New Features and Improvements in Swift 4

Author: Liu Duo (Senior iOS Engineer at Hujiang)

Original article, please credit the author and source when reposting

WWDC 2017 introduced many surprises, and Swift 4 arrived together with the Xcode 9 beta. Because Swift 4 is open‑source, anyone following the swift‑evolution project can already see its new features. This article lists and analyses Swift 4’s new syntax, string handling, standard library changes, and build‑process improvements.

1. Syntax Improvements

Extension can access private properties

Reference the following code:

struct Date: Equatable, Comparable {
    private let secondsSinceReferenceDate: Double
    static func ==(lhs: Date, rhs: Date) -> Bool {
        return lhs.secondsSinceReferenceDate == rhs.secondsSinceReferenceDate
    }
    static func <(lhs: Date, rhs: Date) -> Bool {
        return lhs.secondsSinceReferenceDate < rhs.secondsSinceReferenceDate
    }
}

The code defines a Date struct and implements Equatable and Comparable . In Swift 3 the compiler reports an error because an extension cannot access a private property; you had to change it to fileprivate . Swift 4 expands the scope of private to extensions, eliminating the need for fileprivate .

Combined type and protocol syntax

In Swift 4 you can combine a type and a protocol with & :

protocol Shakeable {
    func shake()
}

extension UIButton: Shakeable { /* ... */ }
extension UISlider: Shakeable { /* ... */ }

func shakeEm(controls: [UIControl & Shakeable]) {
    for control in controls where control.state.isEnabled {
        control.shake()
    }
}

This resolves the earlier need for awkward casts or separate generic constraints.

Associated type with where clause

Swift 4 allows a where clause after an associatedtype declaration, e.g. in Sequence :

protocol Sequence {
    associatedtype Element where Self.Element == Self.Iterator.Element
    associatedtype Iterator: IteratorProtocol where Iterator.Element == Element
    func makeIterator() -> Iterator
}

New Key‑Path syntax

Key‑paths now start with a backslash:

\Kid.nickname
let name = ben[keyPath: \Kid.nickname]
ben[keyPath: \Kid.nickname] = "BigBen"

They are type‑safe, work on structs, and no longer require @objcMembers or dynamic .

Subscript generic support

struct GenericDictionary<Key: Hashable, Value> {
    private var data: [Key: Value]
    init(data: [Key: Value]) { self.data = data }
    subscript<T>(key: Key) -> T? { return data[key] as? T }
}
let dictionary = GenericDictionary(data: ["Name": "Xiaoming"])
let name: String? = dictionary["Name"]

2. String Improvements

Unicode count correctness

Swift 4 counts a composed emoji sequence as a single character, fixing the Swift 3 behaviour.

Faster character processing

Performance for English, French, German, and Spanish strings improves by ~3.5×; for Simplified Chinese and Japanese by ~2.5×.

Removal of characters property

String APIs now operate directly on String without the intermediate characters view.

One‑sided slicing

let values = "abcdefg"
let start = values.index(values.startIndex, offsetBy: 3)
let sub = values[start...]

String as a Collection

Strings now support collection methods such as map , filter , reduce , and can be reversed directly.

Multi‑line string literals

let joke = """
Q: Why does \(name) have \(n) \(character)'s in their name?
A: I don't know…
"""

3. Standard Library Updates

Codable

Swift 4 introduces Codable to replace manual NSCoding implementations.

struct Language: Codable {
    var name: String
    var version: Int
}
let swift = Language(name: "Swift", version: 4)
let encoded = try? JSONEncoder().encode(swift)
let decoded = try? JSONDecoder().decode(Language.self, from: encoded!)

Dictionary and Set enhancements

Initialisation from a Sequence

Allow duplicate keys during construction

mapValues and default‑value subscripting

Grouping and inversion utilities

MutableCollection.swapAt(_:_:)

var arr = [1, 2, 3, 4]
arr.swapAt(1, 2) // [1, 3, 2, 4]

4. Build Process Improvements

New Build System

Xcode 9 introduces a new build system selectable via File → Project Settings… .

Pre‑compiled Bridging Headers

Bridging headers are now compiled once and reused, cutting mixed Swift/Objective‑C build times by ~40%.

Indexing during compilation

Indexing now runs in parallel with compilation, reducing overall wait time.

COW Existential Containers

Copy‑On‑Write containers reduce heap allocations for large existential values.

Reduced implicit @objc inference

Only required places now get implicit @objc , shrinking binary size (Apple Music app down 5.7%).

5. Exclusive Access to Memory

Swift 4 adds compile‑time checks that prevent mutating a collection while simultaneously iterating it, avoiding runtime crashes.

6. Compatibility

Xcode 9 ships with both Swift 3.2 (fully compatible with Swift 3.1) and Swift 4, allowing mixed‑module compilation and a smoother migration path.

7. References

WWDC 2017 Session 402 “What’s New in Swift”

WWDC 2017 Session 212 “What’s New in Foundation”

WWDC 2017 Session 102 “Platforms State of the Union”

Swift Language Programming (Swift 4.0)

swift‑evolution

what’s‑new‑in‑swift‑4

Ray Wenderlich article

Hacking with Swift 4