Avalonia/docs/spec/defining-properties.md

Defining Properties

If you are creating a control, you will want to define properties on your control. The process in Avalonia is broadly similar to other XAML languages with a few differences - the main one being that Perpsex's equivalent of DependencyProperty is called StyledProperty.

Registering Styled Properties

A styled property is analogous to a DependencyProperty in other XAML frameworks.

You register a styled property by calling AvaloniaProperty.Register and storing the result in a static readonly field. You then create a standard C# property to access it.

Here's how the Border control defines its Background property:

    public static readonly StyledProperty<Brush> BackgroundProperty =
        AvaloniaProperty.Register<Border, Brush>(nameof(Background));

    public Brush Background
    {
        get { return GetValue(BackgroundProperty); }
        set { SetValue(BackgroundProperty, value); }
    }

The AvaloniaProperty.Register method also accepts a number of other parameters:

• defaultValue: This gives the property a default value. Be sure to only pass value types and immutable types here as passing a reference type will cause the same object to be used on all instances on which the property is registered.
• inherits: Specified that the property's default value should come from the parent control.
• defaultBindingMode: The default binding mode for the property. Can be set to OneWay, TwoWay, OneTime or OneWayToSource.
• validate: A validation/coercion function of type Func<TOwner, TValue, TValue>. The function accepts the instance of the class on which the property is being set and the value and returns the coerced value or throws an exception for an invalid value.

Using a StyledProperty on Another Class

Sometimes the property you want to add to your control already exists on another control, Background being a good example. To register a property defined on another control, you call StyledProperty.AddOwner:

    public static readonly StyledProperty<Brush> BackgroundProperty =
        Border.BackgroundProperty.AddOwner<Panel>();

    public Brush Background
    {
        get { return GetValue(BackgroundProperty); }
        set { SetValue(BackgroundProperty, value); }
    }

Note: Unlike WPF, a property must be registered on a class otherwise it cannot be set on an object of that class.

Attached Properties

Attached properties are defined almost identically to styled properties except that they are registered using the RegisterAttached method and their accessors are defined as static methods.

Here's how Grid defines its Grid.Column attached property:

    public static readonly AttachedProperty<int> ColumnProperty =
        AvaloniaProperty.RegisterAttached<Grid, Control, int>("Column");

    public static int GetColumn(Control element)
    {
        return element.GetValue(ColumnProperty);
    }

    public static void SetColumn(Control element, int value)
    {
        element.SetValue(ColumnProperty, value);
    }

Readonly AvaloniaProperties

To create a readonly property you use the AvaloniaProperty.RegisterDirect method. Here is how Visual registers the readonly Bounds property:

    public static readonly DirectProperty<Visual, Rect> BoundsProperty =
        AvaloniaProperty.RegisterDirect<Visual, Rect>(
            nameof(Bounds),
            o => o.Bounds);

    private Rect _bounds;

    public Rect Bounds
    {
        get { return _bounds; }
        private set { SetAndRaise(BoundsProperty, ref _bounds, value); }
    }

As can be seen, readonly properties are stored as a field on the object. When registering the property, a getter is passed which is used to access the property value through GetValue and then SetAndRaise is used to notify listeners to changes to the property.

Direct AvaloniaProperties

As its name suggests, RegisterDirect isn't just used for registering readonly properties. You can also pass a setter to RegisterDirect to expose a standard C# property as a Avalonia property.

A StyledProperty which is registered using AvaloniaProperty.Register maintains a prioritized list of values and bindings that allow styles to work. However, this is overkill for many properties, such as ItemsControl.Items - this will never be styled and the overhead involved with styled properties is unnecessary.

Here is how ItemsControl.Items is registered:

    public static readonly DirectProperty<ItemsControl, IEnumerable> ItemsProperty =
        AvaloniaProperty.RegisterDirect<ItemsControl, IEnumerable>(
            nameof(Items),
            o => o.Items,
            (o, v) => o.Items = v);

    private IEnumerable _items = new AvaloniaList<object>();

    public IEnumerable Items
    {
        get { return _items; }
        set { SetAndRaise(ItemsProperty, ref _items, value); }
    }

Direct properties are a lightweight version of styled properties that support the following:

• AvaloniaObject.GetValue
• AvaloniaObject.SetValue for non-readonly properties
• PropertyChanged
• Binding (only with LocalValue priority)
• GetObservable
• AddOwner
• Metadata

They don't support the following:

• Validation/Coercion (although this could be done in the property setter)
• Overriding default values.
• Inherited values

Using a DirectProperty on Another Class

In the same way that you can call AddOwner on a styled property, you can also add an owner to a direct property. Because direct properties reference fields on the control, you must also add a field for the property:

    public static readonly DirectProperty<MyControl, IEnumerable> ItemsProperty =
        ItemsControl.ItemsProperty.AddOwner<MyControl>(
            o => o.Items,
            (o, v) => o.Items = v);

    private IEnumerable _items = new AvaloniaList<object>();

    public IEnumerable Items
    {
        get { return _items; }
        set { SetAndRaise(ItemsProperty, ref _items, value); }
    }

When to use a Direct vs a Styled Property

Direct properties have advantages and disadvantages:

Pros:

• No additional object is allocated per-instance for the property
• Property getter is a standard C# property getter
• Property setter is is a standard C# property setter that raises an event.

Cons:

• Cannot inherit value from parent control
• Cannot take advantage of Avalonia's styling system
• Property value is a field and as such is allocated whether the property is set on the object or not

So use direct properties when you have the following requirements:

• Property will not need to be styled
• Property will usually or always have a value