Wait for ViewPager to be idle in instrumented tests

This fixes the flakiness issues with the intro tests

app/src/androidTest/java/com/beemdevelopment/aegis/IntroTest.java

@@ -13,10 +13,13 @@ import static junit.framework.TestCase.assertNull;
 import static junit.framework.TestCase.assertTrue;
 import static org.hamcrest.Matchers.not;
 
+import androidx.test.espresso.IdlingRegistry;
+import androidx.test.espresso.IdlingResource;
 import androidx.test.espresso.ViewInteraction;
 import androidx.test.ext.junit.rules.ActivityScenarioRule;
 import androidx.test.ext.junit.runners.AndroidJUnit4;
 import androidx.test.filters.LargeTest;
+import androidx.viewpager2.widget.ViewPager2;
 
 import com.beemdevelopment.aegis.rules.ScreenshotTestRule;
 import com.beemdevelopment.aegis.ui.IntroActivity;
@@ -25,6 +28,8 @@ import com.beemdevelopment.aegis.vault.slots.BiometricSlot;
 import com.beemdevelopment.aegis.vault.slots.PasswordSlot;
 import com.beemdevelopment.aegis.vault.slots.SlotList;
 
+import org.junit.After;
+import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.RuleChain;
@@ -39,9 +44,24 @@ import dagger.hilt.android.testing.HiltAndroidTest;
 public class IntroTest extends AegisTest {
     private final ActivityScenarioRule<IntroActivity> _activityRule = new ActivityScenarioRule<>(IntroActivity.class);
 
+    private ViewPager2IdlingResource _viewPager2IdlingResource;
+
     @Rule
     public final TestRule testRule = RuleChain.outerRule(_activityRule).around(new ScreenshotTestRule());
 
+    @Before
+    public void setUp() {
+        _activityRule.getScenario().onActivity(activity -> {
+            _viewPager2IdlingResource = new ViewPager2IdlingResource(activity.findViewById(R.id.pager), "viewPagerIdlingResource");
+            IdlingRegistry.getInstance().register(_viewPager2IdlingResource);
+        });
+    }
+
+    @After
+    public void tearDown() {
+        IdlingRegistry.getInstance().unregister(_viewPager2IdlingResource);
+    }
+
     @Test
     public void doIntro_None() {
         ViewInteraction next = onView(withId(R.id.btnNext));
@@ -56,7 +76,6 @@ public class IntroTest extends AegisTest {
         next.perform(click());
         prev.check(matches(not(isDisplayed())));
         next.perform(click());
-        next.perform(click());
 
         VaultRepository vault = _vaultManager.getVault();
         assertFalse(vault.isEncryptionEnabled());
@@ -93,4 +112,39 @@ public class IntroTest extends AegisTest {
         assertTrue(slots.has(PasswordSlot.class));
         assertFalse(slots.has(BiometricSlot.class));
     }
+
+    // Source: https://stackoverflow.com/a/32763454/12972657
+    private static class ViewPager2IdlingResource implements IdlingResource {
+        private final String _resName;
+        private boolean _isIdle = true;
+        private IdlingResource.ResourceCallback _resourceCallback = null;
+
+        public ViewPager2IdlingResource(ViewPager2 viewPager, String resName) {
+            viewPager.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
+                @Override
+                public void onPageScrollStateChanged(int state) {
+                    _isIdle = (state == ViewPager2.SCROLL_STATE_IDLE || state == ViewPager2.SCROLL_STATE_DRAGGING);
+                    if (_isIdle && _resourceCallback != null) {
+                        _resourceCallback.onTransitionToIdle();
+                    }
+                }
+            });
+            _resName = resName;
+        }
+
+        @Override
+        public String getName() {
+            return _resName;
+        }
+
+        @Override
+        public boolean isIdleNow() {
+            return _isIdle;
+        }
+
+        @Override
+        public void registerIdleTransitionCallback(IdlingResource.ResourceCallback resourceCallback) {
+            _resourceCallback = resourceCallback;
+        }
+    }
 }