例には書いてないけどIPropertyChangedイベントを購読して、イベントが来たらConsole.WriteLine()するんでしょう
で、IPropertyChangedの実装方法が知りたいと


public class Square : INotifyPropertyChanged
{
public event PropertyChangedEventHandler? PropertyChanged;
private void SetProperty< T >( ref T field, T value, [CallerMemberName]string propertyName = null )
{
if ( field.Equals( value ) )
{
return;
}
field = value;
PropertyChanged?.Invoke( this, new PropertyChangedEventArgs( propertyName ) );
}

private double _width;
public double Width
{
set => SetProperty( ref _width, value );
get => _width;
}

private double _height;
public double Height
{
set => SetProperty( ref _height, value );
get => _height;
}
}