worldbeater

public class ViewModel : INotifyPropertyChanged
{
    public ViewModel()
    {
        Clear = new Command(() => Name = string.Empty);
    }
    
    public ICommand Clear { get; }
    
    public string Greeting => $"Hello, {Name}!";

worldbeater

<StackPanel>
    <TextBox Text="{Binding Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
    <TextBlock Text="{Binding Greeting, Mode=OneWay}"/> 
    <Button Content="Clear" Command="{Binding Clear}"/>
</StackPanel>

worldbeater

public class ReactiveViewModel : ReactiveObject
{
    public ReactiveViewModel()
    {
        Clear = ReactiveCommand.Create(() => { Name = string.Empty; });
        this.WhenAnyValue(x => x.Name) 
            .Select(name => $"Hello, {name}!")
            .ToProperty(this, x => x.Greeting, out greeting);
    }

worldbeater

public class ReactivePropertyViewModel
{
    public ReadOnlyReactiveProperty<string> Greeting { get; }
    public ReactiveProperty<string> Name { get; }
    public ReactiveCommand Clear { get; }

    public ReactivePropertyViewModel()
    {
        Clear = new ReactiveCommand();
        Clear.Subscribe(() => Name.Value = string.Empty);

worldbeater

<StackPanel>
    <!-- Note the ".Value" suffix added to "Name" and "Greeting" properties. -->
    <TextBox Text="{Binding Name.Value, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
    <TextBlock Text="{Binding Greeting.Value, Mode=OneWay}"/> 
    <Button Content="Clear" Command="{Binding Clear}"/>
</StackPanel>

worldbeater

[AddINotifyPropertyChangedInterface]
public class FodyReactiveViewModel
{
    public ReactiveCommand<Unit, Unit> Clear { get; }
    public string Greeting { get; private set; }
    public string Name { get; set; } = string.Empty;

    public FodyReactiveViewModel()
    { 
        Clear = ReactiveCommand.Create(() => { Name = string.Empty; });

worldbeater

public sealed class FeedbackViewModel : ReactiveObject, IActivatableViewModel
{
    public ViewModelActivator Activator { get; } = new ViewModelActivator();
    public ReactiveCommand<Unit, Unit> Submit { get; }

    [Reactive] public bool HasErrors { get; private set; }
    [Reactive] public string Elapsed { get; private set; } = string.Empty;

    [Reactive] public string Title { get; set; } = string.Empty;
    [Reactive] public int TitleLength { get; private set; }

worldbeater

 <StackPanel Width="300" VerticalAlignment="Center">
    <TextBlock Text="Feedback" Style="{StaticResource TitleTextBlockStyle}" />
    <TextBox Text="{x:Bind ViewModel.Title, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
             MaxLength="{x:Bind ViewModel.TitleLengthMax}" 
             PlaceholderText="Title" />
    <TextBlock Style="{StaticResource CaptionTextBlockStyle}" Margin="0 5">
        <Run Text="{x:Bind ViewModel.TitleLength, Mode=OneWay}"/>
        <Run Text="letters used from"/>
        <Run Text="{x:Bind ViewModel.TitleLengthMax}"/>
    </TextBlock>

worldbeater

<StackLayout Margin="20">
    <Entry Text="{Binding Title, Mode=TwoWay}" Placeholder="Title"/>
    <StackLayout Orientation="Horizontal">
        <Label Text="{Binding TitleLength, Mode=OneWay}"/>
        <Label Text="letters used from"/>
        <Label Text="{Binding TitleLengthMax, Mode=OneWay}"/>
    </StackLayout>
    <Entry Text="{Binding Message, Mode=TwoWay}" Placeholder="Message"/>
    <StackLayout Orientation="Horizontal">
        <Label Text="{Binding MessageLength, Mode=OneWay}"/>

worldbeater

[Fact]
public void ShouldValidateFormAndSendFeedback()
{
    // Create stubs for all dependencies and inject them.
    var sender = Substitute.For<ISender>();
    var clock = Substitute.For<IClock>();
    var feedback = new FeedbackViewModel(sender, clock);
    feedback.HasErrors.Should().BeTrue();

    // Emulate user input.