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swift-tour.md

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A Swift tour: explore Swift's features and design

Presenter: Allan Shortlidge, Swift Compiler Engineer

Link: https://developer.apple.com/wwdc24/10184

Example app to show features

  • Data model library
  • HTTP server
  • Command-line utility

Value types

  • Use var keyword to introduce variable
  • What are value types?
    • No shared state
    • Equal values are interchangeable
    • Used for more than basic types in Swift
  • Use let keyword for immutable types
  • Structs aggregate multiple values into a single object
  • Arrays are value types in Swift
  • Most Swift types are value types
  • Reference types like classes exist but are used in specialized cases

Errors and optionals

  • Code structure handles all possibilities
  • The throws and try keywords make error-handling explicit
  • Unwrapping optionals ensures that values exist

Errors

  • Sources of errors should be clearly marked
  • Errors contain actionable information for the user
  • Programmer mistakes are not recoverable errors
  • Enums are great error types - they must conform to Error protocol
  • Guard statements are a good way to halt execution when an error occurs

Optionals

  • A value that's either nil or some value
  • Value must be unwrapped
  • Compile-time safety
  • if let syntax to unwrap optional
  • Force-unwrap with !

Code organization

  • Modules and packages

  • Module is a collection of source files that are always built together

  • Modules can have dependencies

  • Swift Package Manager is a tool for managing packages

  • Builds, tests, and runs code

  • Swift Package Index has a list of open-source libraries

  • Access control levels: private, internal, package, public

Classes

  • Reference types for shared mutable state
  • Classes support inheritance
  • Methods can be overridden by class implementation
  • Automatic reference counting (ARC) - ensures an object remains alive as long as there is a reference to it
  • When no more references, things deallocate
  • Avoid reference cycles, which prevent dealloc
  • Can use weak reference, which becomes optional, property becomes nil when thing is deallocated

Protocols

  • Abstract set of requirements for a type
  • To conform to a protocol, it must provide implementations of protocol's requirements
  • Use extensions to add stuff to a type regardless of where the type is defined
  • Collections (array, dictionary, set, string)
  • Every type that conforms to Collections shares features - i.e. iteration through a for loop, accessing an index
  • Shorthand syntax for closures

Concurrency

  • Task is an independent concurrent execution context
  • Tasks are lightweight
  • Support awaiting/cancellation
  • Tasks can execute concurrently
  • When adding a UserStore, concurrency error related to shared mutable state not being Sendable
  • Actors can encapsulate shared mutable state, and protect the state by serializing Task execution

Extensibility

  • Property wrappers
    • Reusable abstraction for property behaviors
    • Eliminate boilerplate via annotation
  • Result builders
    • Build up values iusing a lightweight syntax
    • Create declarative sub-languages
  • Macros
    • Swift code that act as a compiler plugin taking the syntax tree as input and returning transformed code as output