Handling observable object properties with CommunityToolkit.Mvvm

Here are multiple scenarios:

If you only ever replace the entire Person object and don't change its individual properties (you might select a different person from a list) all you need is to decorate the author field with the ObservablePropertyAttribute.

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public partial class MyViewModel : ObservableObject
{
   [ObservableProperty]
   string title;

   [ObservableProperty]
   decimal price;

   [ObservableProperty]
   Person author;
}

This change will be reflected in the UI:

Author = anotherPerson;

and this one will NOT:

Author.FirstName = "James";

If you plan to change the persons individial properties, but keep the instance the same, you need to decorate the individual properties of the Person object with the ObservablePropertyAttribute.

public class Person : ObservableObject
{
    [ObservableProperty]
    string firstName;

    [ObservableProperty]
    string lastName;
}

public partial class MyViewModel : ObservableObject
{
   [ObservableProperty]
   string title;

   [ObservableProperty]
   decimal price;

   public Person Author { get; } = new Person();
}

This change will be reflected in the UI:

Author.FirstName = "James";

and this one will NOT:

Author = anotherPerson;

Or do both and get the UI to reflect any changes, be it changing an individual property or the entire object instance.

public class Person : ObservableObject
{
    [ObservableProperty]
    string firstName;

    [ObservableProperty]
    string lastName;
}

public partial class MyViewModel : ObservableObject
{  
   [ObservableProperty]
   string title;

   [ObservableProperty]
   decimal price;

   [ObservableProperty]
   Person author;
}

Any of these changes will be reflected in the UI:

Author = anotherPerson;
Author.FirstName = "James";