Bug 1240804 - Wait for RDM tool to init first. r=me

MozReview-Commit-ID: Eb1xsE5irMw

devtools/client/responsive.html/index.js

@@ -43,10 +43,11 @@ let bootstrap = {
     this.telemetry.toolOpened("responsive");
     let store = this.store = Store();
     let app = App({
-      onExit: () => window.postMessage({type: "exit"}, "*"),
+      onExit: () => window.postMessage({ type: "exit" }, "*"),
     });
     let provider = createElement(Provider, { store }, app);
     ReactDOM.render(provider, document.querySelector("#root"));
+    window.postMessage({ type: "init" }, "*");
   },
 
   destroy() {

devtools/client/responsive.html/manager.js

@@ -196,6 +196,7 @@ ResponsiveUI.prototype = {
     tabBrowser.loadURI(TOOL_URL);
     yield tabLoaded(this.tab);
     let toolWindow = this.toolWindow = tabBrowser.contentWindow;
+    yield waitForMessage(toolWindow, "init");
     toolWindow.addInitialViewport(contentURI);
     toolWindow.addEventListener("message", this);
   }),
@@ -226,6 +227,21 @@ ResponsiveUI.prototype = {
   },
 };
 
+function waitForMessage(win, type) {
+  let deferred = promise.defer();
+
+  let onMessage = event => {
+    if (event.data.type !== type) {
+      return;
+    }
+    win.removeEventListener("message", onMessage);
+    deferred.resolve();
+  };
+  win.addEventListener("message", onMessage);
+
+  return deferred.promise;
+}
+
 function tabLoaded(tab) {
   let deferred = promise.defer();