UsingValidation

Xamarin.Forms validation sample using INotifyDataErrorInfo

This post is also available in the <a href="https://blogs.msdn.microsoft.com/premier_developer/2017/04/03/validate-input-in-xamarin-forms-using-inotifydataerrorinfo-custom-behaviors-effects-and-prism/" target="_blank">Premier Developer blog</a>. I have recently been investigating the support available in Xamarin.Forms for validation and, in particular, researched the possibility of using INotifyDataErrorInfo to complement the traditional approach of using Behaviors. In simple scenarios, it's possible to perform validation by simply attaching a Behavior to the required view, as shown by the following sample code: <pre title="Sample validation Behavior" class="lang:default decode:true ">public class SampleValidationBehavior:Behavior<Entry> { protected override void OnAttachedTo(Entry entry) { entry.TextChanged += OnEntryTextChanged; base.OnAttachedTo(entry); } protected override void OnDetachingFrom(Entry entry) { entry.TextChanged -= OnEntryTextChanged; base.OnDetachingFrom(entry); } void OnEntryTextChanged(object sender, TextChangedEventArgs args) { var textValue = args.NewTextValue; bool isValid = !string.IsNullOrEmpty(textValue) && textValue.Length >= 5; ((Entry)sender).TextColor = isValid ? Color.Default : Color.Red; } } </pre>

In this case, we are validating the input and modifying the UI if the number of characters entered is less than 5.

What about more articulated scenarios when multiple business rules are required to be checked for the input values?

In these cases, we could take advantage of other types available to make our code more structured and extensible:

<ul> <li>INotifyDataErrorInfo: available in Xamarin.Forms. When implemented it permits specifying custom validation supporting multiple errors per property, cross-property errors and entity-level errors;</li> <li>DataAnnotations decorate the data models using attributes which specify validation conditions to be applied to the specific field;</li> <li>Forms Behaviors specify the specific UI to be applied to the specific validation scenarios, integrating with INotifyDataErrorInfo and DataAnnotations.</li> </ul> To start exploring this approach, I created a new Xamarin.Forms Prism solution using the <a href="https://marketplace.visualstudio.com/items?itemName=BrianLagunas.PrismTemplatePack" target="_blank">Prism Template Pack</a> which generated the following project structure: <img class="size-medium wp-image-8095 aligncenter" src="https://davidezordan.github.io/wp-content/uploads/2017/03/Capture-1-300x150.png" alt="" width="300" height="150" />

Then, I added the following new model to be validated using DataAnnotations and INotifyDataErrorInfo:

<pre title="DataAnntotations" class="lang:default decode:true ">using System.ComponentModel.DataAnnotations; using System.Runtime.CompilerServices; using UsingValidation.Validation; namespace UsingValidation.Models { public class Item : ValidationBase { public Item() { Name = string.Empty; Description = string.Empty; } private string _name; private string _description; [Required(ErrorMessage = "Name cannot be empty!")] public string Name { get { return _name; } set { ValidateProperty(value); SetProperty(ref _name, value); } } [Required(ErrorMessage = "Description cannot be empty!")] [RegularExpression(@"\w{5,}", ErrorMessage = "Description: more than 4 letters/numbers required")] public string Description { get { return _description; } set { ValidateProperty(value); SetProperty(ref _description, value); } } protected override void ValidateProperty(object value, [CallerMemberName] string propertyName = null) { base.ValidateProperty(value, propertyName); OnPropertyChanged("IsSubmitEnabled"); } public bool IsSubmitEnabled { get { return !HasErrors; } } } }</pre>

The model uses attributes declared in the SystemComponentModel.DataAnnotations namespace which can be referenced in the solution modifying the Portable Class Library profile of the UsingValidation common project:

Quick tip: to be able to change the PCL profile I had to remove all the NuGet packages used by the common project, remove the Windows Phone 8 profile and then add back all the removed NuGet packages to the UsingValidation PCL.

