behaviors.md

Behaviors

In Sharp.UI, you can add functionality to user interface controls using behaviors. Behaviors allow you to add functionality to controls without having to subclass them.

You can add a behavior to a control by using the Behaviors method and passing in an instance of the behavior class. For example:

new Entry("Enter text...", out var entry).Text("")
  .Behaviors(new NumericValidationBehavior());

The behavior class, NumericValidationBehavior, implements the OnAttachedTo and OnDetachingFrom methods to subscribe and unsubscribe to the TextChanged event of the Entry control. The behavior also defines the OnEntryTextChanged method, which is triggered when the text in the Entry control changes. This method validates if the text is a valid number, and sets the text color accordingly.

public class NumericValidationBehavior : 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)
    {
        double result;
        bool isValid = double.TryParse(args.NewTextValue, out result);
        ((Entry)sender).TextColor = isValid ? Colors.Green : Colors.Red;
    }
}

Attached Behaviors

If you want to define a behavior using a style, you can use attached properties. You can create attached behaviors using the AttachedBehavior<TView, TBehavior> class.

For example:

public class NumericValidationStyleBehavior : AttachedBehavior<Entry, NumericValidationStyleBehavior>
{
    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)
    {
        double result;
        bool isValid = double.TryParse(args.NewTextValue, out result);
        ((Entry)sender).TextColor = isValid ? Colors.Green : Colors.Red;
    }
}

You can then apply the attached behavior using a style in your ContentPage:

public class AttachedBehaviorPage : ContentPage
{
    public AttachedBehaviorPage()
    {
        Resources = new ResourceDictionary
        {
            new Style<Entry>
            {                
                NumericValidationStyleBehavior.Enable(true)
            }
        };

        Content = new VStack
        {
            new Entry("Enter text...", out var entry).Text("")
        }
        .CenterVertically());
    }
}