ObservableAsPropertyHelper (OAPH)

The ObservableAsPropertyHelper (OAPH) bridges IObservable streams and read-only ViewModel properties. It exposes the latest value via a property and raises INotifyPropertyChanging/Changed notifications for you.

Key points:

• OAPH subscribes to the source observable and surfaces the latest value through Value.
• Delivery of notifications uses the scheduler you pass to ToProperty (defaults to CurrentThreadScheduler), so UI updates can be marshaled to RxApp.MainThreadScheduler when needed.
• You can supply an initial value and optionally defer subscription until the property is first read.
• Ideal for computed/read-only properties; use RaiseAndSetIfChanged for mutable properties.

Basic Example

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

public MyViewModel()
{
    _firstName = this
        .WhenAnyValue(x => x.Name)
        .Where(n => !string.IsNullOrWhiteSpace(n))
        .Select(n => n.Split(' ')[0])
        .ToProperty(this, x => x.FirstName, scheduler: RxApp.MainThreadScheduler);
}

ToProperty Overloads

ToProperty constructs an ObservableAsPropertyHelper<T> from an IObservable<T> and raises change notifications:

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

• initialValue: Used before the first tick (or when deferring, until subscribed).
• deferSubscription: Subscribe on first property access (lazy).
• scheduler: Use RxApp.MainThreadScheduler for UI properties.

nameof Optimization

Avoid expression compilation by using the nameof overload:

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

Deferred Subscription

Delay work until the property is first read. Consider buffering the last value with Replay(1) if the source is hot.

var status = GetStatus().Replay(1).RefCount();
_status = status.ToProperty(this, nameof(Status), deferSubscription: true);

Error Handling

OAPH does not swallow errors. Ensure errors are handled upstream (e.g., Catch/LoggedCatch) or the subscription will terminate.

Using Source Generators

ReactiveUI.SourceGenerators can generate OAPH-backed properties with [ObservableAsProperty]:

using ReactiveUI.SourceGenerators;

public partial class StatusViewModel : ReactiveObject
{
    [ObservableAsProperty]
    private string _status;

    public StatusViewModel()
    {
        _statusHelper = StatusObservable().ToProperty(this, x => x.Status);
    }

    IObservable<string> StatusObservable() => Observable.Return("Ready");
}

When To Use OAPH vs Property

• Use OAPH for computed/read-only values that derive from other observables or properties.
• Use RaiseAndSetIfChanged for writable state your ViewModel mutates directly.

Advanced Notes

• OAPH behaves like a lazy/behavioral observable: it holds the last value and pushes changes on the configured scheduler.
• Combine with WhenAnyValue, Select, Throttle, DistinctUntilChanged for efficient UI updates.