Mastering Delphi Programming: A Complete Reference Guide

Invoker

The next part of the solution is the invoker, which is stored in the Editor.Editor unit. It is defined as a simple class that can execute and undo commands, as shown here:

type  TEditor = class  strict private    FUndo: TStack<TCommand>;  public    constructor Create;    destructor Destroy; override;    procedure Execute(command: TCommand);    procedure Undo(command: TCommand);    function IsUndoEmpty: boolean;    function PopLastCommand: TCommand;  end;

Leaving the boring creation and destruction aside (for more details, see the source code), the interesting methods are implemented as follows:

procedure TEditor.Execute(command: TCommand);begin  command.Execute;  FUndo.Push(command);end;function TEditor.IsUndoEmpty: boolean;begin Result := FUndo.Count = 0; ...