Add RxStore and StoryViewerPage forward navigation.

app/src/test/java/org/thoughtcrime/securesms/util/rx/RxStoreTest.kt

@@ -0,0 +1,70 @@
+package org.thoughtcrime.securesms.util.rx
+
+import io.reactivex.rxjava3.plugins.RxJavaPlugins
+import io.reactivex.rxjava3.schedulers.TestScheduler
+import org.junit.After
+import org.junit.Before
+import org.junit.Test
+
+class RxStoreTest {
+
+  private val testScheduler = TestScheduler()
+
+  @Before
+  fun setUp() {
+    RxJavaPlugins.setInitComputationSchedulerHandler { testScheduler }
+    RxJavaPlugins.setComputationSchedulerHandler { testScheduler }
+  }
+
+  @After
+  fun tearDown() {
+    RxJavaPlugins.reset()
+  }
+
+  @Test
+  fun `Given an initial state, when I observe, then I expect my initial state`() {
+    // GIVEN
+    val testSubject = RxStore(1)
+
+    // WHEN
+    val subscriber = testSubject.stateFlowable.test()
+    testScheduler.triggerActions()
+
+    // THEN
+    subscriber.assertValueAt(0, 1)
+    subscriber.assertNotComplete()
+  }
+
+  @Test
+  fun `Given immediate observation, when I update, then I expect both states`() {
+    // GIVEN
+    val testSubject = RxStore(1)
+
+    // WHEN
+    val subscriber = testSubject.stateFlowable.test()
+    testSubject.update { 2 }
+
+    testScheduler.triggerActions()
+
+    // THEN
+    subscriber.assertValueAt(0, 1)
+    subscriber.assertValueAt(1, 2)
+    subscriber.assertNotComplete()
+  }
+
+  @Test
+  fun `Given late observation after several updates, when I observe, then I expect latest state`() {
+    // GIVEN
+    val testSubject = RxStore(1)
+    testSubject.update { 2 }
+
+    // WHEN
+    testScheduler.triggerActions()
+    val subscriber = testSubject.stateFlowable.test()
+    testScheduler.triggerActions()
+
+    // THEN
+    subscriber.assertValueAt(0, 2)
+    subscriber.assertNotComplete()
+  }
+}