A Universal Link is very similar to a deep link. Deep links allow apps to link users straight into a certain section of an application. Before Universal Links, developers had to use a custom URL scheme to create their deep links.

You might have seen a URL with a custom scheme in the past. These URLs are easily recognized and look as follows:

mustc://FamilyMember/jack 

It's evident that this isn't a regular web URL because web URLs start with a scheme of either http:// or https://. An application can register itself as capable of opening URLs with a certain scheme. So, the MustC app we've been working on could manifest itself as a handler of mustc:// URLs.

However, there are a couple of downsides to this approach. First and foremost, this URL isn't shareable at all. You can't send this URL to any friends that don't have the same app installed. If you were to send this URL to somebody and they didn't have the corresponding app installed, they wouldn't be able to open this URL. This is inconvenient because, for others to access the same content, assuming the content is publicly available on the web, users would have to share a different URL that points to the website. But sharing a link to the website usually means that the content is shown in Safari instead of the app, even if it's installed.

Another problem with custom URL schemes is that any app can register as being capable of opening a certain URL scheme. This means that you could create an application that registers as being capable of opening URLs with any scheme you can come up with and unfortunately, iOS offers the users no control over which application opens what URL scheme.

Universal Links were introduced in iOS 9 to solve all of the problems that exist with custom URL schemes and more. First of all, a Universal Link looks identical to a regular web link. A Universal Link is similar to a regular web link. If you've found a great news article on the web and you share it with somebody who has installed the app that belongs to the news website the link is from, the link will redirect straight to the corresponding app. Safari does not open intermediately; no attempts are made to redirect you from a web page to a custom URL scheme. The user merely is taken from the place where they tap the link, right to the app.

Also, not every app can register as capable of opening a Universal Link. Any app that claims to be able to open a certain Universal Link must be accompanied by the server that hosts the website. This means that, if your app claims to be able to open links from a certain domain, you must own that domain. Apple uses a verification file that you must host on the same domain that your app wants to handle links for, to make sure that your app does not try to open links on behalf of another app or website.

Apart from security benefits, Universal Links also provide a more unified, seamless experience for your users. With Universal Links, Apple didn't just open the door to a better, easier way to link to content inside your app; it also opened up an API that makes it easy for your app and website to share login information securely. Just like tying the links for your app and website together, you can also tie your app to the login credentials stored in Safari for your app. Any user that logs into your website through Safari can automatically be logged into your app.

Now that you're aware of the great features and possibilities of Universal Links, let's see how this all works on the server side.