How can I call a function in an iframe's parent using Greasemonkey

Question

I have a Greasemonkey script which adds an iframe to the page (call it Page 1). The iframe contains another page (call it Page 2). The script runs on Page 2 as well. Pages 1 and 2 are on different domains.

I'd like to allow code running in Page 2 to call a function on Page 1. Given the lower restrictions on Greasemonkey code, is this possible?

Answer 1

The browser will prevent this, because the domains are different.

There are a couple of tricks you can use to communicate between frames:

1. Add a DNS record for the other website to the outer website's domain (www.somewebsite.com and someapp.somewebsite.com). Then use document.domain = "somewebsite.com" in both pages' JavaScript.
2. Use HTML 5 postMessage() to communicate between frames. I know it works in Firefox 3 and Internet Explorer 8, but not in IE7.
3. You can pass simple messages to another page by setting the parent window's URL. Note: It appears browsers prevent setting of the parent URL. This method will this only work for one-way communication from parent to child frame.

Ad 3: You won't be able to read the other frame's URL, but you can set it. If you change the URL to the exact same page, but with an #anchor component to the URL, the page will not actually reload:

window.frames["childFrame"].location.href = "http://www.somewebsite.com/#message"

You'd then need to add a script to the outer page that regularly polls it's location.href and process the messages. Yes, it's ugly, but if done right, it will work in all common browsers.