Add tree navigation via arrow keys

app/src/sortedNodes.tsx

@@ -0,0 +1,19 @@
+import * as q from '../../backend/src/Model'
+import { SettingsState, TopicOrder } from './reducers/Settings'
+import { TopicViewModel } from './model/TopicViewModel'
+
+export function sortedNodes(settings: SettingsState, treeNode: q.TreeNode<any>): Array<q.TreeNode<TopicViewModel>> {
+  const topicOrder = settings.get('topicOrder')
+  let edges = treeNode.edgeArray
+  if (topicOrder === TopicOrder.abc) {
+    edges = edges.sort((a, b) => a.name.localeCompare(b.name))
+  }
+  let nodes = edges.map(edge => edge.target)
+  if (topicOrder === TopicOrder.messages) {
+    nodes = nodes.sort((a, b) => b.leafMessageCount() - a.leafMessageCount())
+  }
+  if (topicOrder === TopicOrder.topics) {
+    nodes = nodes.sort((a, b) => b.childTopicCount() - a.childTopicCount())
+  }
+  return nodes
+}