`BindableReactiveProperty` does not notify value changes in the WinForms binding system

Phenomenon

When you bind an instance of R3.BindableReactiveProperty<T> to a WinForms control using its DataBindings property, changes on the Value property of the instance are not notified to the control. This behavior differs from Reactive.Bindings.ReactiveProperty<T>, which works as expected.

Reproduction

Here's a minimal snippet:

public partial class Form1
{
    // label1 and button1 are defined in the code behind

    private readonly BindableReactiveProperty<string> rp = new("");

    public Form1() {
        InitializeComponent();

        // Bind label1.Text to rp.Value
        label1.DataBindings.Add("Text", rp, "Value");
    }

    public void button1_Click()
    {
        // This change won't be notified to label1
        rp.Value += "X";
    }
}

Cause

The problem lies in event registration, which is eventually achieved by the AddValueChanged method of the ReflectPropertyDescriptor instance corresponding to the Value property. This method checks the following conditions:

Does the component type have an event named <propertyName>Changed? (i.e. ValueChanged)
Does the component type implement INotifyPropertyChanged interface?

The issue arises because the "component type" here is ReactiveProperty<T>, rather than BindableReactiveProperty<T>, since the Value property is defined in the former. Unlike the Reactive.Bindings version, where the event and the property are defined simultaneously, neither condition 1. nor 2. is met. As a result, the property is recognized as "not supporting change events." You can verify this:

var rp = new BindableReactiveProperty<string>("");
var prop = TypeDescriptor.GetProperties(rp).Find("Value", true);
Debug.WriteLine((
    prop.ComponentType.Name,    // "ReactiveProperty`1" (not Bindable)
    prop.SupportsChangeEvents   // false
));

I hope this clarifies the issue and helps you improve the package.

Cysharp / R3