Make all workers run on the Unconfined dispatcher. #851
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kotlin/workflow-core/src/main/java/com/squareup/workflow/Worker.kt
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This is helpful for tests that need to be able to control time. This functionality was covered by the test's dispatcher, but since #851 changed the dispatcher used for workers, we need to be able to configure workers' contexts separately.
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First part of Kotlin implementation of square/workflow#1021. Kotlin counterpart to square/workflow#1174. This implementation intentionally does not run the side effect coroutine on the `workerContext` `CoroutineContext` that is threaded through the runtime for testing infrastructure. Initially, workers ran in the same context as the workflow runtime. The behavior of running workers on a different dispatcher by default (`Unconfined`) was introduced in square/workflow#851 as an optimization to reduce the overhead for running workers that only perform wiring tasks with other async libraries. This was a theoretical optimization, since running on the `Unconfined` dispatcher inherently involves less dispatching work, but the overhead of dispatching wiring coroutines was never actually shown to be a problem. Additionally, because tests often need to be in full control of dispatchers, the ability to override the context used for workers was introduced in square/workflow#940, which introduced `workerContext`. I am dropping that complexity here because it adds a decent amount of complexity to worker/side effect machinery without any proven value. It is also complexity that needs to be documented, and is probably just more confusing than anything. The old behavior for workers is maintained for now to reduce the risk of this change, but side effects will always run in the workflow runtime's context. This is nice and simple and unsurprising and easy to reason about.
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From the kdoc:
Previously, the workers would run on the same dispatcher as the workflow runtime (
Main.immediate), which meant that if the worker was wrapping, e.g., anObservablethat was emitting on a background thread, the coroutine would dispatch back onto the main thread before sending to the channel, which is completely unnecessary, since channels are already thread-safe.This doesn't change the threading behavior for simple side-effect-only workers – since they're started from the workflow runtime, they will be dispatched onto the nested event loop that
Main.immediatecreates for "immediate" dispatches.