iOS products Snapshot Testing

SDSnapshots library provides XCTestCase asserts for UI screenshots comparison with reference image (and reference recording).

• iOS products Snapshot Testing
  • Stack
  • Features
  • Limitations
  • Usage
  • Alternatives
  • License
  • Thanks

Stack

• Xcode 16.1
• iOS Simulator Runtime 18.1
• Swift 6.0

Features

• Works both with UIKit and SwiftUI
• Allows to test multiple devices layout at ones, using the single simulator device launch (cover iPhones, iPads, Split Views in a single test case)
• Respects device orientation and safe area. Applies it from test run SnapshotDevice specification (not simulator's one). May render it as a visible border
• Async API with concurrent implementation for faster test execution
• Produced new.png, diff.png, merge.png on test failure help to visually spot difference between expectation and actual render
• Accessibility snapshots to capture accessibility labels. May be handy during UIKit -> SwiftUI refactor, tests automatically ensures that UI hints still present

Limitations

• Xcode version change may break some tests. Either color components may vary (without visible difference) or layer/text border may shift due to antialiasing algo changes. iOS SDK (bundled with Xcode.app) and iOS Simulator Runtime versions have impact on UIKit behavior
• Xcode version, Simulator device model and version should be fixed and frozen for your test scheme. Xcode.app update will usually require you to commit breaking changes
• Git LFS is strongly advised as tool for .png references storage. Images are significantly larger than text files, storing them directly in commit history will explode repository size
• Animations have to be turned off to stabilize image capture result (use View model toggle). Otherwise test case will be unstable, captured animation frame may shift between test launches
• Snapshot Testing target should have iOS app specified as the TEST_HOST, otherwise UIKit render pipe won't work
• Therefore you need to pack test target code in .xcodeproj. Swift Packages currently do not allow us to declare iOS app targets
• View instance have to be uniq per each prepareSut() invocation inside assert method. Sequential View re-appearance may affect layout

Usage

See example repository for sample project and test code.

Alternatives

• swift-snapshot-testing by PointFree
  • ✅ Do not require Simulator Device launch and TEST_HOST app
  • ✅ Wider capabilities (may assert any objects equality)
  • ❌ Uses CoreGraphics rendering, UIKit views display is inaccurate (layer effects, borders, shadows, opacity may have visual artifacts)

License

Package is under MIT License.

Thanks

• Максим Кузнецов – initial library design
• Бодров Александр – SwiftUI support, respect of safe area, async API, concurrent Matcher, Matcher sensitivity calibration
• Сергей Уразов – accessibility testing