The .NET Command Pattern, Part 2: Listing 1

The CommandStateManager class

using System;
using System.Collections.Generic;
using System.ComponentModel;
using VSMCommandPatternUndo.Commands;
 
namespace VSMCommandPatternUndo
{
    public class CommandStateManager : INotifyPropertyChanged
    {
        private static readonly Lazy<CommandStateManager> _instance = new
Lazy<CommandStateManager>(() => new CommandStateManager()); private Stack<IUndoCommand> _undos = new Stack<IUndoCommand>(); private Stack<IUndoCommand> _redos = new Stack<IUndoCommand>(); public static CommandStateManager Instance { get { return _instance.Value; } } public bool CanUndo { get { return _undos.Count > 0; } } public bool CanRedo { get { return _redos.Count > 0; } } private CommandStateManager() { } public void Executed(IUndoCommand command) { _undos.Push(command); OnNotifyPropertyChanged("CanUndo"); } public void Undo() { if (CanUndo) { IUndoCommand command = _undos.Pop(); _redos.Push(command); command.Undo(); OnNotifyPropertyChanged("CanRedo"); } } public void Redo() { if (CanRedo) { IUndoCommand command = _redos.Pop(); _undos.Push(command); command.Execute(null); OnNotifyPropertyChanged("CanUndo"); } } public event PropertyChangedEventHandler PropertyChanged; private void OnNotifyPropertyChanged(string propertyName) { if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs(propertyName)); } } } }

About the Author

Eric Vogel is a Sr. Software Developer at Kunz, Leigh, & Associates in Okemos, MI. He is the president of the Greater Lansing User Group for .NET. Eric enjoys learning about software architecture and craftsmanship, and is always looking for ways to create more robust and testable applications. Contact him at vogelvision@gmail.com.