Retrieve URL from iframe using window.postMessage

I'm trying to access the URL of an iFrame that I open on my site. The iFrame is from another domain so traditional methods violate the same origin policy and are flagged as cross site scripting. However, I just discovered window.postMessage, but I don't quite know how to pull it all together.

My goal is to capture the final URL in the iFrame. For example, a开发者_运维问答 user clicks a button on my site to open an iFrame. Say that iFrame loads a web site and the user clicks around on the site. When they close the iFrame, I want to capture the URL so the next time they click the button on my site, I can open the iFrame to the same state that the site was in last time they visited. Is this possible with window.postMessage?

Thanks!

document.iframe[name].src 
document.iframe[name].document.location.href