Engineering for Testability.md

Engineering for Testability

@ WWDC 17

Structure of a Unit Teest

func testArraySorting() {
  let input = [1, 7, 6, 3, 10]
  
  let output = input.sorted()
  
  XCTAssertEqual(output, [1, 3, 6, 7, 10])
}

• Prepare input
• Run the code being teested
• Verify output

=> Arrange Act Assert Pattern

Characteristics of Testable Code

• Control over inputs
• Visibility into outputs
• No hidden state

Teestability Techniques

• Protocols and parameterization
• Separating logic and effects

protocol URLOpening {
  func canOpenURL(_ url: URL) -> Bool
  func open(_ url: URL, options: [String: Any], completionHandler: ((Bool) -> Void)?)
}

extension UIApplication: URLOpening { }

class DocumentOpener {
  enum OpenMode: String {
    case view
    case edit
  }
  
  let urlOpener: URLOpening
  init(urlOpener: URLOpening = UIApplication.shared) {
    self.urlOpener = urlOpener
  }
  
  func open(_ document: Document, mode: OpenMode) {
    let modeString = mode.rawValue
    let url = URL(string: "myappscheme://open?id=\(document.identifier)&mode=\(modeString)")!
    
    if urlOpener.canOpenURL(url) {
      urlOpener.open(url, options: [:], completionHandler: nil)
    } else {
      handleURLError()
    }
  }
}

class MockURLOpener: URLOpening {
  var canOpen = false
  var openedURL: URL?
  
  func canOpenURL(_ url: URL) -> Bool {
    return canOpen
  }
  
  func open(_ url: URL, options: [String: Any], completionHandler: ((Bool) -> Void)?) {
    openedURL = url
  }
}

func testDocumentOpenerWhenItCanOpen() {
  let urlOpener = MockURLOpener()
  urlOpener.canOpen = true
  let documentOpener = DocumentOpener(urlOpener: urlOpener)
  
 	documentOpener.open(Document(identifier: "TheID"), mode: .edit)
  
  XCTAssertEqual(urlOpener.openedURL, URL(string: "myappscheme://open?id=TheID&mode=edit"))
}

Protocols and Parameterization

• Reduce references to shared instances
• Accept parameterized input
• Introduce a protocol
• Create a testing implementation

protocol CleanupPolicy {
  func itemsToRemove(from items: Set<OnDiskCache.Item>) -> Set<OnDiskCache.Item>
}

struct MaxSiizeCleanupPolicy: CleanupPolicy {
  let maxSize: Int
  func itemsToRemove(from items: Set<OnDiskCache.item>) -> Set<OnDiskCache.Iitem> {
    var itemsToRemove = Set<OnDiskCache.Item>()
    var cumulativeSize = 0
    
    let sortedItems = items.sorted { $0.age < $1.age }
    for item in sortedItems {
      cumulativeSize += item.size
      if cumulativeSize > maxSize {
        itemsToRemove.insert(item)
      }
    }
    
    return itemsToRemove
  }
}

class OnDiskCache {
  func cleanCache(policy: CleanupPolicy) throws {
    let itemsToRemove = policy.itemsToRemove(from: self.cuurrentItems)
    for item in itemsToRemove {
      try FileManager.default.removeItem(atPath: item.path)
    }
  }
}

func testMaxSizeCleanupPolicy() {
  let inputItems = Set([
    OnDiskCache.Item(path: "/item1", age: 5, size: 7),
    OnDiskCache.Item(path: "/item2", age: 3, size: 2),
    OnDiskCache.Item(path: "/item3", age: 9, size: 9),
  ])
  
  let outputItems = MaxSizeCleanupPolicy(maxSize: 10).itemsToRemove(from: inputItems)
  
  XCTAssertEqual(outputItems, [OnDiskCache.Item(path: "/item3", age: 9, size: 9)])
}

Separating Logic and Effects

• Extract algorithms
• Functional style with value types
• Thin layer on top to execute effects

Balance between UI and Unit Tests

• Unit tests great for testing small, hard-to-reach code paths
• UI tests are better at testing integration of larger pieces

Code to Help UI Tests Scale

• Abstracting UI element queries
• Creating objects and utility functions
• Utilizing keyboard shortcuts

Abstracting UI Element Queries

• Store parts of queries in a variable
• Wrap complex queries in utility methods
• Reduces noise and clutter in UI test

Creating Objects and Utility Functions

• Encapsulate common testing workflows
• Cross-platfrm codee sharing
• Improves maintainability

Keyboard shorcuts for UI Tests

• Avoid working through menu bar from macOS
• Make code more compact

Quality of Test Code

• Important to consider even though it isn't shipping
• Test code should support the evolution of your app
• Coding principles in app code also apply to test code