TipCalcModel.cs

using System;
using System.ComponentModel;
using Xamarin.Forms;

namespace TipCalc
{
	class TipCalcModel : INotifyPropertyChanged
	{
		double subTotal, postTaxTotal, tipPercent, tipAmount, total;

		internal TipCalcModel()
		{
			MessagingCenter.Subscribe<object, InitialAmountArgs>(this, Messages.InitialAmount, (s, e) =>
			{
				SubTotal = e.InitialAmount;
			});
		}

		public event PropertyChangedEventHandler PropertyChanged;

		public double SubTotal
		{
			set
			{
				if (subTotal != value)
				{
					subTotal = value;
					OnPropertyChanged("SubTotal");
					Recalculate();
				}
			}
			get
			{
				return subTotal;
			}
		}

		public double PostTaxTotal
		{
			set
			{
				if (postTaxTotal != value)
				{
					postTaxTotal = value;
					OnPropertyChanged("PostTaxTotal");
					Recalculate();
				}
			}
			get
			{
				return postTaxTotal;
			}
		}

		public double TipPercent
		{
			set
			{
				if (tipPercent != value)
				{
					tipPercent = value;
					OnPropertyChanged("TipPercent");
					Recalculate();
				}
			}
			get
			{
				return tipPercent;
			}
		}

		public double TipAmount
		{
			set
			{
				if (tipAmount != value)
				{
					tipAmount = value;
					OnPropertyChanged("TipAmount");

					// Let any interested parties know that the tip value has changed
					MessagingCenter.Send(Messages.Sender, Messages.Tip, new TipArgs(value));
				}
			}
			get
			{
				return tipAmount;
			}
		}

		public double Total
		{
			set
			{
				if (total != value)
				{
					total = value;
					OnPropertyChanged("Total");
				}
			}
			get
			{
				return total;
			}
		}

		void Recalculate()
		{
			this.TipAmount = Math.Round(this.TipPercent * this.SubTotal / 100, 2);

			// Round total to nearest quarter.
			this.Total = Math.Round(4 * (this.PostTaxTotal + this.TipAmount)) / 4;
		}

		protected void OnPropertyChanged(string propertyName)
		{
			if (PropertyChanged != null)
			{
				PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
			}
		}
	}
}