using System;
using System.Diagnostics;
using System.Threading;
using System.Windows;
using System.Windows.Input;
using System.Windows.Threading;

namespace Kent.Boogaart.MVVM
{
	/// <summary>
	/// A base class for command implementations.
	/// </summary>
	public abstract class Command : ICommand
	{
		private readonly Dispatcher _dispatcher;

		/// <summary>
		/// Constructs an instance of <c>CommandBase</c>.
		/// </summary>
		protected Command()
		{
			if (Application.Current != null)
			{
				_dispatcher = Application.Current.Dispatcher;
			}
			else
			{
				//this is useful for unit tests where there is no application running
				_dispatcher = Dispatcher.CurrentDispatcher;
			}

			Debug.Assert(_dispatcher != null);
		}

		/// <summary>
		/// Occurs whenever the state of the application changes such that the result of a call to <see cref="CanExecute"/> may return a different value.
		/// </summary>
		public event EventHandler CanExecuteChanged
		{
			add { CommandManager.RequerySuggested += value; }
			remove { CommandManager.RequerySuggested -= value; }
		}

		/// <summary>
		/// Determines whether this command can execute.
		/// </summary>
		/// <param name="parameter">
		/// The command parameter.
		/// </param>
		/// <returns>
		/// <see langword="true"/> if the command can execute, otherwise <see langword="false"/>.
		/// </returns>
		public abstract bool CanExecute(object parameter);

		/// <summary>
		/// Executes this command.
		/// </summary>
		/// <param name="parameter">
		/// The command parameter.
		/// </param>
		public abstract void Execute(object parameter);

		/// <summary>
		/// Raises the <see cref="CanExecuteChanged"/> event.
		/// </summary>
		protected virtual void OnCanExecuteChanged()
		{
			if (!_dispatcher.CheckAccess())
			{
				_dispatcher.Invoke((ThreadStart)OnCanExecuteChanged, DispatcherPriority.Normal);
			}
			else
			{
				CommandManager.InvalidateRequerySuggested();
			}
		}
	}
}