Ability to await for closure of any viewmodel

[Adam-Langley]

🚀 Feature Requests

Currently, it is only possible to block until closure of viewmodel that returns a value:

await nav.Navigate<MyViewModel, MyViewModelResult(...);

However, sometimes, I want to perform some work after a child viewmodel has closed - even though it does not return a value.

One might argue "hey, just return a value then!" - however, it means that viewmodel needs to derive from MvxViewModel<TInput, TOutput> instead if MvxViewModel, or MvxViewModel.
As I have my own common viewmodel base class (MyProductsViewModelBase : MvxViewModel), I have to repeat all my code multiple times to have a base class which

1. has no input or output
2. has an input
3. has an input and output

It would be much easier, if the IMvxViewModel just had its own CloseCompletionSource to be notified of when it was closed.

Contextualize the feature

Add a TaskCompletionSource<bool> CloseCompletionSource to the IMvxViewModel contract.
Sure, `IMvxViewModelResult<> would then need to have 2 completion sources (a bool one, and a typed on) - small price to pay.

[entdark]

The ability to await the result in any way got removed: #4299
Feel free to send your callback as a parameter without awaiting for result.

[Cheesebaron]

Yeah, not going to add it back. Might have missed some of the related code.

If you come up with a good lifecycle aware version of it, I might consider it.

[Psjdkhna]

Add a TaskCompletionSource CloseCompletionSource to the IMvxViewModel contract.