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Alex Rupérez

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Published: 04 Dec 10:44
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📸 Delightful Swift snapshot testing.
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📸 SnapshotTesting

Swift 5 iOS/macOS/tvOS CI Linux CI @pointfreeco

Delightful Swift snapshot testing.

Usage

Once installed, no additional configuration is required. You can import the SnapshotTesting module and call the assertSnapshot function.

import SnapshotTesting
import XCTest

class MyViewControllerTests: XCTestCase {
  func testMyViewController() {
    let vc = MyViewController()

    assertSnapshot(matching: vc, as: .image)
  }
}

When an assertion first runs, a snapshot is automatically recorded to disk and the test will fail, printing out the file path of any newly-recorded reference.

🛑 failed - No reference was found on disk. Automatically recorded snapshot: …

open "…/MyAppTests/__Snapshots__/MyViewControllerTests/testMyViewController.png"

Re-run "testMyViewController" to test against the newly-recorded snapshot.

Repeat test runs will load this reference and compare it with the runtime value. If they don't match, the test will fail and describe the difference.

You can record a new reference by setting the record mode to true on the assertion or globally.

assertSnapshot(matching: vc, as: .image, record: true)

// or globally

record = true
assertSnapshot(matching: vc, as: .image)

Snapshot Anything

While most snapshot testing libraries in the Swift community are limited to UIImages of UIViews, SnapshotTesting can work with any format of any value on any Swift platform!

The assertSnapshot function accepts a value and any snapshot strategy that value supports. This means that a view or view controller can be tested against an image representation and against a textual representation of its properties and subview hierarchy.

assertSnapshot(matching: vc, as: .image)
assertSnapshot(matching: vc, as: .recursiveDescription)

View testing is highly configurable. You can override trait collections (for specific size classes and content size categories) and generate device-agnostic snapshots, all from a single simulator.

assertSnapshot(matching: vc, as: .image(on: .iPhoneSe))
assertSnapshot(matching: vc, as: .recursiveDescription(on: .iPhoneSe))

assertSnapshot(matching: vc, as: .image(on: .iPhoneSe(.landscape)))
assertSnapshot(matching: vc, as: .recursiveDescription(on: .iPhoneSe(.landscape)))

assertSnapshot(matching: vc, as: .image(on: .iPhoneX))
assertSnapshot(matching: vc, as: .recursiveDescription(on: .iPhoneX))

assertSnapshot(matching: vc, as: .image(on: .iPadMini(.portrait)))
assertSnapshot(matching: vc, as: .recursiveDescription(on: .iPadMini(.portrait)))

⚠️ Warning: Snapshots must be compared using a simulator with the same OS, device gamut, and scale as the simulator that originally took the reference to avoid discrepancies between images.

Better yet, SnapshotTesting isn't limited to views and view controllers! There are a number of available snapshot strategies to choose from.

For example, you can snapshot test URL requests (e.g., those that your API client prepares).

assertSnapshot(matching: urlRequest, as: .raw)
// POST http://localhost:8080/account
// Cookie: pf_session={"userId":"1"}
//
// email=blob%40pointfree.co&name=Blob

And you can snapshot test Encodable values against their JSON and property list representations.

assertSnapshot(matching: user, as: .json)
// {
//   "bio" : "Blobbed around the world.",
//   "id" : 1,
//   "name" : "Blobby"
// }

assertSnapshot(matching: user, as: .plist)
// <?xml version="1.0" encoding="UTF-8"?>
// <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
// <plist version="1.0">
// <dict>
//   <key>bio</key>
//   <string>Blobbed around the world.</string>
//   <key>id</key>
//   <integer>1</integer>
//   <key>name</key>
//   <string>Blobby</string>
// </dict>
// </plist>

In fact, any value can be snapshot-tested by default using its mirror!

assertSnapshot(matching: user, as: .dump)
// ▿ User
//   - bio: "Blobbed around the world."
//   - id: 1
//   - name: "Blobby"

If your data can be represented as an image, text, or data, you can write a snapshot test for it! Check out all of the snapshot strategies that ship with SnapshotTesting and learn how to define your own custom strategies.

Installation

Carthage

If you use Carthage, you can add the following dependency to your Cartfile:

github "pointfreeco/swift-snapshot-testing" ~> 1.5

⚠️ Warning: Carthage instructs you to drag frameworks into your Xcode project. Xcode may automatically attempt to link these frameworks to your app target. SnapshotTesting.framework is only compatible with test targets, so when you first add it to your project:

  1. Remove SnapshotTesting.framework from any non-test target it may have been added to.
  2. Add SnapshotTesting.framework to any applicable test targets.
  3. Add a New Copy Build Phase to any applicable test targets with Destination set to "Frameworks", and add SnapshotTesting.framework as an item to this phase.
  4. Do not add SnapshotTesting.framework to the "Input Files" or "Output Files" of your app target's Carthage copy-frameworks Run Script Phase.

See Carthage's "Adding frameworks to unit tests or a framework" documentation for more.

CocoaPods

If your project uses CocoaPods, add the pod to any applicable test targets in your Podfile:

target 'MyAppTests' do
  pod 'SnapshotTesting', '~> 1.5'
end

Swift Package Manager

If you want to use SnapshotTesting in a project that uses SwiftPM, add the package as a dependency in Package.swift:

dependencies: [
  .package(url: "https://github.com/pointfreeco/swift-snapshot-testing.git", from: "1.5.0"),
]

Features

  • Dozens of snapshot strategies. Snapshot testing isn't just for UIViews and CALayers. Write snapshots against any value.
  • Write your own snapshot strategies. If you can convert it to an image, string, data, or your own diffable format, you can snapshot test it! Build your own snapshot strategies from scratch or transform existing ones.
  • No configuration required. Don't fuss with scheme settings and environment variables. Snapshots are automatically saved alongside your tests.
  • More hands-off. New snapshots are recorded whether record mode is true or not.
  • Subclass-free. Assert from any XCTest case or Quick spec.
  • Device-agnostic snapshots. Render views and view controllers for specific devices and trait collections from a single simulator.
  • First-class Xcode support. Image differences are captured as XCTest attachments. Text differences are rendered in inline error messages.
  • Supports any platform that supports Swift. Write snapshot tests for iOS, Linux, macOS, and tvOS.
  • SceneKit, SpriteKit, and WebKit support. Most snapshot testing libraries don't support these view subclasses.
  • Codable support. Snapshot encodable data structures into their JSON and property list representations.
  • Custom diff tool integration.

Plug-ins

  • swift-snapshot-testing-nimble adds Nimble matchers for SnapshotTesting.

  • swift-html is a Swift DSL for type-safe, extensible, and transformable HTML documents and includes an HtmlSnapshotTesting module to snapshot test its HTML documents.

Have you written your own SnapshotTesting plug-in? Add it here and submit a pull request!

Related Tools

  • iOSSnapshotTestCase helped introduce screen shot testing to a broad audience in the iOS community. Experience with it inspired the creation of this library.

  • Jest brought generalized snapshot testing to the JavaScript community with a polished user experience. Several features of this library (diffing, automatically capturing new snapshots) were directly influenced.

Learn More

SnapshotTesting was designed with witness-oriented programming.

This concept (and more) are explored thoroughly in a series of episodes on Point-Free, a video series exploring functional programming and Swift hosted by Brandon Williams and Stephen Celis.

Witness-oriented programming and the design of this library was explored in the following Point-Free episodes:

video poster image

License

This library is released under the MIT license. See LICENSE for details.