Widgets: Make a generic postMessage API in WidgetVC

It will be used for https://docs.google.com/document/d/1uPF7XWY_dXTKVKV7jZQ2KmsI19wn9-kFRgQ1tFQP7wQ/edit#.

The Modular postMessage API becomes a specialisation of it.

Riot/Assets/js/postMessageAPI.js

@@ -0,0 +1,67 @@
+/*
+ Copyright 2017 Vector Creations Ltd
+
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+ */
+
+window.riotIOS = {};
+
+// Generic JS -> ObjC bridge
+window.riotIOS.sendObjectMessageToObjC = function(parameters) {
+    var iframe = document.createElement('iframe');
+    iframe.setAttribute('src', 'js:' + JSON.stringify(parameters));
+
+    document.documentElement.appendChild(iframe);
+    iframe.parentNode.removeChild(iframe);
+    iframe = null;
+};
+
+window.riotIOS.events = {};
+
+// Listen to messages posted by the widget
+window.riotIOS.onMessage = function(event) {
+
+    // Do not SPAM ObjC with event already managed
+    if (riotIOS.events[event.data._id]) {
+        return;
+    }
+
+    if (!event.origin) { // stupid chrome
+        event.origin = event.originalEvent.origin;
+    }
+
+    // Keep this event for future usage
+    riotIOS.events[event.data._id] = event;
+
+    riotIOS.sendObjectMessageToObjC({
+                                    'event.data': event.data,
+                                    });
+};
+window.addEventListener('message', riotIOS.onMessage, false);
+
+
+// ObjC -> Widget JS bridge
+window.riotIOS.sendResponse = function(eventId, res) {
+
+    // Retrieve the correspong JS event
+    var event = riotIOS.events[eventId];
+
+    console.log("sendResponse to " + event.data.action + " for "+ eventId + ": " + JSON.stringify(res));
+
+    var data = JSON.parse(JSON.stringify(event.data));
+    data.response = res;
+    event.source.postMessage(data, event.origin);
+
+    // Mark this event as handled
+    riotIOS.events[eventId] = true;
+}