What are the distinctions between the various When

Question

There are several extension methods in Reactive UI for getting observables for property changes.

I think I understand WhenAny and WhenAnyValue. WhenAny is for a series of property change notifications where you want the metadata of which object and property had the change, while WhenAnyValue is for when you really just want the stream of changed values.

First of all, is that an accurate assessment?

What about WhenAnyDynamic, WhenAnyObservable, and ObservableForProperty? I can't really figure out what they're for, or how they're distinct from the first two. Are they all intended for public use? What is their purpose?

Answer 1

I think I understand WhenAny and WhenAnyValue.

Let me demonstrate via code:

// These two statements are 100% identical, but the latter looks nicer.
this.WhenAny(x => x.Foo.Bar, x => x.Value)

this.WhenAnyValue(x => x.Foo.Bar);

What about WhenAnyDynamic, WhenAnyObservable, and ObservableForProperty?

WhenAnyDynamic is like WhenAny but when the things you want to observe aren't constants - you probably won't need it, but RxUI internals does.

WhenAnyObservable lets you get an Observable, but not have to worry about objects changing behind your back. For example

this.SomeChildViewModel.MyCoolCommand
    .Subscribe(x => Console.WriteLine("Clicked!"));

// Later...
this.SomeChildViewModel = new SomeChildViewModel();

// (Hey, why doesn't my Clicked! handler show up anymore! I'm still subscribed
// to the old object but it's super not obvious that's what happened)

Versus

this.WhenAnyObservable(x => x.MyCoolCommand).
    .Subscribe(x => Console.WriteLine("Clicked!"));

// Later...
this.SomeChildViewModel = new SomeChildViewModel();

// Cool, everything still works.

WhenAnyObservable is super useful in the View to Subscribe to Commands.

ObservableForProperty is like WhenAny but doesn't fire when initially subscribed to. You probably should ignore it, it's really just a building block for WhenAny that is around for compatibility reasons.