using System;
using Kent.Boogaart.HelperTrinity.Extensions;

namespace Kent.Boogaart.MVVM
{
	/// <summary>
	/// A command that executes delegates to determine whether the command can execute, and to execute the command.
	/// </summary>
	/// <remarks>
	/// <para>
	/// This command implementation is useful when the command simply needs to execute a method on a view model. The delegate for
	/// determining whether the command can execute is optional. If it is not provided, the command is considered always eligible
	/// to execute.
	/// </para>
	/// </remarks>
	public class DelegateCommand : Command
	{
		private readonly Predicate<object> _canExecute;
		private readonly Action<object> _execute;

		/// <summary>
		/// Constructs an instance of <c>DelegateCommand</c>.
		/// </summary>
		/// <remarks>
		/// This constructor creates the command without a delegate for determining whether the command can execute. Therefore, the
		/// command will always be eligible for execution.
		/// </remarks>
		/// <param name="execute">
		/// The delegate to invoke when the command is executed.
		/// </param>
		public DelegateCommand(Action<object> execute)
			: this(execute, null)
		{
		}

		/// <summary>
		/// Constructs an instance of <c>DelegateCommand</c>.
		/// </summary>
		/// <param name="execute">
		/// The delegate to invoke when the command is executed.
		/// </param>
		/// <param name="canExecute">
		/// The delegate to invoke to determine whether the command can execute.
		/// </param>
		public DelegateCommand(Action<object> execute, Predicate<object> canExecute)
		{
			execute.AssertNotNull("execute");
			_execute = execute;
			_canExecute = canExecute;
		}

		/// <summary>
		/// Determines whether this command can execute.
		/// </summary>
		/// <remarks>
		/// If there is no delegate to determine whether the command can execute, this method will return <see langword="true"/>. If a delegate was provided, this
		/// method will invoke that delegate.
		/// </remarks>
		/// <param name="parameter">
		/// The command parameter.
		/// </param>
		/// <returns>
		/// <see langword="true"/> if the command can execute, otherwise <see langword="false"/>.
		/// </returns>
		public override bool CanExecute(object parameter)
		{
			if (_canExecute == null)
			{
				return true;
			}

			return _canExecute(parameter);
		}

		/// <summary>
		/// Executes this command.
		/// </summary>
		/// <remarks>
		/// This method invokes the provided delegate to execute the command.
		/// </remarks>
		/// <param name="parameter">
		/// The command parameter.
		/// </param>
		public override void Execute(object parameter)
		{
			_execute(parameter);
		}
	}
}