public ShellViewModel()
{
_username = new ObservableProperty <string>(this, () => Username);
_password = new ObservableProperty <string>(this, () => Password);
_logInCommand = new ObservableCommand();
_cancelCommand = new ObservableCommand();
_username.CombineLatest(_password, (username, password) => !string.IsNullOrWhiteSpace(username) && !string.IsNullOrWhiteSpace(password)).Subscribe(_logInCommand);
_logInCommand.Subscribe(ExecuteLogIn);
_cancelCommand.Subscribe(ExecuteCancel);
}