[Spec] ViewModel base class

Provide concrete implementation of INotifyPropertyChanged

MAUI should provide an out of the box base, concrete implementation of the INotifyPropertyChanged pattern with a minimally useful and prescriptive but not overly opinionated set of convenience methods.

This is useful for building view models, and other base models which use the INPC pattern. We currently have nothing out of the box, and pretty much any sample we ship or app we see in the wild has to implement this themselves. Adding something out of the box is very low cost and provides a better experience for new developers.

Xamarin Community Toolkit provides the type ObservableObject which we could choose to use as is:

API

public abstract class ObservableObject: INotifyPropertyChanged
{
    protected virtual bool SetProperty<T>(
			ref T backingStore,
			T value,
			[CallerMemberName] string propertyName = "",
			Action? onChanging = null,
			Action? onChanged = null,
			Func<T, T, bool>? validateValue = null)
    {
        // ...
    }

    public event PropertyChangedEventHandler PropertyChanged;

    public void NotifyPropertyChanged([CallerMemberName]string propertyName)
        => /* ... */
}

Scenarios

C# Example

public class MyViewModel : Microsoft.Maui.Controls.ObservableObject
{
    string name;

    public string Name
    {
        get => name;
        set => Set(ref name, value);
    }
}

Difficulty : low

Questions?

• Should we include anything else?

[Redth]

MAUI is not going to be a traditional nuget package, rather an optional workload component for the dotnet SDK which is a bit different. There is a separation of MAUI core things and BindableObject is not considered a part of that, but rather part of MAUI Controls (and yes you will be able to tell your project you only want Core and not Controls referenced). This type of base class would exist in Controls, not core, as INPC is more closely tied to XAML/MVVM/DataBinding concepts that MAUI Controls uses. Things like Comet/MVU don’t necessarily require or make use of INPC so this doesn’t make sense in that layer.

[dansiegel]

I suggest if something really does need to be done for a better hot reload experience, let’s ensure it’s provided via interface… that way even if there is ultimately some base implementation out of the box, it doesn’t leave anyone out because they’re doing something else 😃