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WatchOS apps can provide four different components for the user: full apps, glances, notifications, and complications. You must always create a full Apple Watch app, which can be opened from the home screen of the Apple Watch.
WatchOS apps are embedded inside iOS apps. When you download and install an iOS app that contains a watchOS app inside it, that app is automatically transferred over the Bluetooth link to the watch. If the watch isn't in Bluetooth range of the iPhone at the time, it's installed later.
WatchOS apps are independent applications that run entirely on the watch: they do their own processing, manage their own memory, and can store files on the watch. However, watchOS apps rely on the parent iPhone for access to any of the user's data that's stored on the device.
Apple Watches require the presence of a parent iPhone. They don't work without one; additionally, they specifically require an iPhone, not an iPod touch or an iPad.
Apps for the Apple Watch have a unique life cycle when compared to iOS or OS X applications. Your app can be launched in a variety of circumstances:
A watchOS app is very similar to an iOS app: it's a bundle of resources and code. The resources include the files that define the UI, any images and media needed by the app, and the compiled binary containing all of the app's code.
The watchOS app is exposed to the user as an icon on the Apple Watch's home screen, which is the grid of icons that you see when you press the Digital Crown from the watch face. In addition to the main app itself, a watchOS app can also include the following:
To communicate with the parent iPhone, you use the WatchConnectivity framework to send and receive files, or small chunks of information. WatchConnectivity is the only way to access information that's kept inside the iOS app—because the Apple Watch and the iPhone are separate devices, there's no shared file storage between them.
The Apple Watch requires you to think about the constraints of the device you're designing for with even more pedantry and attention to detail than is required for the iPhone and iPad. You need to keep the following in mind when designing Apple Watch apps:
In general, as long as you're careful—and pay attention to the design constraints of the watch—you'll probably be fine if you follow the same general approach that is taken for iOS development. That said, it's easy to forget that every single thing that your Apple Watch app does relies on an often unreliable Bluetooth connection to an iPhone. It's especially easy to forget this when you're using the simulator to test things, because the Watch simulator doesn't have to deal with talking over the radio to its simulated counterpart iPhone. This means that the simulator will be significantly faster than a real Apple Watch will be.
There are two ways to run an Apple Watch app: running it on a real device, and running it in the simulator.
Just as with building apps for iPhone and iPad, it's always better to run your code on a real device, for a bunch of reasons: the simulator is faster than the real watch, and responds to user input much more quickly; the simulator is a lot easier to read than the real watch; and apps running on the simulator don't have to compete for attention with other apps. Additionally, when you're running code on the simulator, you're not wearing the app on your wrist, and you're not interacting with it in the same way.
At the same time, though, building and testing your app on the simulator is considerably easier than using a real device—you don't need to worry about pairing, or waiting for the install to complete. You also don't have to own a real device. (Again, though, if you're making apps, you really should own a watch. Given the cost of buying additional hardware, though, it's understandable to want to start building apps on the simulator before getting a device.)
Let's dive into creating an app for the Apple Watch. Because we're starting from scratch, the iOS app that runs on the phone will be mostly empty, and we'll focus our attention on the watchOS app.
To get started, you'll need a copy of Xcode 7.2 or later installed.
When shipping a real app, your iOS app needs to be fully functional. Focus on getting that product complete as well as your Apple Watch—don't make a poor iOS app and put your entire energy into the watchOS app. The first experience your user will have with your apps will be the iOS app.