《GOF设计模式》—备忘录（MEMENTO）—Delphi源码示例：一个反映备忘录模式的迭代接口

说明：
Dylan中的Collection提供了一个反映备忘录模式的迭代接口。Dylan的集合有一个"状态"对象的概念,它是一个表示迭代状态的备忘录。每一个集合可以按照它所选择的任意方式表示迭代的当前状态；该表示对客户完全不可见。

代码：
unit uCollection;

interface

uses Dialogs,Contnrs;

type
    TItem = class
    private
        FValue:string;
    public
        constructor Create(const AValue:string);
        //---
        procedure Process();
    end;

    TIterationState = class
    private
        FCurrent: Integer;
    public
        constructor Create;
    end;

    TCollection = class
    private
        FList: TObjectList;
    public
        constructor Create;
        destructor Destroy; override;
        //---
        function CreateInitialState: TIterationState;
        function Copy(AState: TIterationState): TIterationState;
        //---
        procedure Next(AState: TIterationState);
        function IsDone(AState: TIterationState): boolean;
        function CurrentItem(AState: TIterationState): TObject;
        //---
        procedure Append(AItem: TObject);
        procedure Remove(AItem: TObject);
    end;

implementation

function TCollection.CreateInitialState: TIterationState;
begin
    Result := TIterationState.Create;
end;

function TCollection.Copy(AState: TIterationState): TIterationState;
begin
    Result := TIterationState.Create;
    Result.FCurrent := AState.FCurrent;
end;

constructor TCollection.Create;
begin
    FList := TObjectList.Create;
end;

destructor TCollection.Destroy;
begin
    FList.Free;
    //---
    inherited;
end;

function TCollection.CurrentItem(AState: TIterationState): TObject;
begin
    Result := FList[AState.FCurrent];
end;

function TCollection.IsDone(AState: TIterationState): boolean;
begin
    Result := AState.FCurrent >= FList.Count;
end;

procedure TCollection.Next(AState: TIterationState);
begin
    with AState do
        FCurrent := FCurrent + 1;
end;

procedure TCollection.Append(AItem: TObject);
begin
    FList.Add(AItem);
end;

procedure TCollection.Remove(AItem: TObject);
begin
    FList.Remove(AItem);
end;

constructor TIterationState.Create;
begin
    FCurrent := 0;
end;

constructor TItem.Create(const AValue: string);
begin
    FValue := AValue;
end;

procedure TItem.Process;
begin
    ShowMessage(FValue);
end;

end.


procedure TForm3.Button1Click(Sender: TObject);
var
    ACollection: TCollection;
    AState: TIterationState;
begin
    ACollection := TCollection.Create;
    try
        with ACollection do
        begin
            Append(TItem.Create('1'));
            Append(TItem.Create('2'));
            Append(TItem.Create('3'));
            //---
            AState := CreateInitialState();
            try
                while not IsDone(AState) do
                begin
                    TItem(CurrentItem(AState)).Process();
                    Next(AState);
                end;
            finally
                AState.Free;
            end;
        end;
    finally
        ACollection.Free;
    end;
end;

procedure TForm3.Button2Click(Sender: TObject);
var
    ACollection: TCollection;
    AState,AState1: TIterationState;
begin
    ACollection := TCollection.Create;
    try
        with ACollection do
        begin
            Append(TItem.Create('1'));
            Append(TItem.Create('2'));
            Append(TItem.Create('3'));
            //---
            AState := CreateInitialState();
            try
                while not IsDone(AState) do
                begin
                    TItem(CurrentItem(AState)).Process();
                    Next(AState);
                    //---
                    AState1 := Copy(AState);
                    try
                        while not IsDone(AState1) do
                        begin
                            TItem(CurrentItem(AState1)).Process();
                            Next(AState1);
                        end;
                    finally
                        AState1.Free;
                    end;
                end;
            finally
                AState.Free;
            end;
        end;
    finally
        ACollection.Free;
    end;
end;