22.泛型 Generics Swift官方文档——版纳的笔记

//: Playground - noun: a place where people can play

import UIKit

// # 泛型解决的问题
/*
func swapTwoInts(_ a: inout Int, _ b: inout Int) {
    let temporaryA = a
    a = b
    b = temporaryA
}
var someInt = 3
var anotherInt = 107
swapTwoInts(&someInt, &anotherInt)
print("someInt is now \(someInt), and anotherInt is now \(anotherInt)")
*/

// # 泛型函数
func swapTwoValues(_ a: inout T, _ b: inout T) {
    let temporaryA = a
    a = b
    b = temporaryA
}
var someInt = 3
var anotherInt = 107
swapTwoValues(&someInt, &anotherInt)
var someString = "hello"
var anotherString = "world"
swapTwoValues(&someString, &anotherString)
// trick: swap(_:_:)Swift库已实现

// # 类型形式参数
// 类型形式参数指定并且命名一个占位符类型，紧挨着写在函数名后面的一对尖括号里（比如  ）

// # 命名类型形式参数
// 用大写驼峰命名法,表明是一个占位符

// # 泛型类型
struct Stack {
    var items = [Element]()
    mutating func push(_ item: Element) {
        items.append(item)
    }
    mutating func pop() -> Element {
        return items.removeLast()
    }
    init() {
        // just do nothing here
    }
    init(items: [Element]) {
        self.items = items
    }
}
var stackOfStrings = Stack()
stackOfStrings.push("uno")
stackOfStrings.push("dos")
stackOfStrings.push("tres")
stackOfStrings.push("cuatro")
let fromTheTop = stackOfStrings.pop()

var stackOfInts = Stack(items: [1, 2, 7]) // '' can be inferred so is omittd.


// *** 泛型的类可以被继承为非泛型
let view = UIView()
let label = UILabel()
let topConstraint = label.topAnchor.constraint(equalTo: view.topAnchor)
// YAnchor is a subclass of anchor. Anchor is a generic class.
// 参见泛型1.playground


// # 扩展一个泛型类型
extension Stack {
    var topItem: Element? {
        return items.isEmpty ? nil : items[items.count - 1]
    }
}
if let topItem = stackOfStrings.topItem {
    print("The top item on the stack is \(topItem).")
}

// # 类型约束
// For example, Hashable: a type that provides an integer hash value.
/*
func findIndex(ofString valueToFind: String, in array: [String]) -> Int? {
    // trick: array.enumerated() 返回了一个元组的数组,用for-in提取每个元素(n, x)
    // 若为数组,n是从零开始的序列,x是对应的元素
    // 若为字符串,n是从零开始的序列,x是对应的字符
    for (index, value) in array.enumerated() {
        if value == valueToFind {
            return index
        }
    }
    return nil
}
let strings = ["cat", "dog", "llama", "parakeet", "terrapin"]
if let foundIndex = findIndex(ofString: "llama", in: strings) {
    print("The index of llama is \(foundIndex)")
}
*/
func findIndex(of valueToFind: T, in array:[T]) -> Int? {
    for (index, value) in array.enumerated() {
        if value == valueToFind {
            return index
        }
    }
    return nil
}
let doubleIndex = findIndex(of: 9.3, in: [3.3424, 0.1, 0.25])
let stringIndex = findIndex(of: "Andrea", in: ["Mike", "Malcolm", "Andrea"])

// # 关联类型
protocol Container {
    associatedtype ItemType // ItemType 是一个占位符
    mutating func append(_ item: ItemType)
    var count: Int { get }
    // trick: 下标在协议中的使用
    subscript(i: Int) -> ItemType { get }
}
struct IntStack: Container {
    var items = [Int]()
    mutating func push(_ item: Int) {
        items.append(item)
    }
    mutating func pop() -> Int {
        return items.removeLast()
    }
    typealias ItemType = Int // ???
    mutating func append(_ item: Int) {
        self.push(item)
    }
    var count: Int {
        return items.count
    }
    subscript(i: Int) -> Int {
        return items[i]
    }
}
struct StackAgain: Container {
    var items = [Element]()
    mutating func push(_ item: Element) {
        items.append(item)
    }
    mutating func pop() -> Element {
        return items.removeLast()
    }
    mutating func append(_ item: Element) {
        self.push(item)
    }
    var count: Int {
        return items.count
    }
    subscript(i: Int) -> Element {
        return items[i]
    }
}
// 扩展现有类型来指定关联类型
extension Array: Container {}
// now we can use an Array as a Container type.

// # 泛型where分句
// where语句,冒号+协议语句都是对泛型类型的限制
func allItemsMatch(_ someContainer: C1, _ anotherContainer: C2) -> Bool where C1.ItemType == C2.ItemType, C1.ItemType: Equatable {
    
    // Check that both containers contain the same number of items.
    if someContainer.count != anotherContainer.count {
        return false
    }
    
    // Check each pair of items to see if they are equivalent.
    for i in 0..()
stackOfStringsAgain.push("uno")
stackOfStringsAgain.push("dos")
stackOfStringsAgain.push("tres")
var arrayOfStrings = ["uno", "dos", "tres"]
if allItemsMatch(stackOfStringsAgain, arrayOfStrings) {
    print("All items match.")
} else {
    print("Not all items match.")
}

// # 带有泛型Where分句的扩展
extension Stack where Element: Equatable {
    func isTop(_ item: Element) -> Bool {
        // trick: .last如果为空返回nil,否则返回包裹最后一个元素的可选项
        guard let topItem = items.last else {
            return false
        }
        return topItem == item
    }
}
if stackOfStrings.isTop("tres") {
    print("Top element is tres.")
} else {
    print("Top element is something else.")
}

struct NotEquatable { }
var notEquatableStack = Stack()
let notEquatableValue = NotEquatable()
notEquatableStack.push(notEquatableValue)
// Error when: notEquatableStack.isTop(notEquatableValue)

extension Container where ItemType: Equatable {
    func startsWith(_ item: ItemType) -> Bool {
        return count >= 1 && self[0] == item
    }
}
if [9, 9, 9].startsWith(42) {
    print("Starts with 42.")
} else {
    print("Starts with something else.")
}

extension Container where ItemType == Double {
    func average() -> Double {
        var sum = 0.0
        for index in 0.. Item { get }
    
    associatedtype Iterator: IteratorProtocol where Iterator.Element == Item
    func makeIterator() -> Iterator
}
protocol ComparableContainer: NewContainer where Item: Comparable { }

// # 泛型下标
extension NewContainer {
    subscript(indices: Indices) -> [Item] where Indices.Iterator.Element == Int {
        var result = [Item]()
        for index in indices {
            result.append(self[index])
        }
        return result
    }
}