Backend For FrontEnd API for Xamarin Apps

ApiModels2ViewModels.cs

using System.Threading.Tasks;
using PropertyChanged;

/// <summary>
/// See "Consuming the BFF API" in http://vincenth.net/blog/archive/2016/07/13/building-bff-backend-for-frontend-apis-for-xamarin-mobile-apps.aspx
/// </summary>
namespace MyApp.ApiModels
{
    [ImplementPropertyChanged] // This will make all properties in all partial Contact classes data-bindable
    public partial class Contact : ViewModelBase
    {
        public RelayCommand SaveCommand { get; private set; } 

        async private Task Save() { } // Implement save...
    }

    public abstract class ViewModelBase { } // E.g. implement INotifyPropertyChanged if you want something other than the Fody generated default ...
    public class RelayCommand { } // Implement ICommand ...
}

BffApiDesign.cs

using System;
using Newtonsoft.Json;

/// <summary>
/// See "BFF API Design Artifacts" in http://vincenth.net/blog/archive/2016/07/13/building-bff-backend-for-frontend-apis-for-xamarin-mobile-apps.aspx
/// </summary>
namespace MyApp.ApiModels
{
    [JsonObject(MemberSerialization = MemberSerialization.OptIn)] // When we extend this class to a viewmodel in the client, we do not want to serialize all additional properties (e.g. commands)
    public partial class Contact
    {
        [JsonProperty]
        public Guid Id { get; set; }

        [JsonProperty]
        public string Name { get; set; }

        // More properties ...
    }

    // More API model classes ...
}

namespace MyApp.Api
{
    using ApiModels;

    public class ApiDesignData : IApi
    {
        public ApiResult<Contact> GetContact(Guid contactId)
        {
            return ApiResult<Contact>.FromValue(new Contact
            {
                Id = new Guid("54AEB38C-0820-4BAA-B27B-054CEE4F1199"),
                Name = "Hubert Wolfeschlegelsteinhausenbergerdorff"
            });
        }

        public ApiResult SaveContact(Contact contact)
        {
            return new ApiResult();
        }
    }

    public interface IApi
    {
        ApiResult<Contact> GetContact(Guid contactId);
        ApiResult SaveContact(Contact contact);

        // More API methods ...
    }

    public class ApiResult
    {
        public string Error { get; protected set; }

        public ApiResult(string error = null) { Error = error; }
    }

    public class ApiResult<T> : ApiResult
    {
        public T Value { get; private set; }

        public static ApiResult<T> FromValue(T value) { return new ApiResult<T> { Value = value }; }
        public static ApiResult<T> FromError(string error) { return new ApiResult<T> { Error = error }; }
    }
}

FodyPropertyChangedExample.cs

[ImplementPropertyChanged]
public class Person 
{        
    public string GivenNames { get; set; }
    public string FamilyName { get; set; }
    public string FullName { get { return string.Format("{0} {1}", GivenNames, FamilyName); } }
      // PropertyChanged will automatically be raised for FullName whenever GivenNames or FamilyName change
}