Fetch all unread messages on reconnect

lib/client.js

@@ -276,4 +276,26 @@ export default class Client extends EventTarget {
 		this.ws.close(1000);
 		this.registered = false;
 	}
+
+	/* Execute a command that expects a response. `done` is called with message
+	 * events until it returns a truthy value. */
+	roundtrip(msg, done) {
+		return new Promise((resolve, reject) => {
+			var handleMessage = (event) => {
+				try {
+					var result = done(event);
+					if (result) {
+						this.removeEventListener("message", handleMessage);
+						resolve(result);
+					}
+				} catch (err) {
+					this.removeEventListener("message", handleMessage);
+					reject(err);
+				}
+			};
+
+			this.addEventListener("message", handleMessage);
+			this.send(msg);
+		});
+	}
 }