mono touch 隐藏键盘

/// <summary>
		/// 隐藏键盘
		/// </summary>
		public override void TouchesBegan (NSSet touches, UIEvent evt)
		{
			base.TouchesBegan (touches, evt);
			UITouch touch = touches.AnyObject as UITouch;
			if (touch != null) {
				utxt.ResignFirstResponder ();
			}
		}

utxt是个输入框对象，逻辑判断可以具体一些