Xamarin.Forms - Validation using Fluent Validation in MVVM

In this blog post, you will learn how to use fluent validation in MVVM in Xamarin.Forms App

Introduction

Xamarin.Forms code runs on multiple platforms - each of which has its own filesystem. This means that reading and writing files is most easily done using the native file APIs on each platform. Alternatively, embedded resources are a simpler solution to distribute data files with an app.

Fluent Validation

Fluent Validation is one of the best Libraries for Xamarin.Forms it's used to validate XAML Controls. Fluent Validation is used to create the validation logic separate from business logic. Fluent validation is used to lambda expressions for building validation rules for your business objects.

References

https://github.com/james-russo/XamarinForms-UnobtrusiveValidationPlugin

Prerequisites

• Visual Studio 2017 or later (Windows or Mac)

Let's Start

Setting up a Xamarin.Forms Project

Start by creating a new Xamarin.Forms project. You will learn more by going through the steps yourself. 

Create a new or existing Xamarin forms(.Net standard) Project. With Android and iOS Platform. 

Setup UI

Now, Create a UI for your requirement. I just created a simple login page for validation purposes.

LoginPage.xaml

	<?xml version="1.0" encoding="UTF-8"?>
	<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="XamarinApp.LoginPage">
	<ContentPage.Content>
	<StackLayout HorizontalOptions="Fill" Margin="50,100" VerticalOptions="Start">
	<Label Text="Login" FontSize="Large"/>
	<Entry Placeholder="Username" Text="{Binding UserName}"/>
	<Entry Placeholder="Password" IsPassword="True" Text="{Binding Password}"/>
	<Label TextColor="Red" Text="{Binding LoginError}"/>
	<Button Text="Login" Command="{Binding LoginCommand}"/>
	</StackLayout>
	</ContentPage.Content>
	</ContentPage>

view raw LoginPage.xaml hosted with ❤ by GitHub

LoginPageViewModel

Here, you can write your business logic for your requirement. I just add simple login validation only. LoginCommand will validate the login form.

LoginPageViewModel.cs

	using System;
	using System.ComponentModel;
	using Xamarin.Forms;
	using XamarinApp.Validations;
	namespace XamarinApp.ViewModels
	{
	public class LoginPageViewModel : INotifyPropertyChanged
	{
	#region Fields
	private string _username;
	private Command _LoginCommand;
	LoginValidation loginValidation;
	#endregion
	#region Constructor
	[Obsolete]
	public LoginPageViewModel()
	{
	loginValidation = new LoginValidation();
	}
	#endregion
	#region Properties
	public event PropertyChangedEventHandler PropertyChanged;
	public string UserName
	{
	get { return _username; }
	set
	{
	_username = value;
	OnPropertyChanged(nameof(UserName));
	}
	}
	public string Password { get; set; }
	public string LoginError { get; set; }
	public Command LoginCommand
	{
	get
	{
	return _LoginCommand ?? (_LoginCommand = new Command(ExecuteLoginCommand));
	}
	}
	#endregion
	#region Methods
	public void ExecuteLoginCommand()
	{
	var result = loginValidation.Validate(this);
	if (result.IsValid)
	LoginError = string.Empty;
	}
	protected void OnPropertyChanged(string name)
	{
	PropertyChangedEventHandler handler = PropertyChanged;
	if (handler != null)
	{
	handler(this, new PropertyChangedEventArgs(name));
	}
	}
	#endregion
	}
	}

view raw LoginPageViewModel.cs hosted with ❤ by GitHub

Install Fluent Validation NuGet

Install the following Nuget from Nuget Package Manager In your Visual Studio.

Fluent Validation

Create Fluent Validator

Now, Create a validator class in .Net Standard or PCL Library for your ViewModel or your business logic. I create LoginValidator for my LoginPageViewModel.

LoginValidation.cs

	using System;
	using FluentValidation;
	using XamarinApp.ViewModels;
	namespace XamarinApp.Validations
	{
	public class LoginValidation: AbstractValidator<LoginPageViewModel>
	{
	public LoginValidation()
	{
	RuleFor(x => x.UserName).NotNull().OnFailure(x=>x.LoginError="Username or Password is Missing");
	RuleFor(x => x.Password).NotNull().OnFailure(x => x.LoginError = "Username or Password is Missing");
	}
	}
	}

view raw LoginValidation.cs hosted with ❤ by GitHub

Here I just added the NotNull check validation with the failure message. You can explore more features for the Fluent validation

https://github.com/james-russo/XamarinForms-UnobtrusiveValidationPlugin

Result

The below image for login validation failed.

