After searching a lot about how to test the live data returned by Room DAOs for unit testing, I found it would be a good idea to share it with other people. The language is Kotlin and I am using Robolectric to mock the context; But here, I just focus on the problem itself and I’m not gonna explain how to write the tests. You can of course check the links provided in the text.
I also have to mention that there are also other workarounds to achieve this, like using Mockito, but honestly in my opinion, this was the most clean approach. But if you prefer those ones, good luck with that. 😊
tl;dr
What did I want to do? Check if the LiveData objects returned by a DAO method are correct; And to have 100% test coverage on those files. 😉
What I did? Created a Kotlin extension function for LiveData object in my test class to block the main thread and wait for the observer to call onChange method and return the value. How? Using CountdownLatch. Read the answer to this Stack Overflow post.
private fun <T> LiveData<T>.blockingObserve(): T? {
var value: T? = null
val latch = CountDownLatch(1)
val observer = Observer<T> {
t ->
value = t
latch.countDown()
}
observeForever(observer)
latch.await(2, TimeUnit.SECONDS)
return value
}
The Problem: Although CountDownLatch is used to block the thread and wait for the value, but it would not block the thread and the value was always null. This is probably because of LiveData’s main feature which is usually being used in the main thread and obviously, it is not supposed to block the main thread, never!
Solution: Well, that’s pretty easy! You just need to add this rule to your test class:
@get:Rule val rule = InstantTaskExecutorRule()
It will allow the liveData observer to block the main thread and Viola!
Just don’t forget to add the core-testing dependency which is part of the Architecture library to your project and also match its version to the version of your current Architecture library that you’re using and everything will work like a charm:
testImplementation "android.arch.core:core-testing:1.1.1"
The answer to this post on StackOverflow also explains it.
Now the tests should work fine and as a reminder, I am running the tests as Local Unit Tests using Robolectric and not as Android Instrumented tests.
Good Luck Unit Testing 😊
Ali Kabiri 