Behaviors · FmgLib.MauiMarkup

Behaviors add reusable functionality to controls without subclassing them. FmgLib.MauiMarkup attaches them fluently through the Behaviors(...) method available on every VisualElement.

Attaching a Behavior#

new Entry()
    .Text("Click Item")
    .Behaviors(new YourCustomBehavior());

Multiple behaviors in one call:

new Entry()
    .Placeholder("E-mail")
    .Behaviors(
        new EmailValidationBehavior(),
        new MaxLengthBehavior(64)
    );

Writing a Custom Behavior#

A behavior derives from Behavior<T> and overrides attach/detach hooks:

public class NumericValidationBehavior : Behavior<Entry>
{
    protected override void OnAttachedTo(Entry entry)
    {
        entry.TextChanged += OnTextChanged;
        base.OnAttachedTo(entry);
    }

    protected override void OnDetachingFrom(Entry entry)
    {
        entry.TextChanged -= OnTextChanged;
        base.OnDetachingFrom(entry);
    }

    private void OnTextChanged(object? sender, TextChangedEventArgs e)
    {
        bool isValid = double.TryParse(e.NewTextValue, out _);
        ((Entry)sender!).TextColor = isValid ? Colors.Black : Colors.Red;
    }
}

Usage:

new Entry()
    .Placeholder("Amount")
    .Keyboard(Keyboard.Numeric)
    .Behaviors(new NumericValidationBehavior());

Behavior with Bindable Configuration#

Behaviors are BindableObjects, so they can expose bindable properties — and since the source generator can create fluent methods for any BindableObject, you can even give your behavior its own fluent API:

public class MinLengthBehavior : Behavior<Entry>
{
    public static readonly BindableProperty MinLengthProperty =
        BindableProperty.Create(nameof(MinLength), typeof(int), typeof(MinLengthBehavior), 6);

    public int MinLength
    {
        get => (int)GetValue(MinLengthProperty);
        set => SetValue(MinLengthProperty, value);
    }

    protected override void OnAttachedTo(Entry entry) =>
        entry.TextChanged += (s, e) =>
            entry.BackgroundColor = (e.NewTextValue?.Length ?? 0) >= MinLength
                ? Colors.Transparent
                : Colors.MistyRose;
}

// opt the behavior into fluent generation:
[MauiMarkup(typeof(MinLengthBehavior))]
public class MarkupTargets { }

// then:
new Entry()
    .Behaviors(new MinLengthBehavior().MinLength(8));

Community Toolkit Behaviors#

CommunityToolkit.Maui ships many ready-made behaviors; they attach exactly the same way:

new Entry()
    .Placeholder("E-mail")
    .Behaviors(
        new CommunityToolkit.Maui.Behaviors.EmailValidationBehavior
        {
            InvalidStyle = invalidStyle,
            ValidStyle = validStyle
        }
    );

(If you want fluent methods for toolkit behaviors too, register them with [MauiMarkup(typeof(EmailValidationBehavior))] — see Third-Party Controls.)

Behavior vs. Alternatives#

Need	Best tool
Reusable, self-contained control logic (validation, masking, throttling)	Behavior
One-off event reaction on a single control	`On<Event>` lambda
Declarative property change on a condition	Triggers
Visual reaction to control states (focused, disabled…)	Visual States

Hot Reload Note#

Behaviors attach to the controls created inside Build(); on hot reload the controls are rebuilt and behaviors re-attach naturally. Keep behaviors stateless or reinitialize state in OnAttachedTo so rebuilds stay safe.