Android view events for tests

AndroidViewEvent.kt

import com.jakewharton.rxrelay2.PublishRelay
import io.reactivex.Observable
import io.reactivex.Observer
import org.junit.rules.TestRule
import org.junit.runner.Description
import org.junit.runners.model.Statement

/**
 * [PublishRelay]-like [Observable] enforcing:
 *   1. a unique observer at a time, and
 *   2. that subscription happens on the main thread.
 *
 * In Android views, we bind our events with [RxBinding](https://github.com/JakeWharton/RxBinding).
 * RxBinding enforces the two above listed conditions; but we lose those enforcement when doing
 * testing in our abstracted JVM tests. [AndroidViewEvent] would allow unit tests to fail if a
 * presenter violates any of the conditions `RxBinding` enforces.
 */
class AndroidViewEvent<VE : Any> private constructor() : Observable<VE>(), TestRule {
  private val events = PublishRelay.create<VE>()
  private var mainThreadId = -1L
  private var subscribed = false

  fun accept(viewEvent: VE) {
    events.accept(viewEvent)
  }

  override fun subscribeActual(observer: Observer<in VE>) {
    // Check main thread.
    check(Thread.currentThread().id == mainThreadId)

    check(!subscribed) { "Already subscribed to." }
    subscribed = true

    events.subscribe(observer)
  }

  override fun apply(
    base: Statement,
    description: Description
  ): Statement {
    return object : Statement() {
      override fun evaluate() {
        mainThreadId = Thread.currentThread().id
        base.evaluate()
      }
    }
  }

  companion object {
    fun <VE: Any> create() = AndroidViewEvent<VE>()
  }
}