Compiled Binding in Code

Bindings written in code typically use string paths that are resolved at runtime with reflection, and the overhead of doing this varies from platform to platform. FmgLib.MauiMarkup introduces an additional Getter extension method that defines bindings using a Func argument instead of a string path:

// in before
new Label().Text(e => e.Path("Text"));

// in now
new Label().Text(e => e.Getter(static (Entry entry) => entry.Text));

This compiled binding approach provides the following benefits:

• Improved data binding performance by resolving binding expressions at compile-time rather than runtime.
• A better developer troubleshooting experience because invalid bindings are reported as build errors.
• Intellisense while editing.

Not all methods can be used to define a compiled binding. The expression must be a simple property access expression. The following examples show valid and invalid binding expressions:

// Valid: Property access
static (PersonViewModel vm) => vm.Name;
static (PersonViewModel vm) => vm.Address?.Street;

// Valid: Array and indexer access
static (PersonViewModel vm) => vm.PhoneNumbers[0];
static (PersonViewModel vm) => vm.Config["Font"];

// Valid: Casts
static (Label label) => (label.BindingContext as PersonViewModel).Name;
static (Label label) => ((PersonViewModel)label.BindingContext).Name;

// Invalid: Method calls
static (PersonViewModel vm) => vm.GetAddress();
static (PersonViewModel vm) => vm.Address?.ToString();

// Invalid: Complex expressions
static (PersonViewModel vm) => vm.Address?.Street + " " + vm.Address?.City;
static (PersonViewModel vm) => $"Name: {vm.Name}";

You can easily use multibinding with FmgLib.MauiMarkup. You can add as many BindingBases as you want with the e.Bindings(...) method. In addition, .NET MAUI 9 adds a BindingBase.Create method that sets the binding directly on the object with a Func, and returns the binding object instance:

public partial class MainPage : ContentPage, IFmgLibHotReload
{
    private readonly MainPageViewModel viewModel;
    public MainPage()
    {
        viewModel = new MainPageViewModel();
        this.InitializeHotReload();
    }

    public void Build()
    {
        this
        .BindingContext(viewModel)
        .Content(
            new VerticalStackLayout()
            .Spacing(20)
            .Children(
                new CheckBox()
                .IsChecked(e => 
                    e.Bindings(
			Binding.Create(static (MainPageViewModel model) => model.IsOver16, source: RelativeBindingSource.Self),
		        Binding.Create(static (MainPageViewModel model) => model.HasPassedTest, source: RelativeBindingSource.Self),
		        Binding.Create(static (MainPageViewModel model) => model.IsSuspended, source: RelativeBindingSource.Self)
                    )
                    .Converter(new AllTrueMultiConverter())
                    .FallbackValue("Is Error.")
                    .TargetNullValue("Is Null.")
                ),

                new Label()
                .Text(e => 
                    e.Bindings(
			Binding.Create(static (MainPageViewModel model) => model.Id, source: RelativeBindingSource.Self),
		        Binding.Create(static (MainPageViewModel model) => model.Name, source: RelativeBindingSource.Self),
		        Binding.Create(static (MainPageViewModel model) => model.IsSuspended, source: RelativeBindingSource.Self)
                    )
                    .StringFormat("{0} : {1} : {2}")
                    .FallbackValue("Is Error.")
                    .TargetNullValue("Is Null.")
                )
            )
        );
    }
}