Espresso - how to get current activity to test Fragments?

Here is lelloman's solution with a slight change in Kotlin:

import android.app.Activity
import androidx.test.platform.app.InstrumentationRegistry.getInstrumentation
import androidx.test.runner.lifecycle.ActivityLifecycleMonitorRegistry
import androidx.test.runner.lifecycle.Stage

object EspressoHelper {
    fun getCurrentActivity(): Activity? {
        var currentActivity: Activity? = null
        getInstrumentation().runOnMainSync { run { currentActivity = ActivityLifecycleMonitorRegistry.getInstance().getActivitiesInStage(Stage.RESUMED).elementAtOrNull(0) } }
        return currentActivity
    }
}

I usually get it like this, it looks (and probably is) hacky but, hey, it works

import static android.support.test.InstrumentationRegistry.getInstrumentation;

public class MyTest {

    private Activity getActivityInstance(){
        final Activity[] currentActivity = {null};

        getInstrumentation().runOnMainSync(new Runnable(){
            public void run(){
                Collection<Activity> resumedActivity = ActivityLifecycleMonitorRegistry.getInstance().getActivitiesInStage(Stage.RESUMED);
                Iterator<Activity> it = resumedActivity.iterator();
                currentActivity[0] = it.next();
            }
        });

        return currentActivity[0];
    }
}