Spotify opened up its code to iOS developers on Wednesday, with big implications for music on the iPhone, iPod Touch, and iPad.

Starting today, developers can build Apple iOS apps that play anything from Spotify’s entire 15-million-song-plus music catalog, with support for full-track streaming, playlists, search, and so on.

Want to build an iPad app that plays just about every song in the world whose title starts with the letter ‘B’? Or perhaps you’d rather create an app that lets people collaborate on the perfect music to listen to during a hurricane, like we did last weekend with a web page. Now, you can.

[partner id="evolverfm"]Not only will this give app developers a powerful new way to build music apps incorporating millions of songs (or just a curated slice) without having to secure licenses from every major and independent label and publisher, which is all but impossible for small developers, but it enhances the value of a Spotify subscription.

Evolver.fm confirmed with Spotify spokeswoman Alison Bonny that users must pay for a Spotify Premium subscription in order to use these apps, which stands to reason — after all, they have to pay in order to use Spotify’s own iOS app, too. The difference is that now, a Spotify subscription will likely come with all sorts of extra wacky and ambitious iOS apps that rely on the same catalog, with new interfaces and extra features for sharing, organizing, learning about, and playing that music.

“We hope this will enable a new category of iPhone, iPad and iPod Touch applications with Spotify inside,” wrote Spotify director of platform Sten Garmark, “and allow for more immersive music experiences within iOS apps.”

However, Spotify’s API allows only “non-commercial use” right out of the box. Developers looking to sell apps that incorporate Spotify music — or to release free apps that include ads — are still advised to contact Spotify’s partner team before starting development.

Rdio announced a similar strategy in May, and already powers several third-party apps, so Spotify’s move isn’t new. However, in both cases, only premium subscribers who pay $10 per month can use these third-party apps in order to hear full versions of songs. So now, the race is on between the two services to see which ones can amass the most paying subscribers, because that’s where the largest pool of potential users of third-party apps will pool — and developers will likely follow them.

Spotify plans to hold a meet-up in New York City on September 7 to teach developers how to use this. At some point after that, Spotify subscribers can expect a new generation of free iOS music apps to choose from, all of which rely on the same Spotify subscription they already pay for.

See Also: - Spotify Launches in the U.S at Last