To kick off its annual developers' conference, Apple teased a new programming language, two updated operating systems, and dozens of new features for coders and casual users alike. But in terms of actually changing how people use technology, the most ambitious thing introduced at Monday's keynote was an idea: Continuity across multiple devices. It's an aspiration that could shape a simpler, more intuitive phase in consumer technology.

Continuity was one of the three big talking points Craig Federighi outlined when he introduced OS X Yosemite, right up there in big bright letters with the major categories of "interface" and "apps." It popped up again at the end of the keynote, when Tim Cook reiterated Apple's aim to deliver "an integrated and continuous experience across all of our products."

>Instead of just holding your place, Apple wants to smooth out the immediate experience of moving from one device to another.

So what does it mean? Apple's always touted the cohesive relationship between its software and its hardware. Continuity is the next step: Designing a seamless experience across the many devices in our lives.

Federighi had a few examples of what this might look like. A feature called "Handoff" addresses the basic use case of jumping from one device to another. In the future, Apple gadgets will seek out familiar Apple gadgets in the immediate vicinity. When your iPad is near your Macbook, for instance, a hand off icon will pop up on the tablet's lock screen. If you're reading a web site on your MacBook, you can walk over to your iPad, swipe up, and continue reading the same site. If you're writing an email on your phone, you can hand it off just as easily to your Mac and finish typing it on a real keyboard.

Other features are designed to keep you from needlessly switching between devices in the first place. The next version of OS X will let you place and receive calls on your Mac, for example, simply because your iPhone is nearby. This hints at an even more exciting sort of continuity–one where the devices around you helpfully hand off functionality based upon what you're doing.

These features, though still modest, begin to address a major shortcomings with consumer technology: For all their individual smarts, our devices have painfully little awareness of those around them. Apple intends to fix that problem. Doing so could make technology smarter and simpler in countless ways. But that continuity will come with a cost. To see devices that work together more harmoniously, we should expect product ecosystems that are even more fiercely entrenched than they are today. As so often seems true, a better user experience will require us, the users, to sacrifice choice.

The new Handoff feature lets you pick up where you left off, using another device. Apple

A Different Strategy for Device Integration

We've seen many moves toward solving the greater problem in recent years. Netflix will bookmark your progress on your Xbox and let you pick up where you left off in the iPad app. Browsers eagerly sync tabs from laptops to tablets to phones. Google's entire suite of web apps is geared toward this. So is Dropbox's core service. In general, these fixes involve using the cloud to keep your files fresh and your state saved across devices.

Though some of the updates to iOS and OS X seemed to copy the synching features of Dropbox and What's App, there are new ideas at play. Apple's idea of continuity is both more modest and more ambitious than those of its competitors. Instead of just holding your place on any device you might happen to pick up, Apple's focused on smoothing out the immediate experience of moving from one device to another. Consider Handoff, the feature that will let you fluidly continue a task when picking up a different device. It's a little slicker than the state-saving stuff we've seen so far. With Google Docs, say, you can edit a file on your phone, save it, go to your computer, open Google Docs there, and see that change reflected instantly. But with Handoff, you're shoved directly to the appropriate app, shaving away a few of those steps.

Straight away, however, the ecosystem monster rears its ugly head. Apple says Handoff can be used with "Mail, Safari, Pages, Numbers, Keynote, Maps, Messages, Reminders, Calendar, and Contacts," as well as any third party apps that include it. In other words, Handoff will work like magic if you use Apple's mail app on your iPhone and Apple's mail app on your Mac. If you use Gmail, or Exchange, or Mailbox, or whatever else, it might not work at all.

>Eventually, continuity could devour the entire concept of 'usernames' and authentication.

But Apple's ambitions go beyond shuttling content from one device to another. As we saw yesterday, it's also looking to facilitate a more productive relationship between your devices, eliminating the need for you to switch between them so often in the first place. Right now, using your iPhone as a wireless hotspot means going into your iPhone's settings, turning it on, and verifying the connection by entering a unique passcode on your Mac. But your phone and your computer spend every day side by side. Why the need for a passcode? At some point, after all that time together, shouldn't they simply figure it out?

They should, and with the forthcoming OS X update, they will. Yosemite will let you handle all that hot spot business from your computer, while your phone stays in your pocket, just by virtue of the two devices being nearby. This is the other side of continuity, as Apple sees it: eliminating the annoying and redundant interactions required to juggle the personal ecosystems we rely on every day.

Will It Work? Signs Point to Maybe

For this sort of continuity to be useful, it has to work. Here, Apple's track record is worrisome. From the continually wonky iCloud to iMessage's maddening inability to keep conversations in sync across devices, Apple has shown again and again that it just can't do the cloud on the same level as Google or Dropbox.

All hope may not be lost, however. In cases that don't involve transmitting stuff to and from servers, Apple does personal ecosystem stuff brilliantly. Take AirPlay. When the feature was introduced in 2004 (as AirTunes), playing music on speakers meant plugging something into them. Today, after years of incremental improvements, it's trivial to beam a video you shot on your phone directly to your TV in full resolution. It's a highly useful bit of futuristic functionality that Apple deftly shepherded into existence.

One hopes Apple's efforts toward continuous computing will end up more like AirPlay than iCloud. This greater area of continuous functionality opens up all sorts of possibilities. Today, continuity is what lets you make calls from your Mac and set up your new Apple TV just by tapping your iPhone to it (a feature introduced with an Apple TV software update last fall). Someday soon, it might be about having music playing for you when you arrive home, or stopping it automatically a few minutes after you've left the house for work. Eventually, continuity could devour the entire concept of "usernames" and authentication, handling how your digital identity—your data, your preferences, your files, your settings—moves securely with you from place to place.

As our personal ecosystems continue to expand, continuity only becomes more complex–and more crucial. It's what will let wearables helpfully pass the data they glean to our phones and what will allow our smart homes to respond not just to our commands but to our presence in useful, nuanced ways.

It's good to see Apple thinking about all this stuff early. But in the new Yosemite features, we can also plainly see the unsavory side of this trend. Continuity is bittersweet. It's a future where everything works flawlessly, as long as you don't buy anything from the other guy.