IdlingRegistry doesn't set registerIdleTransitionCallback until a IdlingResourceRegistry sync about rxidler

[technoir42]

I'm having a crash in DelegatingIdlingResourceScheduler.stopWork as well due to callback being null.

[kenyee]

Related to this, we get a similar crash by trying to do the init "(RxJavaPlugins.setInitIoSchedulerHandler(Rx2Idler.create(RXJAVA_IO_SCHEDULER_NAME))" in TestRunner's onCreate (otherwise, the app init creates schedulers before onStart so it will never get overridden w/ the idling one). If we do the init in onStart, the IOScheduler is already started so InitIoSchedulerHandler never gets called (verified w/ breakpoints)

[kenyee]

DelegatingIdlingResourceScheduler.stopWork() probably needs a null check for callback but it also looks like it's not initialized properly...

[danieldello]

I've written a workaround for this error while the issue is not merged, it's far from perfect but it does the trick for me.

class TestRunner : AndroidJUnitRunner() {

    override fun onStart() {
        RxJavaPlugins.setInitComputationSchedulerHandler {
            Rx2Idler.create("RxJava Computation Scheduler").apply(it).also {
                forceIdlingResourceRegistrySync()
            }
        }
        RxJavaPlugins.setInitIoSchedulerHandler {
            Rx2Idler.create("RxJava IO Scheduler").apply(it).also {
                forceIdlingResourceRegistrySync()
            }
        }
        super.onStart()
    }

    @Suppress("DEPRECATION")
    private fun forceIdlingResourceRegistrySync() {
        // Workaround for https://github.com/square/RxIdler/issues/20
        // This forces espresso to sync idling resources, cause the callback to be set for all available
        // resources and preventing RxIdler from crashing
        // By registering and unregistering a new resource we will force the registry to be updated,
        // we don't really care for the resource we are registering itself
        // TODO PLEASE REMOVE THIS ONCE THE BUG ON RXIDLER HAS BEEN FIXED
        val mockIdlingResource = object : IdlingResource {
            override fun getName(): String = "Mock Idling Resource Name"

            override fun isIdleNow(): Boolean = false

            override fun registerIdleTransitionCallback(callback: IdlingResource.ResourceCallback?) {
            }
        }
        Espresso.registerIdlingResources(mockIdlingResource)
        Espresso.unregisterIdlingResources(mockIdlingResource)
    }
}

I'm not sure if a simple call to Espresso.onIdle would suffice, and since I can't seem to reproduce it consistently I went with a flow that I could easily track. If there's a cleaner way to do this please let me know