Android Navigation Instrumentation test throws Method addObserver must be called on the main thread

Question

I have two Fragments, one being a home fragment in my graph. The User will be navigated to the second fragment upon clicking a button. It works as expected by navigating the user to the second fragment and displaying the text. So the graph is good.

Now I wanted to write an instrumentation test.

@RunWith(AndroidJUnit4::class)
class TransitionTest {

    @Test
    fun testNavigationToSecondFragment() {
        
        val navController = TestNavHostController(
            ApplicationProvider.getApplicationContext())

        navController.setGraph(R.navigation.my_graph) <--throws exception

        // the rest of my test continues here
    }
}

However, the line shown above to set the graph throws following exception:

IllegalStateException: Method addObserver must be called on the main thread.

My environment:

fragment_version = 1.2.5 nav_version = 2.3.1 espresso = 3.3.0

Does anyone have any idea what is going on and how to solve it?

Answer 1

I wrapped the setGraph function in runOnUiThread as such, and the test passes. I will update the answer once I find out the real cause and better solution.

runOnUiThread {
    navController.setGraph(R.navigation.my_graph)
}

Answer 2

I faced the same, so I will try to explain the root cause of this. Starting from androidx.lifecycle:lifecycle-*:2.3.0-alpha06. There is a behavior change in LifecycleRegistry . LifecycleRegistry now verifies that its methods are called on the main thread. Now remains another question unsolved whether the instrumentation tests are running in the MainThread? Apparently no otherwise it would have passed. It is using Instrumentation Thread where most of the tests run. As per documentation, you can use @UiThreadTest or runOnUiThread as in the top answer.

Answer 3

I started having this problem as well, after upgrading the package:

implementation 'androidx.activity:activity:1.2.0-alpha06'

to

 implementation 'androidx.activity:activity:1.2.0-beta01'

In my code I am opening a Fragment in a background thread, and this seems to break with new package. The fragment adds an observer during lifecycle:

I think your workaround of wrapping the call to run on UI thread is the only plausible solution, since the observer may be added internally in your case as well.

Answer 4

There is one more solution to solve the issue by using this

InstrumentationRegistry.getInstrumentation().runOnMainSync {
            navController.setGraph(R.navigation.my_graph)
        }

Since @UiThreadTest only work in @Test, @Before and @After so if you have some common test function they might not work as expected