Add API endpoints to get current widget postMessage API version and supported API versions.

src/WidgetMessaging.js

@@ -17,23 +17,23 @@ limitations under the License.
 /*
 Listens for incoming postMessage requests from embedded widgets. The following API is exposed:
 {
-    widgetData: {
-        action: "content_loaded"
-        // additional request fields
-    },
+    api: "widget",
+    action: "content_loaded",
+    // additional request fields
     widgetId: $WIDGET_ID
 }
 
 The complete request object is returned to the caller with an additional "response" key like so:
 {
-    widgetData: {
-        action: "content_loaded"
-        // additional request fields
-    },
+    api: "widget",
+    action: "content_loaded",
+    // additional request fields
     widgetId: $WIDGET_ID
     response: { ... }
 }
 
+The "api" field is required to use this API, and must be set to "widget" in all requests.
+
 The "action" determines the format of the request and response. All actions can return an error response.
 
 A success response is an object with zero or more keys.
@@ -50,6 +50,7 @@ The "message" key should be a human-friendly string.
 
 ACTIONS
 =======
+** All actions must include an "api" field with valie "widget".**
 All actions can return an error response instead of the response outlined below.
 
 content_loaded
@@ -65,13 +66,52 @@ Response:
 }
 Example:
 {
-    widgetData: {
-        action: "content_loaded"
-    },
+    api: "widget",
+    action: "content_loaded",
     widgetId: $WIDGET_ID
 }
+
+
+api_version
+-----------
+Get the current version of the widget postMessage API
+
+Request:
+ - No additional fields.
+Response:
+{
+    api_version: "0.0.1"
+}
+Example:
+{
+    api: "widget",
+    action: "api_version",
+}
+
+supported_api_versions
+----------------------
+Get versions of the widget postMessage API that are currently supported
+
+Request:
+ - No additional fields.
+Response:
+{
+    api: "widget"
+    supported_versions: ["0.0.1"]
+}
+Example:
+{
+    api: "widget",
+    action: "supported_api_versions",
+}
+
 */
 
+const WIDGET_API_VERSION = '0.0.1'; // Current API version
+const SUPPORTED_WIDGET_API_VERSIONS = [
+    '0.0.1',
+];
+
 import dis from './dispatcher';
 
 let listenerCount = 0;
@@ -91,20 +131,30 @@ function onMessage(event) {
     if (
         event.origin.length === 0 ||
         trustedEndpoint(event.origin) ||
-        !event.data.widgetData ||
+        event.data.api !== "widget" ||
         !event.data.widgetId
     ) {
         return; // don't log this - debugging APIs like to spam postMessage which floods the log otherwise
     }
 
-    const widgetData = event.data.widgetData;
+    const action = event.data.action;
     const widgetId = event.data.widgetId;
-    if (widgetData.action == 'content_loaded') {
+    if (action == 'content_loaded') {
         dis.dispatch({
             action: 'widget_content_loaded',
             widgetId: widgetId,
         });
         sendResponse(event, {success: true});
+    } else if (action == 'supported_api_versions') {
+        sendResponse(event, {
+            api: "widget",
+            supported_versions: SUPPORTED_WIDGET_API_VERSIONS,
+        });
+    } else if (action == 'api_version') {
+        sendResponse(event, {
+            api: "widget",
+            version: WIDGET_API_VERSION,
+        });
     } else {
         console.warn("Widget postMessage event unhandled");
         sendError(event, {message: "The postMessage was unhandled"});