Apple has allowed iOS apps to register their own URL schemes on your devices for a while now, but I've never used this functionality in-depth 'till just recently.
Having your app register a URL scheme on a device means that you can open an installed application on a user's iPhone or iPad in ONE CLICK from their email (or the web, or another application). There are all sorts of apps already using this functionality.
We just used this for a project where the client would send confirmation emails to the user after signup. The email would contain an link to a URL. The URL opens in mobile Safari, then launches the app and seamlessly logs the user into the app.
That's one use case; there's probably others (let us know yours). Another one could be a URL embeded in text—say someone is reading on their iPad and the content could prompt them to hop over to the app.
Try It Out
If a user has your app installed on their device, all you have to do is send them to my-great-app://whatever/ and Safari will resign to the background and your app will open. Why don't you try it? I know you've got the Facebook app installed on your iPhone. Just clear your address bar and type (or click the link): fb://profile
Go ahead, I'll wait.
It opened the Facebook app and took you to your profile page (or gave you a big fat cryptic error message if you don't have the Facebook app installed). Facebook allows a LOT of options that you can pass to the application to take you into various parts of the app.
So enough about the Facebook app. What does a developer do if they want to pass a bunch of data to the app—without actually defining what needs to be passed using a bunch of if/else or switch statements?
It's fairly trivial to add url scheme to your application's Info.plist:
Register the url scheme in your Info.plist. Lets say: my-great-app://
In the application delegate, implement this method and return YES:
I wrote a method you can add to your application that allows the passing of data through the url with key/pair values like so:
When the user clicks a link like that, or you auto-redirect them from your site to that URL, you can parse the URL into an NSDictionary with this code. Here's the method and an example implementation along with some pretty badass comments so you know what the heck is going on:
Update for Swift