[BindableProperty] source generators to support partial properties

[stephenquan]

Split from #458 and related to CommunityToolkit.Mvvm 8.4.0 partial property support https://devblogs.microsoft.com/dotnet/announcing-the-dotnet-community-toolkit-840/#partial-properties-support-for-the-mvvm-toolkit-🎉

I've been investigating applying partial property support to BindableProperty and have come up with the following pattern:

/// <summary>
/// CardView developer implementation with proposed partial property with [BindableProperty] attribute.
/// </summary>
public partial class CardView : ContentView
{
    /// <summary>Gets or sets the Card's title.</summary>
    [BindableProperty] public partial string CardTitle { get; set; } = string.empty;
}

// [BindableProperty] source-generated output for the above CardTitle property.
public partial class CardView : ContentView
{
    /// <summary>Bindable property for <see cref="CardTitle"/>.</summary>
    public static readonly BindableProperty CardTitleProperty = BindableProperty.Create(nameof(CardTitle), typeof(string), typeof(CardView),
        defaultValueCreator: (b) => ((CardView)b).OnCardTitleCreateDefaultValue()
    );

    bool IsCardTitleCreatingDefaultValue = false;

    // N.B. xmldoc deliberately omitted. Developer xmldoc is already supplied in the developer's implementation.
    public partial string CardTitle
    {
        get => IsCardTitleCreatingDefaultValue ? field : (string)GetValue(CardView.CardTitleProperty);
        set => SetValue(CardView.CardTitleProperty, field = value);
    }

    object OnCardTitleCreateDefaultValue()
    {
        // The partial property initializer will be used as the BindableProperty default value.
        IsCardTitleCreatingDefaultValue = true;
        object result = CardTitle; 
        IsCardTitleCreatingDefaultValue = false;
        return result;
    }
}

The advantages of the partial property syntax are:

• It is consistent with the new syntax introduced in CommunityToolkit.Mvvm 8.4.0
• It works better with compiled-binding
• It solves an xmldoc problem
• It solves an initializer problem (complex initializers are easily supported)
• The modifiers for the property, as well as the get and set methods, can be set (e.g. public, internal, or private)

Note

In order to use support for partial properties, C# preview is required. You can enable this by adding <LangVersion>preview</LangVersion> to your .csproj file, or to any imported .props file (eg. Directory.Build.props). This is necessary because the generated code makes use of the field keyword.

I have a working implementation on:

• GitHub: https://github.com/stephenquan/SQuan.Helpers.Maui
• Wiki: https://github.com/stephenquan/SQuan.Helpers.Maui/wiki/MVVM
• NuGet: https://www.nuget.org/packages/SQuan.Helpers.Maui.Mvvm

[TheCodeTraveler]

Love this!