Packages and Modules

Access Boundaries

Access control keeps stored details behind public methods and properties.

Hide state behind methods

bonus
access_boundaries.swift
Replay: real traced execution (multi-file project)
struct ScoreBox {
    private var value: Int

    init(start: Int) {
        value = start
    }

    mutating func add(_ amount: Int) {
        value += amount
    }

    var current: Int {
        return value
    }
}

let bonus = 4
var box = ScoreBox(start: 10)
box.add(bonus)
let message = "score=\(box.current)"

print(message)

struct ScoreBox {
    private var value: Int

    init(start: Int) {
        value = start
    }

    mutating func add(_ amount: Int) {
        value += amount
    }

    var current: Int {
        return value
    }
}

let bonus = 0
var box = ScoreBox(start: 10)
box.add(bonus)
let message = "score=\(box.current)"

print(message)

struct ScoreBox {
    private var value: Int

    init(start: Int) {
        value = start
    }

    mutating func add(_ amount: Int) {
        value += amount
    }

    var current: Int {
        return value
    }
}

let bonus = 9
var box = ScoreBox(start: 10)
box.add(bonus)
let message = "score=\(box.current)"

print(message)

  1. bonus ← 4, value ← 10, box ← ScoreBox(value: 10)

    4    init(start: Int) {5        value→ 10 = start106    }78    mutating func add(_ amount: Int) {9        value += amount10    }1112    var current: Int {13        return value14    }15}1617let bonus→ 4 = 4  //@bonus=0, 918var box→ ScoreBox(value: 10) = ScoreBox(start: 10)19boxScoreBox(value: 10).add(bonus4)20let message = "score=\(box.current)"

  2. value ← 14

    8mutating func add(_ amount4: Int) {9    value→ 14 += amount410}

  3. box ← ScoreBox(value: 14), message ← score=14

    12    var current: Int {13        return value1414    }15}1617let bonus = 4  //@bonus=0, 918var box = ScoreBox(start: 10)19box→ ScoreBox(value: 14).add(bonus4)20let message→ score=14 = "score=\(box.current14)"2122print(messagescore=14)
    outputscore=14

  1. bonus ← 0, value ← 10, box ← ScoreBox(value: 10)

    4    init(start: Int) {5        value→ 10 = start106    }78    mutating func add(_ amount: Int) {9        value += amount10    }1112    var current: Int {13        return value14    }15}1617let bonus→ 0 = 018var box→ ScoreBox(value: 10) = ScoreBox(start: 10)19boxScoreBox(value: 10).add(bonus0)20let message = "score=\(box.current)"

  2. mutating func add(_ amount: Int)

    8mutating func add(_ amount0: Int) {9    value10 += amount010}

  3. message ← score=10

    12    var current: Int {13        return value1014    }15}1617let bonus = 018var box = ScoreBox(start: 10)19boxScoreBox(value: 10).add(bonus0)20let message→ score=10 = "score=\(box.current10)"2122print(messagescore=10)
    outputscore=10

  1. bonus ← 9, value ← 10, box ← ScoreBox(value: 10)

    4    init(start: Int) {5        value→ 10 = start106    }78    mutating func add(_ amount: Int) {9        value += amount10    }1112    var current: Int {13        return value14    }15}1617let bonus→ 9 = 918var box→ ScoreBox(value: 10) = ScoreBox(start: 10)19boxScoreBox(value: 10).add(bonus9)20let message = "score=\(box.current)"

  2. value ← 19

    8mutating func add(_ amount9: Int) {9    value→ 19 += amount910}

  3. box ← ScoreBox(value: 19), message ← score=19

    12    var current: Int {13        return value1914    }15}1617let bonus = 918var box = ScoreBox(start: 10)19box→ ScoreBox(value: 19).add(bonus9)20let message→ score=19 = "score=\(box.current19)"2122print(messagescore=19)
    outputscore=19
access control `private` keeps implementation details inside a type while methods expose the operations callers are allowed to perform.