Linting in Swift

Linters define and enforce syntactical conventions across codebases. They enable teams of developers to convey shared values to one another, making new contributions easier and more fun to review and existing code easier to build off of. This talk will explore how Swift compiles, how Swift linting works, and the importance and difficulty of sharing programming patterns and techniques across engineering teams.

Transcript

1. Linting in Swift Tamar Nachmany iOS Engineer, Tumblr @tamarshmallows
2. What is linting?
3. Analyzing code & enforcing custom rules
4. let greeting:String
 let greeting: String Rule: Leave space after colons
5. let library = [ Movie(name: "Casablanca", director: "Michael Curtiz"), Song(name: "Blue Suede Shoes", artist: “Elvis") ] for item in library {
 if let book = item as! Book { // This will crash at runtime.
 } } Rule: Don’t Allow Force Downcasting (as!)
6. 1. Why build automated tools for code review?
 2. Where do Xcode errors come from?
 3. How does SwiftLint supplement the Swift compiler?
 Let’s explore these questions:
7. Why lint?
8. Code review matters.
9. Code review is where engineers work together.
10. Code review is where engineers receive feedback on their work.
11. Maybe code review is even where your engineering culture is formed.
12. Making sure engineers feel like they are always learning is important for retaining talented people.
13. Making sure engineers feel respected and happy is essential for retaining talented people.
14. Linting may elevate your code review process.
15. What do linters get us during the compilation process?
16. What do linters get us during the compilation process? Extra errors.
17. Where do Xcode errors come from?
18. Where do Xcode errors come from? SourceKit, using the Swift Compiler
19. Credit: Chris Lattner & Joe Groff Swift Compilation
20. Swift Compilation Credit: Chris Lattner & Joe Groff
21. Let’s make an error println(“yo”)
23. What information do we need to display this? 1. Error location 2. Error cause 3. Fix-it suggestion 4. Severity (error or warning-level issue)
24. When you enter new code into your project, Xcode sends messages to SourceKit.
 SourceKit responds with some info.
25. SourceKit Response key.diagnostics: [ { key.line: 5, key.column: 9, key.filepath: "/Users/SourceKit-Logging.playground", key.severity: source.diagnostic.severity.error, key.description: "'println' has been renamed to 'print'", key.diagnostic_stage: source.diagnostic.stage.swift.sema, key.fixits: [ { key.offset: 0, key.length: 7, key.sourcetext: "print" } ]
26. lib/Parse Response filename.swift:5:9: error: 'println' has been renamed to 'print' println("yo") ^~~~~~~ print
28. Let’s look at a linter
29. SwiftLint by
30. How does SwiftLint enforce new rules in SourceKit?
31. What does SwiftLint need to do for this to work? • Establish rules • Parse files and assess them based on new rules • Print results in Xcode, the command line, etc
32. .swiftlint.yml Config File whitelist_rules: # Rules to run - variableMaxNameLength
 - comma
33. Rule func validateFile(file: File) -> [StyleViolation] { 
 // Regex 
 }
34. func validateFile(file: File, kind: KindType, dictionary: [String: SourceKitRepresentable]) -> [StyleViolation] { // Analyze file as a dictionary of SourceKitRepresentable strings } ASTRule
35. public func validateFile(file: File) -> [StyleViolation] { return violationRangesInFile(file).map { StyleViolation(ruleDescription: self.dynamicType.description, severity: configuration.severity, location: Location(file: file, characterOffset: $0.location)) } } Comma Rule Analyzes the file using regular expressions
36. public func validateFile(file: File, kind: SwiftDeclarationKind, dictionary: [String: SourceKitRepresentable]) -> [StyleViolation] { return file.validateVariableName(dictionary, kind: kind).map { name, offset in if configuration.excluded.contains(name) { return [] } … } Variable Name Length Rule 
 Needs to know the Swift declaration kind
37. SwiftLint (Realm) SourceKit (Apple) SwiftKitten (Realm) Swift Compiler Instance (Apple)
38. To get set up: • Install SwiftLint locally • Create a config file called (.swiftlint.yml) where rules are defined. • A script that runs SwiftLint • A script that runs your script, perhaps in a build phase for your target(s)
39. In theatre directing, the first thing you say matters.
40. In code reviews, the content of your comments matters.
41. Developing & improving collaboration tools = Very valuable investment of time.
42. What would be a great collaboration tool for your team?
 

43. What would be a great collaboration tool for your team?
 
 You should build it.
44. @tamarshmallows Say hi: