现在使用控件, 更喜欢继承（覆盖控件已有的函数，很奇怪的一种使用方式）

以前写代码, 总是把主单元弄得满满当当; 现在更喜欢把控件比较独立的功能写成一个单元, 改写属性、重载方法...哪怕只有一点点和默认不同, 也喜欢独立出来.

刚刚用到 TListBox, 需要能拖动元素、双击删除.

---

unit ListBox2;

interface

uses
  System.Classes, Vcl.Controls, Vcl.StdCtrls, System.Types;

type
  TListBox2 = class(TCustomListBox)
  protected
    procedure DragOver(Source: TObject; X: Integer; Y: Integer; State: TDragState; var Accept: Boolean); override;
    procedure DblClick; override;
  public
    constructor Create(AOwner: TComponent); override;
    procedure DragDrop(Source: TObject; X: Integer; Y: Integer); override;
  end;

implementation

{ TListBox2 }

constructor TListBox2.Create(AOwner: TComponent);
begin
  inherited;
  DragMode := dmAutomatic;
end;

procedure TListBox2.DblClick;
begin
  inherited;
  Items.Delete(ItemIndex);
end;

procedure TListBox2.DragDrop(Source: TObject; X, Y: Integer);
begin
  inherited;
  Items.Exchange(ItemIndex, ItemAtPos(Point(X,Y), True));
end;

procedure TListBox2.DragOver(Source: TObject; X, Y: Integer; State: TDragState; var Accept: Boolean);
begin
  inherited;
  Accept := True;
end;

end.

---

测试:

---

uses ListBox2;

procedure TForm1.FormCreate(Sender: TObject);
begin
  with TListBox2.Create(Self) do begin
    Parent := Self;
    Align := alLeft;
    Items.CommaText := 'A,B,C,D,E,F,G';
  end;
end;