The Login validation is Success

Conclusion

I hope you have understood how to validate objects using fluent validation in MVVM in Xamarin.Forms App

Thanks for reading. Please share your comments and feedback. Happy Coding :)

Xamarin.Forms - How to use Fody property in MVVM

 In this blog post, you will learn how to use Fody in MVVM in Xamarin.Forms App

Introduction

Xamarin.Forms code runs on multiple platforms - each of which has its own filesystem. This means that reading and writing files is most easily done using the native file APIs on each platform. Alternatively, embedded resources are a simpler solution to distribute data files with an app.

Fody

Fody is a library for IL weaving (the process to manipulate/inject code in IL). Fody will update automatically our code for us to add new cool features.

Fody is Injects code which raises the PropertyChanged event, into property setters of classes which implement INotifyPropertyChanged.

Prerequisites

• Visual Studio 2017 or later (Windows or Mac)

Let's Start

Setting up a Xamarin.Forms Project

 Start by creating a new Xamarin.Forms project. You will learn more by going through the steps yourself.

 Create a new or existing Xamarin forms(.Net standard) Project. With Android and iOS Platform. 

Setup UI

Now, Create a UI for your requirement. I just created a simple UI for LoginPage

LoginPage.xaml

	<?xml version="1.0" encoding="UTF-8"?>
	<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="XamarinApp.LoginPage">
	<ContentPage.Content>
	<StackLayout HorizontalOptions="Fill" Margin="50,100" VerticalOptions="Start">
	<Label Text="Login" FontSize="Large"/>
	<Entry Placeholder="Username" Text="{Binding UserName}"/>
	<Entry Placeholder="Password" IsPassword="True" Text="{Binding Password}"/>
	<Label Text=""/>
	<Button Text="Login"/>
	</StackLayout>
	</ContentPage.Content>
	</ContentPage>

view raw LoginPage.xaml hosted with ❤ by GitHub

Normal Property Creation with Data Binding

The Data Binding is the connection between the XAML and ViewModel of your app. You use it to connect a control (ex: text) of your UI to your logic. Let’s assume that we have an Entry and we want to link it to our ViewModel.

Following code is as we use to create the view model property be like.

LoginPage.Xaml.cs

	public class LoginPageViewModel: INotifyPropertyChanged
	{
	#region Fields
	private string _username;
	#endregion
	#region Constructor
	public LoginPageViewModel()
	{
	}
	#endregion
	#region Properties
	public event PropertyChangedEventHandler PropertyChanged;
	public string UserName {
	get { return _username; }
	set {
	_username = value;
	OnPropertyChanged(nameof(UserName));
	}
	}
	#endregion
	#region Methods
	protected void OnPropertyChanged(string name)
	{
	PropertyChangedEventHandler handler = PropertyChanged;
	if (handler != null)
	{
	handler(this, new PropertyChangedEventArgs(name));
	}
	}
	#endregion
	}

view raw LoginPage.xaml.cs hosted with ❤ by GitHub

Result

Install Fody

Install the following Nuget from Nuget Manager In your Visual Studio.

1. Fody
2. PropertyChanged.Fody

In your project install the Fody plugin (only in your PCL/.Net Standard project, you don’t need to install it in the Android, iOS, UWP projects

Fody NuGet

PropertyChanged.Fody

Add FodyWeavers.xml

Add FodyWeavers.xml file in your .net standard project.

	<?xml version="1.0" encoding="utf-8"?>
	<Weavers xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="FodyWeavers.xsd">
	<PropertyChanged />
	</Weavers>

view raw FodyWeavers.xml hosted with ❤ by GitHub

Fody Properties

Now, Simply add property with { get; set; }. 

	public class LoginPageViewModel: INotifyPropertyChanged
	{
	//Fody
	public string Password { get; set; }
	}

view raw LoginPage.xaml.cs hosted with ❤ by GitHub

Result

Conclusion

I hope you have understood how to use fody in MVVM in Xamarin.Forms App

Thanks for reading. Please share your comments and feedback. Happy Coding :)