Google has released a version of its mobile OS for smartwatches: Android Wear.
The company had been widely expected to come up with a more lightweight version of Android, after Samsung ditched the OS in favour of Tizen.
The OS will let users access information with a "glance or a spoken word", Google said. Smartwatches running Android Wear will take advantage of Google Now to show data "when you need it most" and use the existing voice assistant, letting users say "OK Google" to get its attention.
"The wide variety of Android applications means you’ll receive the latest posts and updates from your favorite social apps, chats from your preferred messaging apps, notifications from shopping, news and photography apps, and more," the company said.
It will also feature health and fitness tools, to help track runs or walks, and can be used to control other devices, such as turning up the volume on your phone or sending a film to your smart TV.
A preview version is now available for developers, with the full SDK set to arrive later this year. The preview lets developers port their existing apps to smartwatches to see how they will work on smaller screens that are round or square-shaped.
By default, notifications from an app can already be viewed on Android Wear devices, but further APIs will allow them to be customised, Google added.
The release also shows Google's vision for what a smartwatch could be.
In the video below, Google imagines such devices being used to quickly book a taxi, send traffic alerts, as a step counter, to display a scannable ticket via a QR code, and to send text messages via voice - suggesting a horrible future where bus riders will shout "OK Google" at their wrists rather than silently type "be there in two minutes".
Google unveiled a list of design principles, revealing what it will be like to use such a device.
The smartwatch face will display "cards", which either suggest information - such as notifications or other data - or show a "demand" interface, which lets users ask for something by saying "OK Google" or tapping the icon to reveal possible actions.
The cards will show images, with notification text popping up to cover the lower half of an image. Google said images must be at least 320 x 320 pixels.
Google said Android Wear devices must be contextually aware and smart, glanceable, helpful and require zero or low interaction.
"By providing a smart connection to the rest of the world while respecting the user’s attention, Android Wear feels personal and global, simple and smart, unobtrusive and ever-ready," the website says. "Notifications that respect these principles will feel most at home in the overall Android Wear experience."
Developers should keep text to a minimum, and only show time-sensitive notifications, for example, leaving less urgent messages - such as daily step count - to only be displayed when asked for.
A developer video shows in better detail how the system will look: