SwiftUI Previews on macOS Catalina and Xcode 11

Written by Mattt October 14^th, 2019

This article has been translated into:: 한국어

Working on a large iOS codebase often involves a lot of waiting: Waiting for Xcode to index your files, waiting for Swift and Objective-C code to compile, waiting for the Simulator to boot and your app to launch…

And after all of that, you spend even more time getting your app into a particular state and onto a particular screen, just to see whether the Auto Layout constraint you just added fixes that regression you found. It didn’t, of course, so you jump back into Xcode, tweak the Content Hugging Priority, hit ⌘R, and start the whole process again.

We might relate our sorry predicament to that one xkcd comic, but for those of us who don’t so much relish in the stop-and-go nature of app development, there’s an old Yiddish joke about Shlemiel the painter (provided below with a few -specific modifications; for the uninitiated, please refer to Joel Spolsky’s original telling):

Shlemiel gets a job as a software developer, implementing a new iOS app. On the first sprint he opens Xcode and implements 10 new screens of the app. “That’s pretty good!” says his manager, “you’re a fast worker!” and pays him a Bitcoin.

The next sprint Shlemiel only gets 5 screens done. “Well, that’s not nearly as good as yesterday, but you’re still a fast worker. 5 screens is respectable,” and pays him a Bitcoin.

The next sprint Shlemiel implements 1 screen. “Only 1!” shouts his manager. “That’s unacceptable! On the first day you did ten times that much work! What’s going on?”

“I can’t help it,” says Shlemiel. “Each sprint I get further and further away from application(_:didFinishLaunchingWithOptions:)!”

Over the years, there have been some developments that’ve helped things slightly, including @IBInspectable and @IBDesignable and Xcode Playgrounds. But with Xcode 11, our wait is finally over — and it’s all thanks to SwiftUI.

Although many of us have taken a “wait and see” approach to SwiftUI, we can start using its capabilities today to radically speed up and improve our development process — without changing a line of code in our UIKit apps.

Consider a subclass of UIButton that draws a border around itself:

final class BorderedButton: UIButton {
    var cornerRadius: CGFloat { … }
    var borderWidth: CGFloat { … }
    var borderColor: UIColor? { … }
}

Normally, if we wanted to test how our UI element performs, we’d have to add it to a view in our app, build and run, and navigate to that screen. But with Xcode 11, we can now see a preview side-by-side with the code editor by adding the following under the original declaration of BorderedButton:

#if canImport(SwiftUI) && DEBUG
import SwiftUI

@available(iOS 13.0, *)
struct BorderedButton_Preview: PreviewProvider {
  static var previews: some View {
    UIViewPreview {
      let button = BorderedButton(frame: .zero)
      button.setTitle("Follow", for: .normal)
      button.tintColor = .systemOrange
      button.setTitleColor(.systemOrange, for: .normal)
      return button
    }.previewLayout(.sizeThatFits)
     .padding(10)
  }
}
#endif

Using a new feature called dynamic replacement, Xcode can update this preview without recompiling — within moments of your making a code change. This lets you rapidly prototype changes like never before.

Want to see how your button handles long titles? Bang away on your keyboard within the call to setTitle(_:for:) in your preview, and test out potential fixes in your underlying implementation without so much as leaving your current file!

UIViewPreview is a custom, generic structure that we created to conveniently host previews of UIView subclasses. Feel free to download the source and add it to your project directly.

Incorporating a proper dependency would be complicated by the conditional import and iOS 13 Deployment Target settings required to make Xcode Previews work for non-SwiftUI apps, so in this particular instance, we think it’s best to embed these files directly.

Expand for the full implementation of UIViewPreview:

import UIKit

#if canImport(SwiftUI) && DEBUG
import SwiftUI
struct UIViewPreview<View: UIView>: UIViewRepresentable {
    let view: View

    init(_ builder: @escaping () -> View) {
        view = builder()
    }

    // MARK: - UIViewRepresentable

    func makeUIView(context: Context) -> UIView {
        return view
    }

    func updateUIView(_ view: UIView, context: Context) {
        view.setContentHuggingPriority(.defaultHigh, for: .horizontal)
        view.setContentHuggingPriority(.defaultHigh, for: .vertical)
    }
}
#endif

Previewing Multiple States

Let’s say our app had a FavoriteButton — a distant cousin (perhaps by composition) to BorderedButton. In its default state, it shows has the title “Favorite” and displays a ♡ icon. When its isFavorited property is set to true, the title is set to “Unfavorite” and displays a ♡̸ icon.

