TestSchedulerRule.kt

package com.some.app.common

import io.reactivex.Scheduler
import io.reactivex.android.plugins.RxAndroidPlugins
import io.reactivex.internal.schedulers.ExecutorScheduler
import io.reactivex.plugins.RxJavaPlugins
import io.reactivex.schedulers.TestScheduler
import org.junit.rules.TestRule
import org.junit.runner.Description
import org.junit.runners.model.Statement

/**
 * After applying such rule in unit test it will replace default schedulers via plugins api
 *
 * This class is especially helpful when testing streams with operators that have default schedulers
 * like [io.reactivex.Observable.delay]
 */
class TestSchedulerRule : TestRule {

    private val immediate = object : Scheduler() {
        override fun createWorker(): Worker {
            return ExecutorScheduler.ExecutorWorker(Runnable::run)
        }
    }

    val testScheduler = TestScheduler()

    override fun apply(base: Statement?, description: Description?): Statement {
        return object : Statement() {
            override fun evaluate() {
                RxJavaPlugins.setIoSchedulerHandler { testScheduler }
                RxJavaPlugins.setComputationSchedulerHandler { testScheduler }
                RxJavaPlugins.setNewThreadSchedulerHandler { testScheduler }
                RxAndroidPlugins.setMainThreadSchedulerHandler { immediate }
                try {
                    base?.evaluate()
                } finally {
                    RxJavaPlugins.reset()
                    RxAndroidPlugins.reset()
                }
            }
        }
    }
}