Coroutine unit test

build.gradle

testImplementation "org.jetbrains.kotlinx:kotlinx-coroutines-test:1.6.4"

MainCoroutineRule.kt

class MainCoroutineRule : TestWatcher(){
  //TestWatcher is class which is use to make JUnit rule.
  val testDispatcher = StandardTestDispatcher()
  
  override fun starting(discription: Description?){
      super.starting(description)
      Dispatchers.setMain(testDispatcher)
  }
  
  override fun finished(description: Description?){
      super.finished(description)
      Dispatchers.resetMain()
  }
  
}

Util.kt

class Util(val dispatcher:CoroutineDispatcher){
  var globalArg = false
  suspend fun getUserName(): AddressDetail(){
      CoroutineScope(dispatcher).launch{
      globalArg = true
      }
  }
}

UtilTest.kt

class UtilTest{

  @get:Rule
  val mainCoroutineRule = MainCoroutineRule()
  
  @Test
  fun testGetUser(){
      val sut = Util(mainCoroutineRule.testDispatcher)
      
      runtest{
          sut.getAddressDetail()
          //When we lounch a coroutine then this test dispatcher will 
          //add this scheduler on a qoue for scheduling but doesn't execute it.
          //now its our responsibility to tell the testDispatcher to execute the coroutine.
          //so below peice of code will execute the coroutine and hold the thread
          //till all the coroutine will not get executed.
          mainCoroutineRule.testDispatcher.scheduler.advanceUntilIdle()
          Assert.assertEquals(expected:true, sut.globalArg)
      }
  }
}