Extend IsChanged for sub viewmodels

[ModernRonin]

I don't know if this is possible with source generators, but if it was it would be pretty useful, I think:

If in a viewmodel you have fields/properties which implement INotifyPropertyChanged and
expose a readable public property with the same name as the one used for [IsChanged], then it would be nice if changes to the sub-viewmodels is-dirty flag would also change the one in the current viewmodel.

Because this would have to work via subscribing to the event, there'd be a potential issue of leaks. But the generated code could just include a method ReleaseSubscriptions() and the writer of a viewmodel could implement IDisposable and call it. A decent DI container (eg Autofac) will automatically call Dispose() at the end of lifetime for objects it created which implement IDisposable, so this would be fairly comfortable.

Should probably be an opt-in feature that one specifies via the [IsChanged] attribute, to avoid breaking existing code.

Cheers,
MR

[canton7]

This is a similar problem to #12: how to safely and reliably get change notifications from the child up to the parent.

I'm somewhat reluctant to introduce behaviour where we leak memory unless the user takes a particular action to mitigate it, particularly if it's the result of a common case that the user can walk into by accident. That said, if the child VM is being assigned to a property which we're generating, we know when that's been unassigned, and we can remove our event subscription at that point.

I rather suspect the best approach is going to be to tackle this at the same time as #12, which means waiting for partial properties. Thankfully there's now a proposal for those!

[caesay]

+1 for this. Would be awesome if this library could subscribe/unsubscribe INPC to classes set to generated properties.