Dynamically switching binding to a custom control does not trigger property changed event of the control

[suugbut]

Description

This repo is intentionally made simple to illustrate the issue.

• A custom control MyControl has a bindable property Value, a Label for displaying Value and a Button for increasing Value.
  MyControl is also backed by a view model called MyControlViewModel nested in MyControl.

• MainPage uses MyControl and has two RadioButton to dynamically bind MyControl to one of two properties (A and B) in MainPageViewModel.

  

Expected Behavior

• Selecting a radio button will bind MyControl to a property in MainPageViewModel.
• Pressing Increase button will increase the bound property.
• MyControl's Label displays the bound property.

Issue and How To Produce It

• From any checked radio button, switch to the other one.
• Press the Increase button at least once.
• Switch to the previous radio button but don't press the Increase button.
• Switch again to the other one. Here MyControl's Label no longer displays the bound property.

Example by assuming that at the beginning the checked radio button is A=0 and unchecked one is B=100.

A ---> B followed by pressing Increase button ---> A but not pressing Increase button ----> B

MyControl's Label:

0 ---> 100 and then becomes 101 ---> 0 ----> 0 (thas should be 101).

My Investigation

The property changed event of the bindable property Value is not triggered
in the last switch explained above (A but not pressing Increase button ----> B).

Details for MyControl

<HorizontalStackLayout
    Spacing="20"
    BindingContext="{x:Reference This}"
    x:DataType="{x:Type local:MyControl}">
    <Button
        Text="Increase"
        Command="{Binding ViewModel.IncreaseCommand}" />
    <Label
        Text="{Binding ViewModel.Value}" />
</HorizontalStackLayout>

public partial class MyControl : ContentView
{
    public MyControlViewModel ViewModel { get; } = new();
    public MyControl()
    {
        InitializeComponent();
        ViewModel.PropertyChanged += (s, e) =>
        {
            Debug.WriteLine("\tViewModel Property Changed");
            if (e.PropertyName == nameof(MyControlViewModel.Value))
            {
                if (Value != ViewModel.Value)
                {
                    Value = ViewModel.Value;
                }
            }
        };
    }

    public static readonly BindableProperty ValueProperty = BindableProperty.Create(
        propertyName: nameof(Value),
        returnType: typeof(int),
        declaringType: typeof(MyControl),
        defaultValue: 0,
        defaultBindingMode: BindingMode.TwoWay,
        propertyChanged: (b, o, n) =>
        {
            Debug.WriteLine("\tValue Property Changed");
            if (b is MyControl me && o is int old && n is int @new)
            {
                if (me.ViewModel.Value != @new)
                {
                    me.ViewModel.Value = @new;
                }
            }
        });

    public int Value
    {
        get => (int)GetValue(ValueProperty);
        set => SetValue(ValueProperty, value);
    }

    public partial class MyControlViewModel : ObservableObject
    {
        [ObservableProperty]
        private int _value;

        [RelayCommand]
        private void Increase() => Value++;
    }
}

Details for MainPage

<VerticalStackLayout
    Padding="30,0"
    Spacing="25">
    <RadioButton
        x:Name="rba"
        Content="{Binding A, StringFormat='A: {0}'}"
        CheckedChanged="RadioButton_CheckedChanged" />
    <RadioButton
        x:Name="rbb"
        Content="{Binding B, StringFormat='B: {0}'}"
        CheckedChanged="RadioButton_CheckedChanged" />
    <local:MyControl
        x:Name="mycontrol" />
</VerticalStackLayout>

public partial class MainPage : ContentPage
{
    public MainPage()
    {
        InitializeComponent();
        BindingContext = new MainPageViewModel
        {
            A = 0,
            B = 100
        };
        rba.IsChecked = true;
    }

    private void RadioButton_CheckedChanged(object sender, CheckedChangedEventArgs e)
    {
        if (e.Value)
        {
            Debug.WriteLine("RadioButton Checked");
            if (sender == rba)
            {
                mycontrol.SetBinding(MyControl.ValueProperty, nameof(MainPageViewModel.A));
            }
            else if (sender == rbb)
            {
                mycontrol.SetBinding(MyControl.ValueProperty, nameof(MainPageViewModel.B));
            }
        }
    }

    public partial class MainPageViewModel : ObservableObject
    {
        [ObservableProperty]
        private int _a;

        [ObservableProperty]
        private int _b;
    }
}

Link to public reproduction project repository

https://github.com/suugbut/MaybeBug

Version with bug

9.0.0 GA

Is this a regression from previous behavior?

Not sure, did not test other versions

Last version that worked well

Unknown/Other

Affected platforms

Android, Windows

Affected platform versions

No response

Did you find any workaround?

private void RadioButton_CheckedChanged(object sender, CheckedChangedEventArgs e)
{
    if (e.Value)
    {
        Debug.WriteLine("RadioButton Checked");
        if (sender == rba)
        {
            mycontrol.SetBinding(MyControl.ValueProperty, nameof(MainPageViewModel.A));
        }
        else if (sender == rbb)
        {
            mycontrol.SetBinding(MyControl.ValueProperty, nameof(MainPageViewModel.B));
        }
        //  by adding either this 
        mycontrol.Value = mycontrol.ViewModel.Value;
       // or this 
       // mycontrol.ViewModel.Value = mycontrol.Value;
    }
}

Relevant log output

[nivetha-nagalingam]

This issue has been verified in Visual Studio 17.14.0 Preview 2.0 (versions 9.0.60 , 9.0.0 & 8.0.100). It can be reproduced on the Windows and Android platform.

[suugbut]

Just a note: I am not trying to replicate the existing Stepper here. The code above is a trivial example to illustrate the issue.