delphi action学习

procedure TEditAction.UpdateTarget(Target: TObject);
begin
  if (Self is TEditCut) then
    Enabled := (GetControl(Target).SelLength > 0) and not GetControl(Target).ReadOnly
  else if (Self is TEditCopy) then
    Enabled := (GetControl(Target).SelLength > 0);
end;

  TEditCopy = class(TEditAction)
  public
    procedure ExecuteTarget(Target: TObject); override;
  end;

  TEditAction = class(TAction)
  private
    FControl: TCustomEdit;
    procedure SetControl(Value: TCustomEdit);
  protected
    function GetControl(Target: TObject): TCustomEdit; virtual;
    procedure Notification(AComponent: TComponent; Operation: TOperation); override;
  public
    destructor Destroy; override;
    function HandlesTarget(Target: TObject): Boolean; override;
    procedure UpdateTarget(Target: TObject); override;
    property Control: TCustomEdit read FControl write SetControl;
  end;

  TEditPaste = class(TEditAction)
  public
    procedure UpdateTarget(Target: TObject); override;
    procedure ExecuteTarget(Target: TObject); override;
  end;

  TEditSelectAll = class(TEditAction)
  public
    procedure ExecuteTarget(Target: TObject); override;
    procedure UpdateTarget(Target: TObject); override;
  end;

执行命令
procedure TEditSelectAll.ExecuteTarget(Target: TObject);
begin
  GetControl(Target).SelectAll;
end;

更新控件状态
procedure TEditSelectAll.UpdateTarget(Target: TObject);
begin
  Enabled := Length(GetControl(Target).Text) > 0;
end;

 UpdateTarget不断的在调用

function TEditAction.GetControl(Target: TObject): TCustomEdit;
begin
  { We could hard cast Target as a TCustomEdit since HandlesTarget "should" be
    called before ExecuteTarget and UpdateTarget, however, we're being safe. }
  Result := Target as TCustomEdit;
end;