Add tree navigation via arrow keys

app/src/model/TopicViewModel.ts

@@ -3,22 +3,37 @@ import { EventDispatcher } from '../../../events'
 
 export class TopicViewModel implements Destroyable {
   private selected: boolean
-  public change = new EventDispatcher<void, TopicViewModel>()
+  private expanded: boolean
+  public selectionChange = new EventDispatcher<void, TopicViewModel>()
+  public expandedChange = new EventDispatcher<void, TopicViewModel>()
 
   public constructor() {
     this.selected = false
+    this.expanded = false
   }
 
   public destroy() {
-    this.change.removeAllListeners()
+    this.selectionChange.removeAllListeners()
   }
 
   public isSelected() {
     return this.selected
   }
 
+  public isExpanded() {
+    return this.expanded
+  }
+
   public setSelected(selected: boolean) {
     this.selected = selected
-    this.change.dispatch()
+    this.selectionChange.dispatch()
+  }
+
+  public setExpanded(expanded: boolean, fireEvent: boolean) {
+    const didChange = this.expanded !== expanded
+    this.expanded = expanded
+    if (didChange && fireEvent) {
+      this.expandedChange.dispatch()
+    }
   }
 }