How to implement INotifyPropertyChanged in Xamarin.Forms

As a design methodology, its better to implement MVVM as a subclass and implement it to your ViewModel.

Sample Implementation:

public class ObservableProperty : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string propertyName)
    {
        var handler = PropertyChanged;
        if (handler != null)
            handler(this, new PropertyChangedEventArgs(propertyName));
    }
}

I also strongly suggest implementing ICommand as a Dictionary structure like:

public abstract class ViewModelBase : ObservableProperty
{
    public Dictionary<string,ICommand> Commands { get; protected set; }

    public ViewModelBase()
    {
        Commands = new Dictionary<string,ICommand>();
    }
}

So all todo in your ViewModel is just inherit the ViewModelBase class and use it

class LoginViewModel : ViewModelBase
{
    #region fields
    string userName;
    string password;
    #endregion

    #region properties
    public string UserName 
    {
         get {return userName;}
        set 
        {
            userName = value;
            OnPropertyChanged("UserName");
        }
     }

    public string Password 
    {
        get{return password;}
        set
        {
            password = value;
            OnPropertyChanged("Password");
        }
    }
    #endregion

    #region ctor
    public LoginViewModel()
    {
        //Add Commands
        Commands.Add("Login", new Command(CmdLogin));
    }
    #endregion


    #region UI methods

    private void CmdLogin()
    {
        // do your login jobs here
    }
    #endregion
}

Finally: Xaml Usage:

<Entry Placeholder="Username"  Text="{Binding UserName}"/>
<Entry Placeholder="Password" Text="{Binding Password}" IsPassword="True"/>
<Button Text="Login" Command="{Binding Commands[Login]}"/>