Issue 3200 - Delay initialization for prerendered documents

safari/ext/background.js

Index: safari/ext/background.js
===================================================================
--- a/safari/ext/background.js
+++ b/safari/ext/background.js
@@ -182,44 +182,59 @@
   var pageCounter = 0;
 
   var Page = function(id, tab, url)
   {
     this.id = id;
     this._tab = tab;
     this._frames = [{url: new URL(url), parent: null}];
 
-    if (tab.page)
-      this._messageProxy = new ext._MessageProxy(tab.page);
-    else
-      // while the new tab page is shown on Safari 7, the 'page' property
-      // of the tab is undefined, and we can't send messages to that page
-      this._messageProxy = {
-        handleRequest: function() {},
-        handleResponse: function() {},
-        sendMessage: function() {}
-      };
-
     this.browserAction = new BrowserAction(this);
     this.contextMenus = new ContextMenus(this);

[kzar, 2017/05/19 12:33:05]

I guess we should assign this._messageProxy to undefined (or null?) here
explicitly, since the modified logic in _getMessageProxy will sometimes return
that?

[Manish Jethani, 2017/05/19 23:58:48]

Yeah, it'll always return undefined or a non-null object. I think I'll just
assign it to null just to make it clear.

   };
   Page.prototype = {
     get url()
     {
       return this._frames[0].url;
     },
     sendMessage: function(message, responseCallback)
     {
       var documentIds = [];
       for (var documentId in this._tab._documentLookup)
         if (this._tab._documentLookup[documentId].pageId == this.id)
           documentIds.push(documentId);
 
-      this._messageProxy.sendMessage(message, responseCallback,
-                                     {targetDocuments: documentIds});
+      var messageProxy = this._getMessageProxy();
+      if (messageProxy)
+      {
+        messageProxy.sendMessage(message, responseCallback,
+                                 {targetDocuments: documentIds});
+      }
+    },
+    _getMessageProxy: function()
+    {
+      // Instantiate the message proxy only if the page object is available.
+      // For prerendered documents, the page object becomes available only once
+      // the document is made visible.
+      if (!this._messageProxy && this._tab.page)
+        this._messageProxy = new ext._MessageProxy(this._tab.page);
+
+      return this._messageProxy;
+    },
+    _handleRequest: function(request, sender)
+    {
+      var messageProxy = this._getMessageProxy();
+      if (messageProxy)
+        messageProxy.handleRequest(request, sender);
+    },
+    _handleResponse: function(response)
+    {
+      var messageProxy = this._getMessageProxy();
+      if (messageProxy)
+        messageProxy.handleResponse(response);
     }
   };
 
   ext.getPage = function(id)
   {
     return pages[id];
   };
 
@@ -419,23 +434,23 @@
 
             ext.onMessage._dispatch(message.payload, sender, sendResponse);
 
             event.message = response;
             break;
         }
         break;
       case "request":
-        sender.page._messageProxy.handleRequest(message, sender);
+        sender.page._handleRequest(message, sender);
         break;
       case "response":
         // All documents within a page have the same pageId and that's all we
         // care about here.
         var pageId = tab._documentLookup[message.targetDocuments[0]].pageId;
-        pages[pageId]._messageProxy.handleResponse(message);
+        pages[pageId]._handleResponse(message);
         break;
       case "replaced":
         // when a prerendered page is shown, forget the previous page
         // associated with its tab, and reset the toolbar item if necessary.
         // Note that it wouldn't be sufficient to do that when the old
         // page is unloading, because Safari dispatches window.onunload
         // only when reloading the page or following links, but not when
         // the current page is replaced with a prerendered page.