Discovering a Hidden iPhone URL Scheme

As an iPhone developer, one of the simplest and easiest mechanisms you have to interact with other applications is through the use of iPhone URL Schemes. These are so important that I’ve created a wiki page where I keep track of those I come across, including code samples that help me exchange data with them.

However, not all editors document the URL schemes they support in their apps, and this blocks reuse and collaboration. I recently came into such a problem, trying to use TwitterFon from my own apps, to post messages to Twitter. The TwitterFon site only specifies the following iPhone URL scheme:

twitterfon:///post?this%20is%20a%20test

The problem is, this URL scheme does not perform an URL-decoding on the message parameter, which means that a phrase like “this is a test” will appear in TwitterFon URL-encoded, that is, as “this%20is%20a%20test”. Clearly not acceptable.

However, thanks to Ashley Mills, I learnt that the USA Today iPhone app is able to use TwitterFon to share articles via Twitter, and does this properly, without URL-encoded characters. How do they do that? Obviously, they are using an URL scheme exported by TwitterFon, but not documented anywhere (*). I finally discovered that the URL scheme sought is the following (“message” instead of “post”!):

twitterfon:///message?some%20text%20here

This is how I found out: I impersonated TwitterFon in my own iPhone with an ad-hoc app created in Xcode, that shows me the URL used by USA Today to launch TwitterFon.

These are the steps required:

- (BOOL)application:(UIApplication *)application
      handleOpenURL:(NSURL *)url
{
    viewController.viewer.text = [url absoluteString];
    return YES;
}

You can download a zip file with the Xcode project created above if you want. Be careful if you run it on your own device!

Voilà! Finally, delete your own impersonated app, go to the App Store, re-install the application you’ve impersonated (normally it’s a free download, even for non-free apps), and you are done. The same mechanism could be used to find out similar, hidden URL mechanisms in other apps.

(*) Actually this URL scheme is only shortly mentioned in the changelog of the application…