Adding CancellationBag

[hfossli]

As per discussion on https://forums.swift.org/t/how-can-the-composable-architecture-really-become-composable/43045/2

This can be instead or in addition to #335

Let me know if you approve of this direction and I'll spend time writing unit tests and documentation. ☀️

[hfossli]

This is of course an incremental non breaking change.

[chrisballinger]

@hfossli Thank you for this! I think your solution is much more ergonomic and developer-friendly than the LifecycleAction approach.

In my case I was hitting this issue (e.g. Action was received by an optional reducer when its state was "nil") for what I think would be a relatively common scenario of:

• starting up a database observation, based on a database identifier in State during onAppear
• tearing down the observation during onDisappear

So far using your approach in this PR has fixed the crashes that I was seeing related to that workflow, and I like that the cancelAll approach ensures that any other dangling Cancellables don't get missed.

[gkaimakas]

TCA already provides an extension on NSRecursiveLock for sync located in Internal/Locking.swift

[hfossli]

Yup, just wanted a simple standalone file that could be copied out and in to any project

[andreyz]

@hfossli I think it's possible to extend Reducer with a method (that calls optional internally), that will simply cancel all local effects on nilling of the passed in reducer. To me this seems like a straightforward approach that doesn't require introduction of a new concept like "bags".

[hfossli]

Interesting! I'm all ears. Do you have some example? I couldn't find any simple solutions.

[ddeneka]

Shouldn't this be:

    return cancelInFlight ? .concatenate(.cancel(id: id, bag: bag), effect) : effect

[stephencelis]

@hfossli I'm going to close this out in tandem with #335 (comment)

Thanks again!