Issue 3823 - Split up message responder code

lib/messaging.js

+/*
+ * This file is part of Adblock Plus <https://adblockplus.org/>,
+ * Copyright (C) 2006-2016 Eyeo GmbH
+ *
+ * Adblock Plus is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * Adblock Plus is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @module messaging */
+
+"use strict";
+
+/**
+ * Communication port wrapping ext.onMessage to receive messages.
+ *
+ * @constructor
+ */
+function Port()
+{
+  this._callbacks = Object.create(null);
+  this._onMessage = this._onMessage.bind(this);
+  ext.onMessage.addListener(this._onMessage);
+};
+
+Port.prototype = {
+  _onMessage: function(message, sender, sendResponse)
+  {
+    let async = false;
+    let callbacks = this._callbacks[message.type];
+
+    if (callbacks)
+    {
+      for (let callback of callbacks)
+      {
+        let response = callback(message, sender);
+
+        if (response && typeof response.then == "function")
+        {
+          response.then(
+            sendResponse,
+            reason => {
+              console.error(reason);
+              sendResponse(undefined);
+            }
+          );
+          async = true;
+        }
+        else if (typeof response != "undefined")
+        {
+          sendResponse(response);
+        }
+      }
+    }
+
+    return async;
+  },
+
+  /**
+   * Function to be called when a particular message is received.
+   *
+   * @callback Port~messageCallback
+   * @param {object} message
+   * @param {object} sender
+   * @return The callback can return undefined (no response),
+   *         a value (response to be sent to sender immediately)
+   *         or a promise (asynchronous response).
+   */
+
+  /**
+   * Adds a callback for the specified message.
+   *
+   * The return value of the callback (if not undefined) is sent as response.
+   * @param {string}   name
+   * @param {Port~messageCallback} callback
+   */
+  on: function(name, callback)
+  {
+    if (name in this._callbacks)
+      this._callbacks[name].push(callback);
+    else
+      this._callbacks[name] = [callback];
+  },
+
+  /**
+   * Removes a callback for the specified message.
+   *
+   * @param {string}   name
+   * @param {Port~messageCallback} callback
+   */
+  off: function(name, callback)
+  {
+    let callbacks = this._callbacks[name];
+    if (callbacks)
+    {
+      let idx = callbacks.indexOf(callback);
+      if (idx != -1)
+        callbacks.splice(idx, 1);
+    }
+  }
+};
+
+/**
+ * The default port to receive messages.
+ *
+ * @type {Port}
+ */
+exports.port = new Port();
+
+/**
+ * Creates a new port that is shutdown when the given window is unloaded.
+ *
+ * @param {Window} window
+ * @return {Port}
+ */
+exports.getPort = function(window)
+{
+  let port = new Port();
+  window.addEventListener("unload", () =>
+  {
+    ext.onMessage.removeListener(port._onMessage);
+  });

[Wladimir Palant, 2016/03/21 15:50:13]

Please consider caching the port per window so that it won't be created multiple
times unnecessarily.

Also, maybe have Port.disconnect() method that's called here like in Firefox?
Note that I export Port constructor. This way the API allows you to create and
disconnect a port manually if necessary.

[Sebastian Noack, 2016/03/21 17:15:42]

Done.

+  return port;
+};
+