Add RxStore and StoryViewerPage forward navigation.

app/src/main/java/org/thoughtcrime/securesms/util/rx/RxStore.kt

@@ -0,0 +1,30 @@
+package org.thoughtcrime.securesms.util.rx
+
+import io.reactivex.rxjava3.core.Flowable
+import io.reactivex.rxjava3.processors.BehaviorProcessor
+import io.reactivex.rxjava3.schedulers.Schedulers
+import io.reactivex.rxjava3.subjects.PublishSubject
+
+/**
+ * Rx replacement for Store.
+ * Actions are run on the computation thread.
+ */
+class RxStore<T : Any>(defaultValue: T) {
+
+  private val behaviorProcessor = BehaviorProcessor.createDefault(defaultValue)
+  private val actionSubject = PublishSubject.create<(T) -> T>().toSerialized()
+
+  val state: T get() = behaviorProcessor.value!!
+  val stateFlowable: Flowable<T> = behaviorProcessor
+
+  init {
+    actionSubject
+      .observeOn(Schedulers.computation())
+      .scan(defaultValue) { v, f -> f(v) }
+      .subscribe { behaviorProcessor.onNext(it) }
+  }
+
+  fun update(transformer: (T) -> T) {
+    actionSubject.onNext(transformer)
+  }
+}