AvaloniaProperty default value factory

Is your feature request related to a problem? Please describe. It should be possible to set default value factory.

Describe the solution you’d like

public static StyledProperty<TValue> Register<TOwner, TValue>(string name, Func<TValue> defaultValueFactory, bool inherits = false, BindingMode defaultBindingMode = BindingMode.OneWay, Func<TValue, bool> validate = null, Func<IAvaloniaObject, TValue, TValue> coerce = null, Action<IAvaloniaObject, bool> notifying = null) where TOwner : IAvaloniaObject;
public static AttachedProperty<TValue> RegisterAttached<TOwner, THost, TValue>(string name, Func<TValue> defaultValueFactory, bool inherits = false, BindingMode defaultBindingMode = BindingMode.OneWay, Func<TValue, bool> validate = null, Func<IAvaloniaObject, TValue, TValue> coerce = null) where THost : IAvaloniaObject;
public static AttachedProperty<TValue> RegisterAttached<THost, TValue>(string name, Type ownerType, Func<TValue> defaultValueFactory, bool inherits = false, BindingMode defaultBindingMode = BindingMode.OneWay, Func<TValue, bool> validate = null, Func<IAvaloniaObject, TValue, TValue> coerce = null) where THost : IAvaloniaObject;

Additional context WPF doesn’t have this feature. But UWP does: CreateDefaultValueCallback Delegate

Property = DependencyProperty.Register("MyProperty",
    propertyType, ownerType,
    PropertyMetadata.Create(createDefaultValue));

Possible use cases

1. Storing mutable objects inside of property, and default value should be new for each avalonia object having it - https://devblogs.microsoft.com/oldnewthing/20191002-00/?p=102950 (actually it’s a pretty BAD idea, and this topic describes why).
2. When default value is read from global settings class or any other static source that can be mutated, and default value should be recalculated.

[grokys]

The UWP implementation has a serious problem: https://devblogs.microsoft.com/oldnewthing/20191002-00/?p=102950

It may be possible for Avalonia to avoid that problem, but it’s a bit tricky.

EDIT: Oh I just noticed you already linked to that, sorry, Anyway I think that unless we can avoid the problem described there I’d be 👎 on adding default value factories.

[robloo]

I would be willing to bet Microsoft has changed the intended use/functionality of this. Originally, it might have been a full factory created to facilitate instantiating objects used on the UI thread from another thread. Then they did some backtracking and clarified in the documents it should only be used for the very narrow case. That could explain why everyone has different impressions and tests dont always match the docs. It could also be that it never worked correctly so they narrowed scope to the only tested and validated use case.

Reguardless, treating this as a factory in Avalonia would be great. It could be used to instantiate collections for each dependency object (equivalent in Avalonia) without doing so in the constructor which is cleaner IMO.