Change reconnect delay to be a random amount between 3s and 15s (#164)

Author: mingweiw

src/directLine.ts

@@ -807,11 +807,16 @@ export class DirectLine implements IBotConnection {
             // WebSockets can be closed by the server or the browser. In the former case we need to
             // retrieve a new streamUrl. In the latter case we could first retry with the current streamUrl,
             // but it's simpler just to always fetch a new one.
-            .retryWhen(error$ => error$.delay(3000).mergeMap(error => this.reconnectToConversation()))
+            .retryWhen(error$ => error$.delay(this.getRetryDelay()).mergeMap(error => this.reconnectToConversation()))
         )
         .flatMap(activityGroup => this.observableFromActivityGroup(activityGroup))
     }
 
+    // Returns the delay duration in milliseconds
+    private getRetryDelay() {
+        return Math.floor(3000 + Math.random() * 12000);
+    }
+
     // Originally we used Observable.webSocket, but it's fairly opionated  and I ended up writing
     // a lot of code to work around their implemention details. Since WebChat is meant to be a reference
     // implementation, I decided roll the below, where the logic is more purposeful. - @billba