Apple has announced the latest version of WatchOS.

Called WatchOS 4, it'll replace WatchOS 3 and is meant for Apple Watch. It debuted at Apple's Worldwide Developers Conference in June 2017. It's a significant update that brings new watch faces, including a Siri watch face and Disney watch faces, an improved Music app, a new dock, new fitness features, and more.

Here's everything we know so far about WatchOS 4.

WatchOS is an Apple-developed operating system that powers Apple Watch devices. WatchOS 4 is the latest version of WatchOS. Each major upgrade to WatchOS typically adds new software features.

Apple released WatchOS 4 to developers on 5 June. Apple will likely seed several other betas, including a golden master version, before it releases the final version for consumers sometime this autumn. Like all of Apple's software updates, WatchOS 4 will be free to download and use.

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First off, there's a new Siri watch face. It uses machine learning to display information specific to you. It reminds us of the proactive app and contact suggestions in iOS. You should see information and app shortcuts that are relevant, and you can rotate the Digital Crown to scroll vertically through the various notifications and apps on the face.

Second, there's a kaleidoscope watch face, which you can rotate using the Digital Crown. Doing so produces a hippy effect that rotates and changes shape. There are also new Disney watch faces. Expanding upon the popular Micky Mouse and Minnie Mouse faces available, Apple added Woody, Jessie, and Buzz Lightyear. They're all from Toy Story, and each one has a different animation.

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They feature a number of longer animations that behave differently when you rotate your wrist. Apple called this "vignettes". For instance, you may see a character climbing on to the watch face from below.

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Activity and Workout are popular apps on Apple Watch, and they've been updated with more features. Notifications in Activity are now more personalised. You may see one when you're about to hit an achievement, for instance. There's also "smart coaching" that will serve up monthly challenges, based on your history and activity records. There's some new animations, too.

You will see a sparkly animation when you've filled your rings in Activity. Apple has further changed the way pool swim workouts work. So, if you're taking a rest at one end of the pool, your watch will automatically detect it and mark it as the end of one set of laps and the start of another. This is useful for getting accurate timings. Apple also introduced two-way data exchange.

This works specifically with gym equipment. Just tap the NFC tag to connect your watch with the gym equipment, meaning you'll need to find a gym that supports the tech first. Your heart rate can be read by the watch and sent to the equipment, and data and speed and other info is sent to your watch. You can also pick a playlist that automatically starts with your workout.

And finally, there's a new High Intensity Interval Training workout available that uses your heart rate to push you to your limits.

Nope. The Music app on Apple Watch has been redesigned, too. You will see album art and can rotate the Digital Crown to scroll. The app also tries to guess what you want to hear, so you'll see music and playlists that you listen to automatically updated on your watch. WatchOS 4 will support native core Bluetooth too. That means you will have the ability to link and work with smaller wireless products.

The company teased that glucose monitoring is coming to the Apple Watch, but details were scant. We'll update this article when we know more. There's also a redesigned dock interface so that you can vertically scroll through recent apps using the Digital Crown. And lastly, Apple added a new blinking light you can activate via Control Centre so that your night runs can be safer.