Add files for the reactive example

ReactiveExample/LoginViewModel.cs

@@ -0,0 +1,57 @@
+using System;
+using System.Reactive;
+using System.Reactive.Linq;
+using System.Runtime.Serialization;
+using System.Threading.Tasks;
+using ReactiveUI;
+using ReactiveUI.Fody.Helpers;
+
+namespace ReactiveExample {
+	[DataContract]
+	public class LoginViewModel : ReactiveObject {
+		readonly ObservableAsPropertyHelper<int> _usernameLength;
+		readonly ObservableAsPropertyHelper<int> _passwordLength;
+		readonly ObservableAsPropertyHelper<bool> _isValid;
+		
+		public LoginViewModel () {
+			var canLogin = this.WhenAnyValue (
+				x => x.Username, 
+				x => x.Password,
+				(username, password) =>
+					!string.IsNullOrWhiteSpace (username) &&
+					!string.IsNullOrWhiteSpace (password));
+			
+			_isValid = canLogin.ToProperty (this, x => x.IsValid);
+			Login = ReactiveCommand.CreateFromTask (
+				() => Task.Delay (TimeSpan.FromSeconds (1)),
+				canLogin);
+			
+			_usernameLength = this
+				.WhenAnyValue (x => x.Username)
+				.Select (name => name.Length)
+				.ToProperty (this, x => x.UsernameLength);
+			_passwordLength = this
+				.WhenAnyValue (x => x.Password)
+				.Select (password => password.Length)
+				.ToProperty (this, x => x.PasswordLength);
+		}
+		
+		[Reactive, DataMember]
+		public string Username { get; set; } = string.Empty;
+		
+		[Reactive, DataMember]
+		public string Password { get; set; } = string.Empty;
+		
+		[IgnoreDataMember]
+		public int UsernameLength => _usernameLength.Value;
+		
+		[IgnoreDataMember]
+		public int PasswordLength => _passwordLength.Value;
+
+		[IgnoreDataMember]
+		public ReactiveCommand<Unit, Unit> Login { get; }
+		
+		[IgnoreDataMember]
+		public bool IsValid => _isValid.Value;
+	}
+}