Facebook-backed spec lets apps link to other apps on mobes

Look, ma, we've invented the web – only with no browser

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Facebook and a handful of partners have lent their support to App Links, a new, cross-platform, open source system for linking content from mobile apps directly to other mobile apps.

Deep linking is easy on the desktop, because every link is handled by the browser. But on mobile platforms, people often prefer to use platform-specific apps to view content. Instead of watching video embedded in a browser window, for example, they'd rather use their device's dedicated YouTube app.

Mobile browsers offer ways of launching apps for specific URLs, but so far there hasn't been any consistent way to launch one app directly from another. App Links, announced at Facebook's F8 developer conference on Wednesday, aims to provide developers an easy way to do just that.

What Facebook and its partners have come up with is more of a protocol than an SDK. Some reference code is available, but what the App Links specification mainly does is define a standard set of procedures for discovering and navigating to mobile apps.

The spec provides a set of metadata tags that developers can add to the HTML of their web pages to specify whether there is an app for a given mobile platform that can be used to view a particular piece of content.

For example, a page might include App Links tags indicating where to find an Android app, an iPhone app, and a separate app for iPads, plus another tag that specifies a fallback web URL. Mobile apps can then use this information to figure out the appropriate way to view that content.

If an app is available and it's installed on the device, it can be launched. Failing that, the web URL can be forwarded to the mobile browser – or, in the case of mobile-only apps, the link can fail, if that's what the developer specifies.

Exactly how this process works depends on the mobile platform. Facebook has provided a reference code library called Bolts for Android and iOS, and it has also published documentation for using App Links on Windows Phone, but the protocol is simple enough that developers shouldn't have trouble writing their own implementations for these or other platforms.

In addition, Facebook has been scouring the web to build its own, public index of App Links metadata. Developers can optionally query this index via Facebook's Graph API to find metadata for arbitrary URLs, which may be faster than scanning the HTML of individual pages for tags.

The catch, of course, is that for App Links to become the norm depends on developers building support into their apps. It's too early yet to know how many will be willing to do so, but a number of prominent companies have already signed on, including Dropbox, Flixster, Hulu, Pinterest, and Spotify, among others – and some were already serving App Links HTML tags on their web pages as of Wednesday. ®

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