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Thursday, September 26, 2019

Building a Cross Platform MVVM Pattern with ReactiveUI and Xamarin.Forms

As a Mobile Developer, when we start a new project, we always search and speak about application architecture. One of the most common choices in Xamarin.Forms is MVVM, this is due to the small amount of work to implement it, if we use ReactiveUI, we can additionally write applications in a reactive manner. It’s a time to check reactive UI how we can implement in Xamairn Forms project.
Reactive Extensions have been around for many years, and is available to most development environments. In this post, we are specifically going to look at Rx in terms of .NET development in Xamarin Forms. 
Rx is just a library for composing asynchronous, and event-based code with observables, and configuring it via LINQ. You can use LINQ to define criteria of when you want to perform an action, on the event. Rx can do more, but we will only look at the very basics in this post.
ReactiveUI allows you to combine the MVVM pattern with Reactive Programming using such features, as WhenAnyValue, ReactiveCommand, ObservableAsPropertyHelper, Binding and WhenActivated.
Create New Xamarin.Forms Application
In order to learn the ReactiveUI, let’s start creating a new Xamarin.Forms project using Visual Studio 2019 or VS for Mac. When using Visual Studio 2019 on a Windows machine, you will need to pair the mac machine for run and build the iOS platform.

Open Visual Studio 2019 >>Create a New Project or select "Open Recent Application" and the available templates will appear on a Windows like below. Select Xamarin.Form app and click on “Next”

ReactiveUI Nuget Package:
To implement ReactiveUI in our applications we will need to install the library. 
Step 1: Right click on Project and Click on Manage NuGet Packages for Solution option. 
Step 2: Search for “ReactiveUI.XamForms”.
Step 3: Install it for all of our projects with each platform. 

Create ViewModel :
ReactiveUI syntax for read-write properties is to notify Observers that a property has changed. Otherwise we would not be able to know when it was changed.
  • In cases when we don't need to provide for two-way binding between the View and the ViewModel, we can use one of many ReactiveUI Helpers, to notify Observers of a changing read-only value in the ViewModel.
  • RaiseAndSetIfChanged fully implements a Setter for a read-write property on a ReactiveObject, using CallerMemberName to raise the notification and the ref to the backing field to set the property.
  • ReactiveCommand is a Reactive Extensions and asynchronous aware implementation of the ICommand interface. ICommand is often used in the MVVM design pattern to allow the View to trigger business logic defined in the ViewModel
Let we create following property and command in the following ViewModel 
using System;
using System.Threading.Tasks;
using ReactiveUI;
using System.Reactive;
using System.Text.RegularExpressions;
using System.Collections.Generic;
using System.Reactive.Linq;

namespace ReactiveUIXamarin.ViewModel
    public class MainPageViewModel: ReactiveObject
        private string _result;
        public string Result
            get => _result;
            set => this.RaiseAndSetIfChanged(ref _result, value);
        private string _username;
        public string UserName
            get => _username;
            set => this.RaiseAndSetIfChanged(ref _username, value);
        private string _password;
        public string Password
            get => _password;
            set => this.RaiseAndSetIfChanged(ref _password, value);
        private string _address;
        public string Address
            get => _address;
            set => this.RaiseAndSetIfChanged(ref _address, value);
        private string _phone;
        public string Phone
            get => _phone;
            set => this.RaiseAndSetIfChanged(ref _phone, value);
        public ReactiveCommand<Unit, Unit> RegisterCommand { get; }

        public MainPageViewModel()
            RegisterCommand = ReactiveCommand

        private IObservable<Unit> ExecuteRegisterCommand()
            Result = "Hello" + UserName + " Registration Success";
            return Observable.Return(Unit.Default);

Create UI View:
ReactiveUI allows you to create views using two different approaches. The recommended approach is using type-safe ReactiveUI bindings that can save you from memory leaks and runtime errors. The second approach is using XAML markup bindings.
The following sample UI created by recommended approach using type-safe ReactiveUI .
        <Entry x:Name="Username" Placeholder="Username"/>
        <Entry x:Name="Password" Placeholder="Password"  />
        <Entry x:Name="Address" Placeholder="Address"  />
        <Entry x:Name="Phone" Placeholder="Phone Number" />
        <Button x:Name="Register" Text="Register" TextColor="White" BackgroundColor="Gray" />
        <Label x:Name="Result" />

  • ContentPage should inherit from ReactiveContentPage<TViewModel> and we are going to use ReactiveUI Binding to bind our ViewModel to our View. 
  • Reactive binding is a cross platform way of consistently binding properties on your ViewModel to controls on your View. 
  • The ReactiveUI binding has a few advantages over the XAML based binding. The first advantage is that property name changes will generate a compile error rather than runtime errors.
  • One important think needs to follows while Binding, always dispose bindings via WhenActivated, or else the bindings leak memory.
using ReactiveUI;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Forms;
using ReactiveUIXamarin.ViewModel;
using ReactiveUI.XamForms;
using System.Reactive.Disposables;

namespace ReactiveUIXamarin

    public partial class MainPage : ReactiveContentPage<MainPageViewModel>
        public MainPage()
            ViewModel = new MainPageViewModel();

            // Setup the bindings.
            // Note: We have to use WhenActivated here, since we need to dispose the
            // bindings on XAML-based platforms, or else the bindings leak memory.
            this.WhenActivated(disposable =>
                this.Bind(ViewModel, x => x.UserName, x => x.Username.Text)
                this.Bind(ViewModel, x => x.Password, x => x.Password.Text)
                this.Bind(ViewModel, x => x.Address, x => x.Address.Text)
                this.Bind(ViewModel, x => x.Phone, x => x.Phone.Text)
                this.BindCommand(ViewModel, x => x.RegisterCommand, x => x.Register)
                this.Bind(ViewModel, x => x.Result, x => x.Result.Text)

You can download the source code from github repository. When you run the application in an iPhone device, you will get the following output and click on Register, it will show the confirmation message like below screen.
This article has demonstrated and covers a little bit of what you can do when you combine the power of Reactiveui in Xamarin.Forms. I hope this article will help you to get started with awesome Framework.


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