Как в WPF привязать активность Button к наличию значений в TextBox?

Question

romaro @romaro

C#
WPF

Как в WPF привязать активность Button к наличию значений в TextBox?

Я создал контекст, в котором есть два поля для текстбоксов и поле статуса формы (PasswordBox пока намеренно не использую, т.к. с ним отдельные проблемы):

namespace GuiBuyerDesktop.Windows.LoginWindow {
    internal class LoginWindowContext : BaseViewModel {
        private string _Login;
        private string _Password;

        public string Login
        {
            get => _Login;
            set => Set(ref value, _Login);
        }

        public string Password
        {
            get => _Password;
            set => Set(ref value, _Password);
        }

        public bool IsFormReadyToSend
        {
            get {
                return (_Login is not null && _Password is not null);
            }
        }
    }
}

BaseViewModel:

namespace WPFCore {
    public abstract class BaseViewModel : INotifyPropertyChanged {
        // All bindings subscribed to this event
        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChaged([CallerMemberName] string PropertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(PropertyName));
        }

        protected virtual bool Set<T>(ref T field, T value, [CallerMemberName] string PropertyName = null)
        {
            if (Equals(field, value)) return false;

            field = value;
            OnPropertyChaged(PropertyName);

            return true;
        }
    }
}

XAML:

<Window x:Class="GuiBuyerDesktop.Windows.LoginWindow.LoginWindowView"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:vm="clr-namespace:GuiBuyerDesktop.Windows.LoginWindow"
        mc:Ignorable="d"
        Title="Вход"
        Width="350"
        SizeToContent="Height"
        ResizeMode="CanMinimize">
    <Window.DataContext>
        <vm:LoginWindowContext/>
    </Window.DataContext>
    <StackPanel>
        <TextBlock 
            Background="#fcec5e" 
            Height="65" 
            FontFamily="{StaticResource AR}" 
            FontSize="45" 
            TextAlignment="Center" 
            Padding="0,2,0,0">
            Главпоставка
        </TextBlock>
        <StackPanel Margin="30,10,30,25">
            <StackPanel.Resources>
                <Style x:Key="BaseInputStyle" TargetType="{x:Type Control}">
                    <Setter Property="Margin" Value="0,15,0,0"/>
                    <Setter Property="Height" Value="40"/>
                    <Setter Property="BorderBrush" Value="#d4b2ff"/>
                    <Setter Property="BorderThickness" Value="3"/>
                    <Setter Property="FontSize" Value="17"/>
                    <Setter Property="Padding" Value="4,4,0,0"/>
                </Style>
            </StackPanel.Resources>
            <TextBox
                Style="{StaticResource BaseInputStyle}"
                Text="{Binding Path=Login, UpdateSourceTrigger=LostFocus, Mode=OneWayToSource}"
                >
            </TextBox>
            <TextBox
                Style="{StaticResource BaseInputStyle}"
                Text="{Binding Path=Password, UpdateSourceTrigger=LostFocus, Mode=OneWayToSource}"
                >
            </TextBox>
            <!--<PasswordBox
                Style="{StaticResource BaseInputStyle}"
                Password="{Binding Path=Password, UpdateSourceTrigger=PropertyChanged}"
                ></PasswordBox>-->
            <Button 
                Background="#fcec5e" 
                BorderBrush="Black" 
                BorderThickness="2" 
                Height="45" 
                Margin="0,15,0,0"
                FontFamily="{StaticResource AR}" 
                FontSize="22"
                IsEnabled="{Binding Path=IsFormReadyToSend}"
                >
                Вход
            </Button>
        </StackPanel>
    </StackPanel>
</Window>

Почему кнопка остается не активной, когда оба поля заполнены?