To use the capability offered by INotifyDataErrorInfo, the model needs to implements 3 members defined in the interface:

<ul> <li>GetErrors() returns an IEnumerable sequence of strings containing the error messages triggered by validation;</li> <li>the HasErrors property returns a boolean value indicating if there are validation errors;</li> <li>ErrorsChanged event can be triggered to Notify if the validation errors have been updated.</li> </ul> This interface is quite flexible and is designed to be customised depending on the different scenarios needed: I took as a starting point <a href="https://github.com/MaulikParmar/XamarinForms/tree/master/ValidationDemo" target="_blank">this implementation</a> available on GitHub and modified it accordingly: I decided to separate the implementation of INotifyDataErrorInfo in a different base class called ValidationBase which contains the following code using a Dictionary<string, List<string>> needed for storing the generated validation errors: <pre title="ValidationBase class" class="lang:default decode:true">public class ValidationBase : BindableBase, INotifyDataErrorInfo { private Dictionary<string, List<string>> _errors = new Dictionary<string, List<string>>();

public ValidationBase()
{
    ErrorsChanged += ValidationBase_ErrorsChanged;
}

private void ValidationBase_ErrorsChanged(object sender, DataErrorsChangedEventArgs e)
{
    OnPropertyChanged("HasErrors");
    OnPropertyChanged("ErrorsList");
}

#region INotifyDataErrorInfo Members

public event EventHandler&lt;DataErrorsChangedEventArgs&gt; ErrorsChanged;

public IEnumerable GetErrors(string propertyName)
{
	if (!string.IsNullOrEmpty(propertyName))
	{
		if (_errors.ContainsKey(propertyName) &amp;&amp; (_errors[propertyName].Any()))
		{
		   return _errors[propertyName].ToList();
		}
		else
		{
		   return new List&lt;string&gt;();
		}
	}
	else
	{
	   return _errors.SelectMany(err =&gt; err.Value.ToList()).ToList();
	}
}

public bool HasErrors
{
    get
    {
    return _errors.Any(propErrors =&gt; propErrors.Value.Any());
    }
}

#endregion

</pre> The validation is performed by this method which evaluates the DataAnnotations decorating the model using the Validator available in the System.ComponentModel.DataAnnotations namespace and then stores the error messages in the dictionary: <pre title="Performing validation" class="lang:default decode:true ">protected virtual void ValidateProperty(object value, [CallerMemberName] string propertyName = null) { var validationContext = new ValidationContext(this, null) { MemberName = propertyName };

var validationResults = new List&lt;ValidationResult&gt;();
Validator.TryValidateProperty(value, validationContext, validationResults);
RemoveErrorsByPropertyName(propertyName);

HandleValidationResults(validationResults);

}</pre> At this stage, I needed a solution for linking the UI to the model, and modifying the visuals depending on the presence or not of validation errors.

The ViewModel for the sample page, contains only a property storing an instance of the item defined in the model:

<pre title="MainPage ViewModel definition" class="lang:default decode:true ">public class MainPageViewModel : BindableBase, INavigationAware { public MainPageViewModel() { DemoItem = new Item(); } private Item _item; public Item DemoItem { get { return _item; } set { SetProperty(ref _item, value); } } public void OnNavigatedFrom(NavigationParameters parameters) { } public void OnNavigatedTo(NavigationParameters parameters) { DemoItem.Name = string.Empty; DemoItem.Description = string.Empty; } }</pre>

Then, the corresponding XAML contains two Entry views used for input and a ListView used for showing the validation errors:

<pre title="MainPage XAML definition" class="lang:default decode:true "><?xml version="1.0" encoding="utf-8" ?> <ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" xmlns:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms" xmlns:validation="clr-namespace:UsingValidation.Validation" xmlns:effects="clr-namespace:UsingValidation.Effects" prism:ViewModelLocator.AutowireViewModel="True" x:Class="UsingValidation.V

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UsingValidation