ChangeCanExecute.cs

  //Esse código é apenas para demonstração da funcionalidade do ChangeCanExecute
	public class ViewModelBase : INotifyPropertyChanged
	{
		public event PropertyChangedEventHandler PropertyChanged;

		public void Notify(string propertyName)
		{
			PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
		}
	}

	public class ViewModel : ViewModelBase
	{
		string username;
		public string Username
		{
			get
			{
				return username;
			}

			set
			{
				username = value;
				Notify("Username");
				this.LoginCommand.ChangeCanExecute();
			}
		}

		string password;
		public string Password
		{
			get
			{
				return password;
			}

			set
			{
				password = value;
				Notify("Password");
				this.LoginCommand.ChangeCanExecute();
			}
		}

		public Command LoginCommand
		{
			get;
			set;
		}

		public ViewModel ()
		{
			this.LoginCommand = new Command(this.Excetute, this.CanExecute);
		}

		private void Excetute(object obj)
		{
			//efetua login
		}

		private bool CanExecute(object obj)
		{
			return !string.IsNullOrEmpty(this.Password) &&
				   !string.IsNullOrEmpty(this.Username);
		}
 }