Advanced scenarios

Sharp.UI provides two attributes, PropertyCallbacks and DefaultValue, to allow developers to customize properties and their behavior.

PropertyCallbacks Attribute

The PropertyCallbacks attribute allows you to specify four different callbacks for a property: propertyChanged, validateValue, coerceValue, and defaultValueCreator.

• propertyChanged callback is invoked when the value of the property changes.
• validateValue callback is used to validate the value of the property before it is set.
• coerceValue callback is used to adjust the value of the property, for example, to ensure that it falls within a certain range.
• defaultValueCreator callback is used to create the default value for the property.

DefaultValue Attribute

The DefaultValue attribute allows you to specify the default value for a property. This attribute provides a convenient way to set a default value for a property without having to create a defaultValueCreator callback.

Example

[BindableProperties]
public interface IAngleViewModelProperties
{
    [PropertyCallbacks(propertyChanged: "OnAngleChanged")]
    public double RawAngle { get; set; }

    [PropertyCallbacks(coerceValue: "CoerceAngle")]
    public double Angle { get; set; }

    [DefaultValue(360.0)]
    public double MaximumAngle { get; set; }
}

Class declaration

[SharpObject]
public partial class AngleViewModel : BindableObject, IAngleViewModelProperties
{
    static void OnAngleChanged(BindableObject bindable, object oldValue, object newValue)
    {
        var viewModel = bindable as AngleViewModel;
        viewModel.Angle = (double)newValue;
    }

    static object CoerceAngle(BindableObject bindable, object value)
    {
        var viewModel = bindable as AngleViewModel;
        double input = (double)value;

        if (input > viewModel.MaximumAngle)
            input = viewModel.MaximumAngle;

        return input;
    }
}

Usage

public class AngleViewModelPage : ContentPage
{
    AngleViewModel viewModel = new AngleViewModel();

    public AngleViewModelPage()
    {
        this.BindingContext = viewModel;
        this.Content = new VStack
        {
            e => e.VerticalOptions(LayoutOptions.Center),

            new Slider(0, 500)
                .Value(e => e.Path("RawAngle")),

            new Label()
                .Text(e => e.Path("RawAngle").StringFormat("Raw angle {0}"))
                .FontSize(30),

            new Label()
                .Text(e => e.Path("Angle").StringFormat("Angle {0}"))
                .Rotation(e => e.Path("Angle"))
                .FontSize(30)

        };
    }
}