class in delphi. 但是我需要根据需要更改特定的组件类. 这是一些伪演示代码： unit Unit1;TForm1 = class(TForm) ImageList1: TImageList; ImageList2: TImageList;private ImageList3: TImageList;end;p

我的问题类似于这里的想法： Replacing a component class in delphi.
但是我需要根据需要更改特定的组件类.
这是一些伪演示代码：

unit Unit1;

TForm1 = class(TForm)
  ImageList1: TImageList;
  ImageList2: TImageList;
private
  ImageList3: TImageList;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  ImageList3 := TImageList.Create(Self);
  // all instances of TImageList run as usual
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  Unit2.MakeSuperImageList(ImageList2);
  Unit2.MakeSuperImageList(ImageList3);
  // from now on ONLY ImageList2 and ImageList3 are TSuperImageList
  // ImageList1 is unchanged
end;

unit Unit2;

type
  TSuperImageList = class(Controls.TImageList)
  protected
    procedure DoDraw(Index: Integer; Canvas: TCanvas; X,Y: Integer;
      Style: Cardinal; Enabled: Boolean = True); override;
  end;

procedure TSuperImageList.DoDraw(Index: Integer; Canvas: TCanvas; X,Y: Integer;
  Style: Cardinal; Enabled: Boolean = True);
var
  Icon: TIcon;
begin
  Icon := TIcon.Create;
  try
    Self.GetIcon(Index,Icon);
    Canvas.Draw(X,Y,Icon);
  finally
    Icon.Free;
  end;
end;

procedure MakeSuperImageList(ImageList: TImageList);
begin
  // TImageList -> TSuperImageList
end;

注意：要清楚,我想更改一些实例,但不是全部,所以interposer class将不会.

解决方法

这更容易想到(感谢 Hallvard’s Blog – Hack#14: Changing the class of an object at run-time)：

procedure PatchInstanceClass(Instance: TObject; NewClass: TClass);
type
  PClass = ^TClass;
begin
  if Assigned(Instance) and Assigned(NewClass)
    and NewClass.InheritsFrom(Instance.ClassType)
    and (NewClass.InstanceSize = Instance.InstanceSize) then
  begin
    PClass(Instance)^ := NewClass;
  end;
end;

type
  TMyButton = class(TButton)
  public
    procedure Click; override;
  end;

procedure TMyButton.Click;
begin
  ShowMessage('Click!');
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  PatchInstanceClass(Button1,TMyButton);
end;

（编辑：李大同）

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