The ObservableAsPropertyHelper (OAPH) is a class that simplifies the interop between an IObservable and a property on your ViewModel. It allows you to have a property which reflects the latest value that has been sent through the IObservable stream.

It is important to note that OAPH will not set the property value immediately, but will rather schedule it on the provided or default scheduler. The [Reactive] property with .BindTo() should be used for business-critical code.

It will invoke the INotifyPropertyChanged and INotifyPropertyChanging based events for the View Model when the observable emits a new value. ObservableAsPropertyHelper<T> is very similar to a Lazy in so far as it provides a Value member which provides the latest value of the Observable. They are often read-only and reflect the IObservable stream. It is common to combine ObservableAsPropertyHelper with the WhenAny extensions.

Example

First, we need to declare an Output Property, using a class called ObservableAsPropertyHelper<T>:

readonly ObservableAsPropertyHelper<string> firstName;
public string FirstName => firstName.Value;

Similar to read-write properties, this code should always be 100% boilerplate. Next, we'll use a helper method ToProperty to initialize firstName in the constructor. ToProperty has two overloads, one where it directly returns the OAPH and another where the OAPH is returned in a out parameter. We generally recommend using the former approach due to better readability of the code:

firstName = this
    .WhenAnyValue(x => x.Name)
    .Select(name => name.Split(' ')[0])
    .ToProperty(this, x => x.FirstName);

Here, ToProperty creates an ObservableAsPropertyHelper instance which will signal that the FirstName property has changed. If you prefer using the overload where the OAPH is returned in a out parameter, then use the following code:

this.WhenAnyValue(x => x.Name)
    .Select(name => name.Split(' ')[0])
    .ToProperty(this, x => x.FirstName, out firstName);

ToProperty()

ToProperty allows you to construct an ObservableAsPropertyHelper<T> from an IObservable<T>. When a new value has been added to the IObservable<T>, it will use the overload methods in the IReactiveObject interface to trigger the required events.

ToProperty is an extension method on IObservable<T> and semantically acts like a "Subscribe".

public static ObservableAsPropertyHelper<TRet> ToProperty<TObj, TRet>(
   this IObservable<TRet> target,
   TObj source,
   Expression<Func<TObj, TRet>> property,
   TRet initialValue = default(TRet),
   bool deferSubscription = false,
   IScheduler? scheduler = null)

The parameters of ToProperty allows you to specify the source of the property, often just the current class.

• An expression or a string to the Property that is exposed.
• Optionally the initial value being exposed by the Property before any values are emitted by the source observable. If you are deferring the subscription this value may be the first value since the observable won't be immediately subscribed to.
• Optionally if you defer subscription that will not subscribe to the observable until the user accesses the Property. By default we will subscribe to the observable immediately so that you have the latest value.
• Optionally a scheduler, by default this will be CurrentThreadScheduler from where the call is made.

Property vs ObservableAsPropertyHelper

You should use a property and RaiseAndSetIfChanged if you are intending to mutate the value.

ObservableAsPropertyHelper properties are useful for when you have "calculated" values, for example, if their value is solely the result of other properties. You will also use the ObservableAsPropertyHelper when you want to expose the latest value from a IObservable<T>.

ObservableAsPropertyHelper properties are helpful to remove spaghetti code where different methods and components may be mutating multiple locations. They also clearly define what values are used in the calculation of the value and help describe the dependent properties for the ObservableAsPropertyHelper, unlike settable properties where you have to search the code base further for location where the value is mutating.

Performance considerations

nameof() instead of using default Index.

For performance based solutions you can also use the nameof() operator override of ToProperty() which won't use the Expression.

firstName = this
    .WhenAnyValue(x => x.Name)
    .Select(name => name.Split(' ')[0])
    .ToProperty(this, nameof(FirstName));

Defer Subscription

If you are creating a large number of OAPH, consider deferring your subscription. ToProperty also allows you to deferSubscription to the underlying IObservable<T>. Deferring the subscription not have the OAPH Subscribe to the base IObservable<T> until the Value property has been accessed. This is especially useful if you have more complex IObservable<T> because this approach is very close to the approach that Lazy<T> takes.

// nameStatusObservable is IObservable<string>
var nameStatusObservable = this
    .WhenAnyValue(x => x.Name)
    .Select(name => GetLatestStatus(name));

// name is ObservableAsPropertyHelper<string>
name = nameStatusObservable
    .ToProperty(this, nameof(Name), deferSubscription: true);

// nameStatusObservable won't be subscribed until the 
// Name property is accessed.
private readonly ObservableAsPropertyHelper<string> name;
public string Name => name.Value; 

One caveat of deferring the subscription is if you aren't careful you'll have an invalid value until a new value is added to the IObservable<T>. If using Hot Observables consider using a ReplaySubject<T> and limiting the ReplaySubject<T> to 1 previous value in the constructor.

public class StatusViewModel : ReactiveObject
{
    readonly ObservableAsPropertyHelper<string> status;
    public string Status => status.Value;
    
    public StatusViewModel()
    {
        var replayStatus = new ReplaySubject<string>(1);
        // .OnNext() the 'replayStatus' subject somewhere...

        // Note, that 'replayStatus' is also IObservable<string>, 
        // so we are allowed to add a call to .ToProperty() to
        // convert it to ObservableAsPropertyHelper<string>.
        status = replayStatus.ToProperty(this, nameof(Status), deferSubscription: true);
    }
}