Coroutine flow support

[emmano]

This should help with adding support for coroutines as mentioned in #231

A couple of things to note:

• I had to use the experimental channel field in callbackFlow{} in order to comply to the onError argument for FormulaRuntime. Using cancel() on the ProducerScope is not possible since it takes a CancellableException and not a Throwable.
• The documentation for ViewModel integration suggests to dispose of the "producer" observable inside onClear() and to dispose of the consumer subscirption inside onDestroy() of the Activity. If I am not mistaken it seems that this can lead to two dangerous scenarios:
• 1. The producer stream can trigger UI updates when the Activity/Fragment is not visible. This can be solved by disposing of the consumer inside onStop().
• 2. The producer stream might be wasting resources since it is kept alive until the ViewModel is cleared. For example, if the producer stream depends on a stream that monitors network connectivity, the producer stream will still emit events even if the consumer has been disposed, hence wasting resources (i.e. battery). To address this issue, in one of the samples I added, I am using repeatOnLifecycle{} in combination with SharingStarted.WhileSubscribed(). This approach not only stops/starts collection, but also cancels the producer on the required lifecycle when there are no more collectors. repeatOnLifecycle{} uses an alpha release of the lifecycle runtime dependency.
• I added a test for StopWatchFormula just as an example of how testing a flow might look like. It is probably better to add Flow testing capabilities to formula-test just like it is done for rx.

[Laimiux]

It would be good to have such a tests within formula-coroutines module to cover toFlow() and FlowStream.fromFlow {}.

[emmano]

I added tests for FlowRuntime and FlowStream. I know you were trying to decide if it still made sense to have per runtime specific tests, but we can always remove them. I added TestFlow to serve the purpose of Rx's TestObserver. I know square came out with Turbine, but it felt like an overkill.

[Laimiux]

First of all, thank you for this contribution 🙌 Overall, it looks pretty solid.

• The documentation for ViewModel integration suggests to dispose of the "producer" observable inside onClear() and to dispose of the consumer subscirption inside onDestroy() of the Activity.

We generally want Formula to survive configuration changes and forward navigation.

• We want to maintain state across configuration changes.
• We want network requests to persist across configuration changes as well.

This approach has been working well for us, but the issues you bring up are valid.

• For UI updates, we control the subscription from Activity or Fragment, so we can unsubscribe even though Formula is still running.
• Regarding wasteful/heavy resources, we haven't run into this issue much, but one option is to control it at the resource level. Formula keeps running, but the resource stream receives Activity lifecycle events and pauses when app is in the background.

I added a test for StopWatchFormula just as an example of how testing a flow might look like. It is probably better to add Flow testing capabilities to formula-test just like it is done for rx.

I have been thinking about this a little bit, not yet sure what is the best option here. For more context, the RxJava version is used only internally within formula-test. My current thinking is that tests shouldn't really care about which runtime is used as they should behave equivalently. One caveat is that Coroutines has better support for nullable inputs and outputs where RxJava has to force non-null values currently.

[Laimiux]

Thank you for your contribution @emmano!! I'm going to take it over from here, finish up and merge the changes in #240.

[emmano]

Thank you for your contribution @emmano!! I'm going to take it over from here, finish up and merge the changes in #240.

No problem! Glad to help!

[Laimiux]

Closing this as we merged #240