Ugly attempt to Idle Espresso test until view is visible. DO NOT USE THIS. Code sample for discussion here: https://issuetracker.google.com/issues/79626762

IdleUntilVisibleIdlingResource.java

package com.example.app.tests;

import android.support.test.espresso.IdlingResource;
import android.support.test.espresso.ViewFinder;
import android.support.test.espresso.ViewInteraction;
import android.util.Log;
import android.view.View;

import java.lang.reflect.Field;

import static android.support.test.espresso.Espresso.onView;
import static android.support.test.espresso.matcher.ViewMatchers.withId;

/**
 * An implementation of {@link IdlingResource} that is idle until view (provided by viewId) is visible.
 */
public class IdleUntilVisibleIdlingResource implements IdlingResource {
    private static final String TAG = IdleUntilVisibleIdlingResource.class.getSimpleName();

    private ResourceCallback resourceCallback;
    private final int viewId;

    public IdleUntilVisibleIdlingResource(int viewId) {
        this.viewId = viewId;
    }

    @Override
    public String getName() {
        return TAG;
    }

    @Override
    public boolean isIdleNow() {
        // i don't know what activity is current, so I can't just call findViewById()
        View viewOfInterest;
        try {
            Field field = ViewInteraction.class.getDeclaredField("viewFinder");
            field.setAccessible(true);
            ViewInteraction viewInteraction = onView(withId(viewId));
            ViewFinder viewFinder = (ViewFinder) field.get(viewInteraction);
            viewOfInterest = viewFinder.getView();
        } catch (IllegalAccessException | NoSuchFieldException e) {
            Log.e(TAG, "Reflection failed", e);
            // if reflection fails, just return false and eventually we'll hit our IdlingPolicy timeout
            return false;
        }

        boolean isIdle = viewOfInterest != null && viewOfInterest.getVisibility() == View.VISIBLE;
        if (isIdle) {
            resourceCallback.onTransitionToIdle();
        }
        return isIdle;
    }

    @Override
    public void registerIdleTransitionCallback(ResourceCallback callback) {
        resourceCallback = callback;
    }
}

TestCases.java

@RunWith(AndroidJUnit4.class)
public final class TestCases {

    @Rule
    public ActivityTestRule<IntroActivity> activityRule = new ActivityTestRule<>(IntroActivity.class);

    @Before
    public void setUp() {
        IdlingRegistry.getInstance().register(new IdleUntilVisibleIdlingResource(R.id.some_view_of_interest));
    }

    @Test
    public void someTest() {
        // 1. test starts on Intro Activity
        // 2. click on a button that navigates off IntroActivity and to a second, unknown activity
        // 3. idle until some view is visible on the second activity; see setUp() method
        // 4. do some view assertions on the second activity

        // In step #3, I need to get access to this view. However because I don't know which activity is the current
        // activity, I can't use findViewById(). So I use a hack to get the view directly from Espresso (which has
        // already done most of the ugly/hacky/relfective work to get access to the view tree)
    }
}