类的惰性属性

{在我们定义的对象（联系人）中包含的字段是对象（电话列表）时。往往这个字段不经常使用到。这样的字段称为惰性字段也就是惰性属性。
本例中的FPhoneNumbers就是这样情况。
}
unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    ComboBox1: TComboBox;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

  TContact = class
  private
    FName: string;
    FPhoneNumbers: TStrings;
    function CreateFPhoneNumbers: TStrings;
    function GetPhoneNumbers: TStrings;
    procedure SetPhoneNumbers(const Value: TStrings);
  public
    constructor Create; virtual;
    destructor Destroy; override;
    property Name: string read FName write FName;
    property PhoneNumbers: TStrings read GetPhoneNumbers write SetPhoneNumbers;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
  with TContact.Create do
  begin
    Name := Button1.Caption;
    PhoneNumbers := ComboBox1.Items;
    ShowMessage(PhoneNumbers.Text);
  end;

end;

{ TContact }

constructor TContact.Create;
begin
  inherited;
  FPhoneNumbers := nil;
end;

function TContact.CreateFPhoneNumbers: TStrings;
begin
  if (not Assigned(FPhoneNumbers)) then
    FPhoneNumbers := TStringList.Create;
  Result := FPhoneNumbers;

end;

destructor TContact.Destroy;
begin
  FPhoneNumbers.Free;
  inherited;
end;

function TContact.GetPhoneNumbers: TStrings;
begin
  Result := CreateFPhoneNumbers;
end;

procedure TContact.SetPhoneNumbers(const Value: TStrings);
begin
  if (Value = FPhoneNumbers) then Exit;
  PhoneNumbers.Assign(Value);
end;

end.