《GOF设计模式》—单件(Singleton)—Delphi源码示例：创建Singleton类的子类（重载Instance方法）

示例：创建Singleton类的子类（重载Instance方法）

说明：

另一个选择Singleton的子类的方法是将Instance的实现从父类中分离出来并将它放入子类。

代码：

unit uSingleton3;

 

interface

 

uses

    SysUtils, Dialogs;

 

type

    TSingleton = class

    private

        class var

            FInstance: TSingleton;

    public

        constructor Create;   

        destructor Destroy; override;

        //---

        class function Instance: TSingleton;virtual;

        procedure Operate;virtual;

    end;

    TSingletonA = class(TSingleton)

    public

        class function Instance: TSingleton; override;

        procedure Operate; override;

    end;

    TSingletonB = class(TSingleton)

    public

        class function Instance: TSingleton; override;

        procedure Operate; override;

    end;

 

implementation

 

constructor TSingleton.Create;

begin

    if FInstance = nil then

        FInstance := Self

    else

        abort;

end;

 

destructor TSingleton.Destroy;

begin

    FInstance := nil;

    //---

    inherited;

end;

 

class function TSingleton.Instance: TSingleton;

begin

    if FInstance = nil then

        FInstance := TSingleton.Create;

    //---

    Result := FInstance;

end;

 

procedure TSingleton.Operate;

begin

    ShowMessage('Singleton');

end;

 

class function TSingletonA.Instance: TSingleton;

begin

    if FInstance = nil then

        FInstance := TSingletonA.Create;

    //---

    Result := FInstance;

end;

 

procedure TSingletonA.Operate;

begin

    ShowMessage('SingletonA');

end;

 

class function TSingletonB.Instance: TSingleton;

begin

    if FInstance = nil then

        FInstance := TSingletonB.Create;

    //---

    Result := FInstance;

end;

 

procedure TSingletonB.Operate;

begin

    ShowMessage('SingletonB');

end;

 

end.

 

procedure TForm1.Button5Click(Sender: TObject);

begin

    TSingleton.Instance.Operate;

    TSingleton.Instance.Free;

    //---

    TSingletonA.Instance.Operate;

    TSingletonA.Instance.Free;

    //---

    TSingletonB.Instance.Operate;

    TSingletonB.Instance.Free;

end;