Today Extension Tutorial: Getting Started

Update Note:This tutorial has been updated to iOS 10 and Swift 3 by Michael Katz. The original tutorial was written by Chris Wagner.

iOS 8 introduced App Extensions: a way for you to share your app’s functionality with other apps or the OS itself.

One of these types of extensions is a Today Extension, also known as a Widget. These allow you to present information in the Notification Center and Lock Screen, and are a great way to provide immediate and up-to-date information that the user is interested in. Today Extensions can also appear on the Search screen, and on the quick action menu on the Home screen when using 3D Touch.

In this tutorial, you’ll write an interactive Today Extension that renders the current and recent market prices of a Bitcoin based on the United States Dollar.

Never has it been so easy to deliver valuable information so quickly to your users. Let’s get started!

Introducing Bitcoin

If you’re not familiar with Bitcoin, the short explanation is that it’s a digital cryptocurrency that’s still in its infancy. Aside from using it for peer-to-peer exchanges and purchases, Bitcoin trading allows the user to exchange it for a number of other cryptocurrencies like Dogecoin and Litecoin, and flat currency such as the US Dollar and the Euro.

As a relatively new currency, its market value fluctuates by the minute; there have been huge peaks and troughs in its short lifetime. Thus, it’s a perfect candidate for a Today Extension since investors will want up-to-the-second price quotes!

Introducing Crypticker

Since you’re writing a today extension, you’ll first need a host app to extend; meet Crypticker.

Crypticker is a simple app that displays the current Bitcoin price, the difference between yesterdays price and the current price, as well as a price history chart. The chart includes 30 days of history; tapping or swiping your finger on the chart reveals the exact price for a specific day in the past.

The extension will contain all of these features. Note that the swipe gesture often triggers sliding between the Today and Notifications sections within Notification Center, so it doesn’t really provide the best or most reliable user experience, but a single tap works quite well.

Getting Started

Download the Crypticker starter project to get started. The project contains the entire Crypticker app as described above, but please note that this tutorial will not focus on the development of the container app.

Build and run the project to see what you’re starting with:

The app looks very similar to the screenshot above; the data displayed will, of course, depend on how the Bitcoin market looks right now. Touching the chart near the bottom of the view will draw a line and display the price for the relevant day.

BTC widget

For the unfamiliar, BTC is shorthand for Bitcoin; much like USD stands for United States Dollar. The Today Extension will render a scaled down version of Crypticker’s primary view.

Theoretically, the Crypticker app has the ability to show pricing for multiple Cryptocurrencies, but your today extension is specific to BTC. Therefore, its name shall be BTC Widget.

Note: Today Extensions, by nature, have just one simple purpose. If you wanted to provide information for another cryptocurrency, like Dogecoin, it would be best to package a second widget with the app or design your UI appropriately, perhaps like the Stocks widget.

By the end of the tutorial, your Today Extension will look something like this:

Extensions are packaged as a separate binary from their host app. So you’ll need to add a Today Extension target to the Crypticker project.

In Xcode’s Project Navigator, select the Crypticker project and add a new target by selecting Editor\Add Target… When the template picker appears, choose iOS\ Application Extension, and then Today Extension. Click Next.

Set the Product Name to BTC Widget, and verify that the language is Swift, the project is Crypticker and Embed in Application is also Crypticker. Click Finish.

When prompted, activate the BTC Widget scheme. As the text indicates, another Xcode scheme will be created for you.

Congratulations! BTC Widget will now appear in your list of targets.

Make sure you select BTC Widget, then the General tab, and then press the + button under Linked Frameworks and Libraries.

Select CryptoCurrencyKit.framework and click Add.

CryptoCurrencyKit is a custom framework used by the Crypticker app to retrieve currency prices from the web and display them in a beautiful chart. Luckily for you, the incredibly kind and thoughtful developer behind Crypticker modularized the code into a framework, so that it can be shared between multiple targets. :]

In order to share code between a host app and its extensions you must use a custom framework. If you don’t, you’ll find yourself duplicating a lot of code and violating an important rule of software engineering: DRY – or, Don’t Repeat Yourself. I’ll say it again: “Don’t repeat yourself”.

This tutorial won’t go into much detail on frameworks themselves, as there’s enough information on them to fill their own tutorial. And wouldn’t you know we’ve done exactly that? ;] If you’d like to know more about creating and managing your own custom frameworks, check out this tutorial.

At this point, you’re ready to begin implementing the extension.

Notice there’s now a group in the Project navigator named after your new target, BTC Widget. This is where the Today Extension’s code is grouped, by default.

Expand the group and you’ll see there is a view controller, a storyboard file and an Info.plist file. Its target configuration also tells it to load its interface from MainInterface.storyboard, which contains a single view controller with the class set to TodayViewController.swift.

You’ll notice some files you might expect to see are missing from the Today Extension template; like an app delegate for instance. Remember that today extensions run inside another host app, so they don’t go through the traditional app lifecycle.

In essence, the lifecycle of the today extension is mapped to the lifecycle of the TodayViewController. For example, TodayViewController‘s viewDidLoad method is called when the widget is launched, just like application(_:didFinishLaunchingWithOptions:) is called when the main app launches.

Open MainInterface.storyboard. You’ll see a clear view with a light Hello World label.

Make sure the BTC Widget scheme is selected in Xcode’s toolbar and build and run. This will launch the iOS Simulator and open the Notification Center, which in turn launches your widget. Notification Center is effectively the host app for Today Extensions. This also causes Xcode to attach its debugger to the widget’s process.

Behold your widget. Cool, right? Whilst this is super-exciting stuff, the widget clearly needs a little work. It’s time to make it do some interesting things!

Note: The name of the widget may be ‘CRYPTICKER’ – once you’ve run the host app the widget uses that name instead.