XE2做单实例

unit Unit11;

interface

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

type
  TMyTestClass = class
  private
    //class var n : integer;
    class var MyTestClass : TMyTestClass;
  public
    function test : string;
    class function NewInstance: TObject; override;
    class function GetInstance : TMyTestClass;
  end;



  TForm11 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form11: TForm11;

implementation

{$R *.dfm}

procedure TForm11.Button1Click(Sender: TObject);
var
  a : TMyTestClass;
begin
  {两种方式均可，但GetInstance貌似更能清晰表面这个类是单实例。}
  {a := TMyTestClass.Create;}
  a := TMyTestClass.GetInstance;
  if a <> nil then
    Application.MessageBox(PChar(a.test), '提示信息', mb_OK);
end;

{ TMyTestClass }

class function TMyTestClass.GetInstance: TMyTestClass;
begin
  Result := TMyTestClass.Create;
end;

class function TMyTestClass.NewInstance: TObject;
begin
//  if n = 1 then
//  begin
//    Result := nil;
//    raise Exception.Create('只能创建1个实例');
//  end
//  else
//  begin
//    Result := inherited NewInstance;
//    Inc(n);
//  end;
  if MyTestClass <> nil then
  begin
    Result := MyTestClass;
  end
  else
  begin
    Result := inherited NewInstance;
    MyTestClass := Result as TMyTestClass;
  end;
end;

function TMyTestClass.test: string;
begin
  Result := Self.ClassName;
end;

end.

类变量+NewInstance实现单实例，真是太Easy了。