Fix memory leaks in activation.

[jlaanstra]

This PR fixes two memory leaks that cause views to stick around forever. This also fixes #684.

Memory Leak No. 1:

The ActivationForViewFetcher for Xaml wants to be notified of changes to the IsHitTestVisible property and subscribes to it, but this subscription sticks around, even if the view is unloaded. Because IsHitTestVisible is a dependency property, this leaks the view.

This has been fixed by only listening to IsHitTestVisible when the view is loaded.

Memory Leak No. 2:

The activation logic subscribes to the ViewModel property of a view to be notified of changes. Similar to leak no. 1, this subscription sticks around, even if a view is unloaded.

This has been fixed by disposing this subscription when the view is unloaded.

Long-term Improvement

These fixes made the already complicated activation logic a little more complicated. I believe it has been discussed before why the activation logic consists of two observables, instead of one and I cannot remember what the reasons for having two were. Now might be a good time to refactor this into a single observable returning a bool indicating active or inactive.

Thoughts?

[anaisbetts]

Now might be a good time to refactor this into a single observable returning a bool indicating active or inactive.

I'm not sure we can do that, it'd be a breaking change, no? Or do you just mean the internals for activation

[jlaanstra]

I'm not sure we can do that, it'd be a breaking change, no? Or do you just mean the internals for activation.

There would indeed be a breaking change in the IActivationForViewFetcher interface. We should not change it if people are actually implementing this themselves, but I think most people use the builtin stuff. Is anyone implementing this himself?

The behavior of the activation stuff will not change.

[jlaanstra]

The interface has been adjusted. Paging @rikbosch to check if I did not fuck up Winforms activation.

[rikbosch]

@jlaanstra it's looking OK.

I've removed the unused observable declarations

[jlaanstra]

@rikbosch Thanks missed them apparently. @paulcbetts

[anaisbetts]

Cool! Thanks @jlaanstra @rikbosch!