A JUnit 4 test rule for working with testing coroutines.

CoroutinesRule.kt

package com.abercrombie.testing.ui.rules

import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.test.TestCoroutineDispatcher
import kotlinx.coroutines.test.TestCoroutineScope
import kotlinx.coroutines.test.resetMain
import kotlinx.coroutines.test.runBlockingTest
import kotlinx.coroutines.test.setMain
import org.junit.rules.TestRule
import org.junit.runner.Description
import org.junit.runners.model.Statement

/**
 * Rule to add a simple way to set a test main dispatcher and a custom test scope and reseting
 * them to their defaults after the evaluation is done:
 *
 * https://proandroiddev.com/how-to-unit-test-code-with-coroutines-50c1640f6bef
 * https://github.com/Kotlin/kotlinx.coroutines/tree/master/kotlinx-coroutines-test
 */
@ExperimentalCoroutinesApi
class CoroutinesRule(
  private val testDispatcher: TestCoroutineDispatcher = TestCoroutineDispatcher(),
  private val testCoroutineScope: TestCoroutineScope = TestCoroutineScope(testDispatcher)
) : TestRule {

  override fun apply(base: Statement, description: Description?) = CoroutinesStatement(base)

  fun runBlockingTest(block: suspend TestCoroutineScope.() -> Unit) {
    testCoroutineScope.runBlockingTest { block() }
  }

  inner class CoroutinesStatement(private val base: Statement) : Statement() {
    override fun evaluate() {
      Dispatchers.setMain(testDispatcher)

      base.evaluate()

      Dispatchers.resetMain()
      testCoroutineScope.cleanupTestCoroutines()
    }
  }
}