[Spec] Implement LabeledBy property for accessibility

[rachelkang]

Implement LabeledBy property for accessibility

Recreate a property that is reminiscent of the Xamarin.Forms AutomationProperties.LabeledBy property.

And make sure there is an implementation for iOS.

API

SemanticProperties.LabeledBy

Properties

API	Description
LabeledBy	iOS: custom implementation, Android: labelFor

Implementation Details

As with AutomationProperties.LabeledBy, the API and behavior will be very much the same.

The AutomationProperties.LabeledBy attached property allows another element to define accessibility information for the current element. For example, a Label next to an Entry can be used to describe what the Entry represents. This can be accomplished in XAML as follows:

<Label x:Name="label" Text="Enter your name: " />
<Entry AutomationProperties.IsInAccessibleTree="true"
       AutomationProperties.LabeledBy="{x:Reference label}" />

Alternatively, it can be set in C# as follows:

var nameLabel = new Label { Text = "Enter your name: " };
var entry = new Entry();
AutomationProperties.SetIsInAccessibleTree(entry, true);
AutomationProperties.SetLabeledBy(entry, nameLabel);

On Android, it will get mapped down to the android:labelFor attribute, also as before.

On iOS, there is unfortunately no platform equivalent (hence why this property was not previously implemented on XF). However, we will pull some magic strings to make sure .NET MAUI supports this on iOS as well.

Related: #469

[rachelkang]

Closing as there are no current plans to implement this property. LabeledBy is only built into Android and WinUI, and the difference in behavior it provides, when compared against other screen reader APIs (i.e. contentDescription) was found to be negligible. If needed in the future, handlers may be leveraged.