[Proposal] Add RelativeBindingSource Support to Typed Bindings Extensions

[brminnick]

Feature name

Add RelativeBindingSource Support to Typed Bindings

Link to discussion

None

Progress tracker

• Android Implementation
• iOS Implementation
• MacCatalyst Implementation
• Windows Implementation
• Tizen Implementation
• Unit Tests
• Samples
• Documentation

Summary

This Proposal improves support for RelativeBindingSource to our TypedBinding extensions.

This is not a breaking change and improves our API surface to better match .NET MAUI's TypedBinding API for its TypedBinding.Source property.

RelativeBindingSource is the recommended way for .NET MAUI developers to creating a binding referencing themselves or an ancestor without needing to pass in an instance of that object

• Self Binding
  • RelativeBindingSource.Self
• Binding to an ancestor in the UI hierarchy
  • new RelativeBindingSource(RelativeBindingSourceMode.FindAncestor, typeof(Page)))
• Binding to an ancestor in the BindingContext hierarchy
  • new RelativeBindingSource(RelativeBindingSourceMode.FindAncestorBindingContext, typeof(MyViewModel))

Current Workaround

The current workaround to use RelativeBindingSource with our existing TypedBinding APIs is to first use .Assign() to assign the control to a variable, then pass that variable in as the source parameter.

.Assign(out CarouselView carouselView)
.Bind(CarouselView.CurrentItemChangedCommandParameterProperty, static (CarouselView view) => view.CurrentItem, source: carouselView)

Motivation

All of our TypedBinding APIs (example below) require the source parameter type to match the TBindingContext used in by the getter and setter parameter:

• Expression<Func<TBindingContext, TSource>> getter
• Action<TBindingContext, TSource>? setter = null
• TBindingContext? source = default // Cannot be of type RelativeBindingSource

public static TBindable Bind<TBindable, TBindingContext, TSource>(
		this TBindable bindable,
		BindableProperty targetProperty,
		Expression<Func<TBindingContext, TSource>> getter,
		Action<TBindingContext, TSource>? setter = null,
		BindingMode mode = BindingMode.Default,
		string? stringFormat = null,
		TBindingContext? source = default) where TBindable : BindableObject);

Detailed Design

The best way to support RelativeBindingSource, is to lower the Type of the API's source parameter from TBindingContext? to object?.

Using object? for the source parameter matches the .NET MAUI API for the TypedBinding.Source property.

Current Source Parameter

TBindingContext? source = default

Updated Source Parameter

object? source = null

Example Updated API

/// <summary>Bind to a specified property</summary>
public static TBindable Bind<TBindable, TBindingContext, TSource>(
  this TBindable bindable,
  BindableProperty targetProperty,
  Expression<Func<TBindingContext, TSource>> getter,
  Action<TBindingContext, TSource>? setter = null,
  BindingMode mode = BindingMode.Default,
  string? stringFormat = null,
  object? source = null) // Lowered from TBindingContext? source = default
where TBindable : BindableObject;

Usage Syntax

.Bind(CarouselView.CurrentItemChangedCommandParameterProperty, static (CarouselView view) => view.CurrentItem, source: RelativeBindingSource.Self)

Drawbacks

I don't see any drawbacks.

We will need to update our Unit Tests to validate RelativeBindingSource.

We will also need to update our Unit Tests with invalid source parameters to ensure .NET MAUI throws an exception if a user provides an invalid source parameter to our API now that it supports any object type.

Alternatives

An alternative to lowering our existing API surface from TBindingContext? source = default to object? source = null would be to create new APIs specific to RelativeBindingSource (example below).

I do not recommend this approach for two reasons:

• It doubles our API surface for TypedBinding support, increasing our maintenance cost
  • Requires one TypedBinding API for TBindingContext? source = default and a second TypedBinding API for object? source = null
• Lowering the parameter Type to object? better matches .NET MAUI's TypedBinding.Source API (public object? Source)

Example API using RelativeBindingSource

public static TBindable Bind<TBindable, TBindingContext, TSource>(
		this TBindable bindable,
		BindableProperty targetProperty,
		Expression<Func<TBindingContext, TSource>> getter,
		Action<TBindingContext, TSource>? setter = null,
		BindingMode mode = BindingMode.Default,
		string? stringFormat = null,
		RelativeBindingSource? source = null) where TBindable : BindableObject);

Unresolved Questions

None