Add UserData ioDevice for setting and getting username from blocks

src/blocks/scratch3_sensing.js

@@ -308,9 +308,8 @@ class Scratch3SensingBlocks {
         return 0;
     }
 
-    getUsername () {
-        // Logged out users get empty string. Return that for now.
-        return '';
+    getUsername (args, util) {
+        return util.ioQuery('userData', 'getUsername');
     }
 }
 

src/engine/runtime.js

@@ -21,6 +21,7 @@ const DeviceManager = require('../io/deviceManager');
 const Keyboard = require('../io/keyboard');
 const Mouse = require('../io/mouse');
 const MouseWheel = require('../io/mouseWheel');
+const UserData = require('../io/userData');
 const Video = require('../io/video');
 
 const StringUtil = require('../util/string-util');
@@ -259,6 +260,7 @@ class Runtime extends EventEmitter {
             keyboard: new Keyboard(this),
             mouse: new Mouse(this),
             mouseWheel: new MouseWheel(this),
+            userData: new UserData(),
             video: new Video(this)
         };
 

src/io/userData.js

@@ -0,0 +1,24 @@
+class UserData {
+    constructor () {
+        this._username = '';
+    }
+
+    /**
+     * Handler for updating the username
+     * @param {object} data Data posted to this ioDevice.
+     * @property {!string} username The new username.
+     */
+    postData (data) {
+        this._username = data.username;
+    }
+
+    /**
+     * Getter for username. Initially empty string, until set via postData.
+     * @returns {!string} The current username
+     */
+    getUsername () {
+        return this._username;
+    }
+}
+
+module.exports = UserData;