A guided project from chapter 1 of Apple's App Development with Swift book:
So far in this lesson, you've learned the basics of Xcode and Interface Builder. You'll now apply your knowledge of these tools by building a project.
By the end of this Guided Project, you'll have created an app, called Light, that changes the screen from black to white, and back again, whenever the user taps a button. To successfully build the light, you'll need to use Xcode documentation, set breakpoints, and create outlets and actions.