Remove BindablePropertyAttribute.DefaultValue

[TheCodeTraveler]

Description of Change

This PR removes the DefaultValue property from [BindableProperty] for the following reasons now that it is not longer required thanks to #2987:

1. Setting the DefaultValue has been the largest source of pain and workarounds in BindablePropertySourceGenerator
   • You can see how the helper method creating the generated code continues to grow every time we discover a new not-yet-supported edge case
   • We can now remove that nasty helper method, saving maintenance costs and avoid future bugs from yet-to-be encountered edge cases
   • Initializers already support every edge case
2. The DefaultValue property only supported compile-time constants, while partial property initializer can use static readonly values
   • For example, [BindableProperty(DefaultValue = Colors.Transparent)] would generate a compiler error because Colors.Transparent is public static readonly
3. Improve the developer experience
   • It's a bit confusing to give developers two ways to initialize / set the default value of the bindable property
   • If we keep BindablePropertyAttribute.DefaultValue, we would need to write an additional Analyzer to generate a compiler error when we detect a developer is using both DefaultValue and the partial property initializer

PR Checklist

• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard

Additional information

TouchBehavior Refactoring

I refactored TouchBehavior's properties to be null by default so that we can determine whether or not a developer as set each property.

I was forced into this refactor because the underlying GestureManager was relying on BindableProeprety.IsSet to determine whether or not a developer had set a value. This logic only works with BindableProperty.DefaultValue. Now that partial property initializers use DefaultValueCreator, BindableProperty.IsSet always returned false until .NET MAUI only calls DefaultValueCreator when the BindableProperty is first accessed (e.g. BindableProperty.GetValue(DefaultOpacityProperty).

This refactor of TouchBehavior should not cause any breaking changes for developers, as noted by the fact that I did not need to modify any of the following code:

• TouchBehaviorPage
• TouchBehaviorTests
  • (other than testing the Default Values of its properties)

ImageTouchBehavior Refactoring

I also refactored ImageTouchBehavior (same reasons as TouchBehavior).

This refactor required adding an OnPropertyChanged method for DefaultImageSource, HoveredImageSource and PressedImageSource to support the scenario where the ImageSource? was set to null when the image was active.

This refactor of TouchBehavior should not cause any breaking changes for developers, as noted by the fact that I did not need to modify any of the following code:

• ImageTouchBehaviorPage
• ImageTouchBehaviorTests
  • I added additional Assertions to ImageTouchBehaviorTests .VerifyImageSourceStateMachine
  • I created ImageTouchBehaviorTests .VerifyImageSourceStateMachineWhenImageSourceSetToNullWhilstActive
  • I modified the DefaultValues test

[ne0rrmatrix]

Do we want to add tests for most common system types that we will see used? Like DateTime and others? Or add them as we update our sample app?

[TheCodeTraveler]

Do we want to add tests for most common system types that we will see used? Like DateTime and others?

@ne0rrmatrix Done! I've added these to this Unit Test: GenerateBindableProperty_WithComplexDefaultValues_GeneratesCorrectCode

[ne0rrmatrix]

This was amazing. Learned a few new things reading this PR. It is exactly as discussed in this months meeting and is exactly what we are looking for. Very good job. I have just that one question about how we are handling touch behavior null issue that I mentioned. If you feel it is a non issue I think we can go ahead and merge this. I am excited to see this one get added!

It fixes the one issue I was struggling with and solves a few others with regards to default values.