sendMessage from background script to content script in app fails - google-chrome

Question

UPDATE
From what I can tell, it is impossible to send a message from the background script to the content script using the "sendMessage" function. However there is a horrible workaround,
In your content script's window.onload, send a message to the background script:
chrome.runtime.sendMessage( { action: "messaging", window: "app" }, this.listenForFutureMessages );
Also in the content script, have the following function:
listenForFutureMessages: function(someAction)
{
//Take some action based on the message
//If we want the background script to be able to contact
//us again, we need to give them another callback. This
//is because Chrome only allows one use per callback
chrome.runtime.sendMessage( { action: "messaging", window: "app" }, this.listenForFutureMessages );
},
In the background script, have a listener that does something like this:
chrome.runtime.onMessage.addListener(
function(request, sender, sendResponse)
{
if ( request.action === "messaging" )
{
//Save the callback for later
this.listeners[ request.window ] = sendResponse;
//Tell chrome we will be using the callback later
return true;
}
}
);
When your background script wants to send the content script a message, simply call it like this:
this.listeners[ "app" ]( { someProperty: "some value" } );
This is a stupid way to do this, but it makes this actually possible. Hope this helps anyone else who needs this functionality.
ORIGINAL
I'm unable to send a message from my background script to a content script. When I try to find the tab id, it tells me I don't have permissions even though my app has that permission. And when I receive a message from the content script, and print out the sender object, it shows tab.id = -1. The API to send a message to a content script requires a tab id!
chrome.tabs.sendMessage(integer tabId, any message, function responseCallback)
The error:
chrome.tabs is not available: You do not have permission to access this API. Ensure that the required permission or manifest property is included in your manifest.json.
Error in event handler for 'undefined': Cannot call method 'sendMessage' of undefined TypeError: Cannot call method 'sendMessage' of undefined
at chrome-extension://panoaieakcofaegcjfbmhndaekfgpijh/scripts/background.js:109:16
at Event.dispatchToListener (event_bindings:356:21)
at Event.dispatch_ (event_bindings:342:27)
at Event.dispatch (event_bindings:362:17)
at miscellaneous_bindings:167:33
at Event.dispatchToListener (event_bindings:356:21)
at Event.dispatch_ (event_bindings:342:27)
at Event.dispatch (event_bindings:362:17)
at Object.chromeHidden.Port.dispatchOnMessage (miscellaneous_bindings:253:22)
So how do I contact my content script? (I have multiple windows and need to be able to contact them individually)
My manifest:
{
"manifest_version": 2,
"name": "App",
"description": "App",
"version": "0.75",
"minimum_chrome_version": "27",
"offline_enabled": true,
"icons":
{
"16": "images/icon16.png",
"48": "images/icon48.png",
"128": "images/icon128.png"
},
"app":
{
"background":
{
"scripts":
[
"scripts/background.js"
]
}
},
"permissions":
[
"unlimitedStorage",
"fullscreen",
{
"fileSystem":
[
"write"
]
},
"background",
"<all_urls>",
"tabs"
],
"update_url": "http://192.168.1.121/app.xml"
}

Answer 1

There's not such a thing called "Content scripts" in a Chrome app. Your manifest file looks like a mixture of a Chrome extension. Open chrome://extensions/, enable developer mode, and you would see a warning that the "background" and "tabs" permissions are invalid for a Chrome app.
If you're implementing a Chrome app, just use chrome.runtime.sendMessage and chrome.runtime.onMessage. These messages can be send from and to your event page and the main page. For example:
// event page (aka background page)
chrome.app.runtime.onLaunched.addListener(function() {
chrome.app.window.create('main.html');
});
// Later, when you want to notify the app window
chrome.runtime.sendMessage(" ... any message ... ");
<!-- main.html -->
<script src="main.js"></script>
// main.js
chrome.runtime.onMessage.addListener(function(message, sender, sendResponse) {
// Do something with the message
});