Defining a ViewModel

A ViewModel should be a ObservableObject

How Avalonia Do by Default

Default template of avalonia.mvvm generates a class ViewModelBase as base observable type to share across the project.

using CommunityToolkit.Mvvm.ComponentModel;

namespace ToDoList.ViewModels;

public class ViewModelBase : ObservableObject // nah I don't like it.
{
}

But this is rather confusing for beginners. so manually inherit from ObservableObject is a good start.

using CommunityToolkit.Mvvm.ComponentModel;

namespace ToDoList.ViewModels;

public partial class MainWindowViewModel : ObservableObject
{
}

What's behind ObservableObject

ObservableObject is both INotifyPropertyChanged and INotifyPropertyChanging, implementing the two events.

public abstract class ObservableObject : INotifyPropertyChanged, INotifyPropertyChanging { /* ... */ }

There's also a SetProperty method for performing the observer pattern. Notice a ref modifier appears here, it's just for making sure we can bind a value type instance.

protected bool SetProperty<T>([NotNullIfNotNull(nameof(newValue))] ref T field, T newValue, [CallerMemberName] string? propertyName = null)
{
    // We duplicate the code here instead of calling the overload because we can't
    // guarantee that the invoked SetProperty<T> will be inlined, and we need the JIT
    // to be able to see the full EqualityComparer<T>.Default.Equals call, so that
    // it'll use the intrinsics version of it and just replace the whole invocation
    // with a direct comparison when possible (eg. for primitive numeric types).
    // This is the fastest SetProperty<T> overload so we particularly care about
    // the codegen quality here, and the code is small and simple enough so that
    // duplicating it still doesn't make the whole class harder to maintain.
    if (EqualityComparer<T>.Default.Equals(field, newValue)) 
    { 
        return false;
    } 

    OnPropertyChanging(propertyName);

    field = newValue;

    OnPropertyChanged(propertyName);

    return true;
}

NOTE

There's multiple overloads of SetProperty, it's just one of the most commonly used.