Disable laziness in ReactiveCommand and ToProperty in unit test runner

[anaisbetts]

Consider the following VM constructor code:

ToasterList = new ReactiveList<Toaster>();
ToasterList.CollectionCount
    .Select(x => String.Format("{0} toasters!", x))
    .ToProperty(this, x => x.ToasterPrompt, out toasterPrompt);

Then the following test:

viewModel = new MyViewModel();
viewModel.ToasterList.Add(new Toaster());

// Fails! Why????
Assert.Equal("1 toasters!", viewModel.ToasterPrompt);

This fails because in RxUI 6.0, ObservableAsPropertyHelper is now lazy, and doesn't subscribe to its source until the Value is accessed. Since CollectionCount is a Hot Observable, OAPH won't update until the collection count changes one more time.

In the application, this is a good thing - since the person subscribing to the VM is usually a View (via Bind et al), we don't waste subscriptions on things that aren't being displayed to the user.

However, in a test runner, we don't need that efficiency, and we're confusing users who make reasonable assertions then are surprised when they don't work

[anaisbetts]

/cc @reactiveui/owners, any objections?

[tberman]

I think making the test harness not the same as the app is not a good idea.

[anaisbetts]

I think making the test harness not the same as the app is not a good idea.

I had the same feeling initially, but then I realized - the test harness is already not the same as the app, because there's no View attached to your VMs. This change should make the test runner more similar to the app in general. While there is a risk that you'll now have bugs that only surface in-app, I think that case is pretty rare.

[tberman]

Maybe add a log.debug or something each time so people understand whats going on?

[tberman]

(In the test-runner case)

[anaisbetts]

We really need to add a FAQ in the docs about test runners vs. in-app