In this article by Hossam Ghareeb, author of the book Application Development with Swift, we will talk about a new technology, WatchKit, and a new era of wearable technologies. Now technology is a part of all aspects of our lives, even wearable objects. You can see smart watches such as the new Apple watch or glasses such as Google glass. We will go through the new WatchKit framework to learn how to extend your iPhone app functionalities to your wrist.

(For more resources related to this topic, see here.)

Apple watch

Apple watch is a new device on your wrist that can be used to extend your iPhone app functionality; you can access the most important information and respond in easy ways using the watch. The watch is now available in most countries in different styles and models so that everyone can find a watch that suits them.

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When you get your Apple watch, you can pair it with your iPhone. The watch can't be paired with the iPad; it can only be paired with your iPhone. To run third-party apps on your watch, iPhone should be paired with the watch. Once paired, when you install any app on your iPhone that has an extension for the watch, the app will be installed on the watch automatically and wirelessly.

WatchKit

WatchKit is a new framework to build apps for Apple watch. To run third-party apps on Apple watch, you need the watch to be connected to the iPhone. WatchKit helps you create apps for Apple watch by creating two targets in Xcode:

The WatchKit app: The WatchKit app is an executable app to be installed on your watch, and you can install or uninstall it from your iPhone. The WatchKit app contains the storyboard file and resources files. It doesn't contain any source code, just the interface and resource files.
The WatchKit extension: This extension runs on the iPhone and has the InterfaceControllers file for your storyboard. This extension just contains the model and controller classes. The actions and outlets from the previous WatchKit app will be linked to these controller files in the WatchKit extension.

These bundles—the WatchKit extension and WatchKit app—are put together and packed inside the iPhone application. When the user installs the iPhone app, the system will prompt the user to install the WatchKit app if there is a paired watch.

Using WatchKit, you can extend your iOS app in three different ways:

The WatchKit app

As we mentioned earlier, the WatchKit app is an app installed on Apple watch and the user can find it in the list of Watch apps. The user can launch, control, and interact with the app. Once the app is launched, the WatchKit extension on the iPhone app will run in the background to update a user interface, perform any logic required, and respond to user actions.

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Note that the iPhone app can't launch or wake up the WatchKit extension or the WatchKit app. However, the WatchKit extension can ask the system to launch the iPhone app and this will be performed in the background.

Glances

Glances are single interfaces that the user can navigate between. The glance view is just read-only information, which means that you can't add any interactive UI controls such as buttons and switches. Apps should use glances to display very important and timely information. The glance view is a nonscrolling view, so your glance view should fit the watch screen. Avoid using tables and maps in interface controllers and focus on delivering the most important information in a nice way. Once the user clicks on the glance view, the watch app will be launched. The glance view is optional in your app.

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The glance interface and its interface controller files are a part of your WatchKit extension and WatchKit app. The glance interface resides in a storyboard, which resides in the WatchKit app. The interface controller that is responsible for filling the view with the timely important information is located in the WatchKit extension, which runs in the background in the iPhone app, as we said before.

Actionable notifications

For sure, you can handle and respond to local and remote notifications in an easy and fast way using Apple watch. WatchKit helps you build user interfaces for the notification that you want to handle in your WatchKit app. WatchKit helps you add actionable buttons so that the user can take action based on the notification. For example, if a notification for an invitation is sent to you, you can take action to accept or reject the notification from your wrist. You can respond to these actions easily in interface controllers in WatchKit extension.

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Working with WatchKit

Enough talking about theory, lets see some action.

Go to our lovely Xcode and create a new single-view application and name it WatchKitDemo. Don't forget to select Swift as the app language. Then navigate to File | New | Target to create a new target for the WatchKit app:
After you select the target, in the pop-up window, from the left side under iOS choose Apple Watch and select WatchKit App. Check the following screenshot:
After you click on Next, it will ask you which application to embed the target in and which scenes to include. Please check the Include Notification Scene and Include Glance Scene options, as shown in the following screenshot:
Click on Finish, and now you have an iPhone app with the built-in WatchKit extension and WatchKit app.

Xcode targets

Now your project should be divided into three parts. Check the following screenshot and let's explain these parts:

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As you see in this screenshot, the project files are divided into three sections. In section 1, you can see the iPhone app source files, interface files or storyboard, and resources files. In section 2, you can find the WatchKit extension, which contains only interface controllers and model files. Again, as we said before, this extension also runs in iPhone in the background. In section 3, you can see the WatchKit app, which runs in Apple watch itself. As we see, it contains the storyboard and resources files. No source code can be added in this target.

Interface controllers

In the WatchKit extension of your Xcode project, open InterfaceController.swift. You will find the interface controller file for the scene that exists in Interface.storyboard in the WatchKit app.

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The InterfaceController file extends from WKInterfaceController, which is the base class for interface controllers. Forget the UI classes that you were using in the iOS apps from the UIKit framework, as it has different interface controller classes in WatchKit and they are very limited in configuration and customization. In the InterfaceController file, you can find three important methods that explain the lifecycle of your controller: awakeWithContext, willActivate, and didDeactivate. Another important method that can be overridden for the lifecycle is called init, but it's not implemented in the controller file. Let's now explain the four lifecycle methods:

init: You can consider this as your first chance to update your interface elements.
awakeWithContext: This is called after the init method and contains context data that can be used to update your interface elements or to perform some logical operations on these data. Context data is passed between interface controllers when you push or present another controller and you want to pass some data.
willActivate: Here, your scene is about to be visible onscreen, and its your last chance to update your UI. Try to put simple UI changes here in this method so as not to cause freezing in UI.
didDeactivate: Your scene is about to be invisible and, if you want to clean up your code, it's time to stop animations or timers.

Summary

In this article, we covered a very important topic: how to develop apps for the new wearable technology, Apple watch. We first gave a quick introduction about the new device and how it can communicate with paired iPhones. We then talked about WatchKit, the new framework, that enables you to develop apps for Apple watch and design its interface. Apple has designed the watch to contain only the storyboard and resources files. All logic and operations are performed in the iPhone app in the background.