We can preview both at once by wrapping two UIViewPreview instances within a single SwiftUI Group:

Group {
  UIViewPreview {
    let button = FavoriteButton(frame: .zero)
    return button
  }
  UIViewPreview {
    let button = FavoriteButton(frame: .zero)
    button.isFavorited = true
    return button
  }
}.previewLayout(.sizeThatFits)
 .padding(10)

Previewing Dark Mode

With Dark Mode in iOS 13, it’s always a good idea to double-check that your custom views are configured with dynamic colors or accommodate both light and dark appearance in some other way.

An easy way to do this would be to use a ForEach element to render a preview for each case in the ColorScheme enumeration:

ForEach(ColorScheme.allCases, id: \.self) { colorScheme in
    UIViewPreview {
      let button = BorderedButton(frame: .zero)
      button.setTitle("Subscribe", for: .normal)
      button.setImage(UIImage(systemName: "plus"), for: .normal)
      button.setTitleColor(.systemOrange, for: .normal)
      button.tintColor = .systemOrange
      return button
  }.environment(\.colorScheme, colorScheme)
   .previewDisplayName("\(colorScheme)")
}.previewLayout(.sizeThatFits)
 .background(Color(.systemBackground))
 .padding(10)

Previewing Dynamic Type Size Categories

We can use the same approach to preview our views in various Dynamic Type Sizes:

ForEach(ContentSizeCategory.allCases, id: \.self) { sizeCategory in
  UIViewPreview {
      let button = BorderedButton(frame: .zero)
      button.setTitle("Subscribe", for: .normal)
      button.setImage(UIImage(systemName: "plus"), for: .normal)
      button.setTitleColor(.systemOrange, for: .normal)
      button.tintColor = .systemOrange
      return button
  }.environment(\.sizeCategory, sizeCategory)
   .previewDisplayName("\(sizeCategory)")
}.previewLayout(.sizeThatFits)
 .padding(10)

Previewing Different Locales

Xcode Previews are especially time-saving when it comes to localizing an app into multiple languages. Compared to the hassle of configuring Simulator back and forth between different languages and regions, this new approach makes a world of difference.

Let’s say that, in addition to English, your app supported various right-to-left languages. You could verify that your RTL logic worked as expected like so:

let supportedLocales: [Locale] = [
  "en-US", // English (United States)
  "ar-QA", // Arabic (Qatar)
  "he-IL", // Hebrew (Israel)
  "ur-IN"  // Urdu (India)
].map(Locale.init(identifier:))

func localizedString(_ key: String, for locale: Locale) -> String? { … }

return ForEach(supportedLocales, id: \.identifier) { locale in
  UIViewPreview {
    let button = BorderedButton(frame: .zero)
    button.setTitle(localizedString("Subscribe", for: locale), for: .normal)
    button.setImage(UIImage(systemName: "plus"), for: .normal)
    button.setTitleColor(.systemOrange, for: .normal)
    button.tintColor = .systemOrange
    return button
  }.environment(\.locale, locale)
   .previewDisplayName(Locale.current.localizedString(forIdentifier: locale.identifier))
}.previewLayout(.sizeThatFits)
 .padding(10)

Previewing View Controllers on Different Devices

SwiftUI previews aren’t limited to views, you can also use them with view controllers. By creating a custom UIViewControllerPreview type and taking advantage of some new UIStoryboard class methods in iOS 13, we can easily preview our view controller on various devices — one on top of another:

#if canImport(SwiftUI) && DEBUG
import SwiftUI

let deviceNames: [String] = [
    "iPhone SE",
    "iPhone 11 Pro Max",
    "iPad Pro (11-inch)"
]

@available(iOS 13.0, *)
struct ViewController_Preview: PreviewProvider {
  static var previews: some View {
    ForEach(deviceNames, id: \.self) { deviceName in
      UIViewControllerPreview {
        UIStoryboard(name: "Main", bundle: nil)
            .instantiateInitialViewController { coder in
            ViewController(coder: coder)
        }!
      }.previewDevice(PreviewDevice(rawValue: deviceName))
        .previewDisplayName(deviceName)
    }
  }
}
#endif

Although most of us are still some years away from shipping SwiftUI in our apps (whether by choice or necessity), we can all immediately benefit from the order-of-magnitude improvement it enables with Xcode 11 on macOS Catalina.

By eliminating so much time spent waiting for things to happen, we not only get (literally) hours more time each week, but we unlock the possibility of maintaining an unbroken flow state during that time. Not only that, but the convenience of integrated tests fundamentally changes the calculus for testing: instead of being a rare “nice to have,” they’re the new default. Plus: these inline previews serve as living documentation that can help teams both large and small finally get a handle on their design system.

It’s hard to overstate how much of a game-changer Xcode Previews are for iOS development, and we couldn’t be happier to incorporate them into our workflow.