Support backstack read operations from a background thread

magellan-library/src/main/java/com/wealthfront/magellan/navigation/CopyOnWriteArrayDeque.kt

@@ -0,0 +1,147 @@
+package com.wealthfront.magellan.navigation
+
+import java.util.Deque
+import java.util.concurrent.CopyOnWriteArrayList
+
+internal class CopyOnWriteArrayDeque<E> : Deque<E> {
+
+  private val list = CopyOnWriteArrayList<E>()
+
+  override fun add(element: E): Boolean {
+    return list.add(element)
+  }
+
+  override fun addAll(elements: Collection<E>): Boolean {
+    return list.addAll(elements)
+  }
+
+  override fun clear() {
+    list.clear()
+  }
+
+  override fun iterator(): MutableIterator<E> {
+    return list.iterator()
+  }
+
+  override fun remove(): E {
+    return list.removeFirst()
+  }
+
+  override fun remove(element: E?): Boolean {
+    return list.remove(element)
+  }
+
+  override fun removeAll(elements: Collection<E>): Boolean {
+    return list.removeAll(elements.toSet())
+  }
+
+  override fun retainAll(elements: Collection<E>): Boolean {
+    return list.retainAll(elements.toSet())
+  }
+
+  override fun contains(element: E?): Boolean {
+    return list.contains(element)
+  }
+
+  override fun containsAll(elements: Collection<E>): Boolean {
+    return list.containsAll(elements)
+  }
+
+  override fun isEmpty(): Boolean {
+    return list.isEmpty()
+  }
+
+  override fun offer(e: E): Boolean {
+    return list.add(e)
+  }
+
+  override fun poll(): E? {
+    return list.removeFirstOrNull()
+  }
+
+  override fun element(): E {
+    return list.first()
+  }
+
+  override fun peek(): E? {
+    return list.firstOrNull()
+  }
+
+  override fun addFirst(e: E) {
+    list.add(0, e)
+  }
+
+  override fun addLast(e: E) {
+    add(e)
+  }
+
+  override fun offerFirst(e: E): Boolean {
+    list.add(0, e)
+    return true
+  }
+
+  override fun offerLast(e: E): Boolean {
+    return list.add(e)
+  }
+
+  override fun removeFirst(): E {
+    return list.removeFirst()
+  }
+
+  override fun removeLast(): E {
+    return list.removeLast()
+  }
+
+  override fun pollFirst(): E? {
+    return list.removeFirstOrNull()
+  }
+
+  override fun pollLast(): E? {
+    return list.removeLastOrNull()
+  }
+
+  override fun getFirst(): E {
+    return list.first()
+  }
+
+  override fun getLast(): E {
+    return list.last()
+  }
+
+  override fun peekFirst(): E? {
+    return list.firstOrNull()
+  }
+
+  override fun peekLast(): E? {
+    return list.lastOrNull()
+  }
+
+  override fun removeFirstOccurrence(o: Any?): Boolean {
+    return list.remove(o)
+  }
+
+  override fun removeLastOccurrence(o: Any?): Boolean {
+    val lastIndex = list.lastIndexOf(o)
+    return if (lastIndex >= 0) {
+      list.removeAt(lastIndex)
+      true
+    } else {
+      false
+    }
+  }
+
+  override fun push(e: E) {
+    addFirst(e)
+  }
+
+  override fun pop(): E {
+    return removeFirst()
+  }
+
+  override fun descendingIterator(): MutableIterator<E> {
+    throw NotImplementedError("descendingIterator() is not supported.")
+  }
+
+  override val size: Int
+    get() = list.size
+}

magellan-library/src/main/java/com/wealthfront/magellan/navigation/NavigationDelegate.kt

@@ -32,7 +32,7 @@ public open class NavigationDelegate(
 
   protected var containerView: ScreenContainer? = null
   protected val navigationPropagator: NavigationPropagator = NavigationPropagator
-  public val backStack: Deque<NavigationEvent> = ArrayDeque()
+  public val backStack: Deque<NavigationEvent> = CopyOnWriteArrayDeque()
 
   protected val Deque<NavigationEvent>.currentNavigable: NavigableCompat?
     get() {