As Apple's mobile operating system matures, each year’s version brings new capabilities and challenges for developers. Last year’s iOS 7 launch meant app makers had to redesign their apps to fit Apple’s new aesthetic, a hefty undertaking for some. But with iOS 8, the challenge is centered more around functionality. Apple introduced 4,000 new APIs developers can take advantage of, and to implement some of them, developers are having to rethink their app's identity within the iOS realm.

"Apple is enabling this Jetsons-esque future where everything talks to everything," Matt Johnston, chief strategy officer of app analytics company Applause, told WIRED. “It’s not only a bigger challenge for Apple, it’s an order of magnitude more complex for app companies."

Apps used to be siloed on a specific device, functioning in their own protected little bubble. But things are now far less straightforward. With features like Handoff and Continuity, developers now have to worry about an app working from iPhone to iPad to Mac. If an app has an OS X and iOS version, users will grow to expect that, like with Safari or Mail, this app too will let you leave and pick up where you left off, no matter what device you're on.

Apps also now have to share data and functionality with one another in an intricate web of interdependencies. iOS 8's share extension is a specific example of this: This feature makes custom capabilities of your app available to users while they’re in other apps via iOS's share button. For the team at popular note-sharing app Evernote, this caused numerous headaches.

“The share extension called for the team to work within an entirely new set of constraints, different from those in the main app, using technologies that they don't work with on a daily basis,” Evernote’s VP of mobile products Jamie Hull told WIRED via email. “We couldn't just take what we had done for desktop browser extensions and apply it to the iOS app without severely compromising both performance and usability, so the team had to build and test several approaches in parallel until we found something that worked.”

With additional iOS 8 functionalities on the way, Evernote currently includes several iOS 8-specific features, including a "Today" widget for adding new notes, the aforementioned share extension for clipping content to your account, and Touch ID to unlock the app instead of using a passcode. Dealing with the larger iPhone 6 Plus form factor has also been difficult.

“While the new phone screen sizes were actually relatively straightforward to support, the larger form factors open up a lot of questions about the ideal UX for the app,” Hull said. The team had to decide whether the largest-size phone would operate more like a tablet, and whether some onscreen items should be given greater emphasis now that the iPhone keyboard has built-in formatting buttons.

For Flexibits’ premiere title, Fantastical 2, co-founder Michael Simmons said getting the widget right was his team’s biggest obstacle.

“We were a month late to the party and that was because of the widget,” Simmons told WIRED. “We could have made a simple list, but we really wanted it done right.”

Widgets in Notification Center’s "Today View" have to follow a set of specifications, including a maximum height. For a calendar app, the team had to solve problems like: How do you make something helpful and beautiful in such a compressed view? What is most beneficial to app users, a list of to-do items, or a calendar view of the month ahead? What does a user most want to see when they swipe Notification Center down from the top of their device's screen?

Taking their time to get it right paid off though: Fantastical 2 is now the number one app in the App Store’s Productivity category and among the top 50 in paid apps.

For Flipboard, which also recently updated its iOS app, iOS 8 introduced another new concern for the team: version support. Android has an app compatibility library that makes it easy for apps to get newer APIs on older system versions. On iOS, Flipboard co-founder Evan Doll said, they had to decide how far back to support: Just iOS 7 and iOS 8? iOS 6? iOS 5? That’s pretty old in iPhone years. (Flexibits got around this particular issue by deciding that Fantastical 2 would be an iOS 8 exclusive.)

But it’s important to note that the added complexity of a new OS isn’t always a negative for developers—many are excited about taking advantage of new technology in their apps, even if it requires a few all nighters.

“With new features like the Today widget, interactive notifications, and app sharing extensions, iOS 8 adds a lot of functionality that makes an app much more productive,” Flexibits' Simmons said. And as Fantastical’s current perfect five star rating shows, if you get it right, all that extra work pays off.