Swift 4 Cheat Sheet Pro

@(编程笔记)[Swift]

这个是在 Swift 4 Cheat Sheet Advanced 的基础上再次进阶的小抄，内容并没有多高端，只是 Cheat Sheet Advanced 更多是常用语法，而这里搜集的是一些很有用但是又需要一定基础的语法。如果滥用，既容易降低代码可读性也容易出错。

Protocols

https://docs.swift.org/swift-book/LanguageGuide/Protocols.html

Conditionally Conforming to a Protocol

extension Array: TextRepresentable where Element: TextRepresentable {
    var textualDescription: String {
        let itemsAsText = self.map { $0.textualDescription }
        return "[" + itemsAsText.joined(separator: ", ") + "]"
    }
}
let myDice = [d6, d12]
print(myDice.textualDescription)
// Prints "[A 6-sided dice, A 12-sided dice]"

Generics

func swapTwoValues<T>(_ a: inout T, _ b: inout T) {
    let temporaryA = a
    a = b
    b = temporaryA
}

func someFunction<T: SomeClass, U: SomeProtocol>(someT: T, someU: U) {
    // function body goes here
}

https://docs.swift.org/swift-book/LanguageGuide/Generics.html

Advanced Operators

https://docs.swift.org/swift-book/LanguageGuide/AdvancedOperators.html

Bitwise NOT Operator

let initialBits: UInt8 = 0b00001111
let invertedBits = ~initialBits  // equals 11110000

Bitwise AND Operator

let firstSixBits: UInt8 = 0b11111100
let lastSixBits: UInt8  = 0b00111111
let middleFourBits = firstSixBits & lastSixBits  // equals 00111100

Bitwise OR Operator

let someBits: UInt8 = 0b10110010
let moreBits: UInt8 = 0b01011110
let combinedbits = someBits | moreBits  // equals 11111110

Bitwise XOR Operator

let firstBits: UInt8 = 0b00010100
let otherBits: UInt8 = 0b00000101
let outputBits = firstBits ^ otherBits  // equals 00010001

Bitwise Left and Right Shift Operators

Shifting Behavior for Unsigned Integers

let shiftBits: UInt8 = 4   // 00000100 in binary
shiftBits << 1             // 00001000
shiftBits << 2             // 00010000
shiftBits << 5             // 10000000
shiftBits << 6             // 00000000
shiftBits >> 2             // 00000001

Map, FlatMap, compactMap, Filter, Reduce And Sort

Map

let values = [2.0,4.0,5.0,7.0]
let squares2 = values.map({
  (value: Double) -> Double in
  return value * value
})
// [4.0, 16.0, 25.0, 49.0]

FlatMap

使用 flatMap 函数替代 map 函数的原因在于前者能够忽略可选值为 nil 的情况。例如 flatMap([0,nil,1,2,nil]) 的结果是 [0,1,2]。

其实整个 flatMap 方法可以拆解成两步：

第一步像 map 方法那样，对元素进行某种规则的转换。
第二步，执行 flatten 方法，将数组中的元素一一取出来，组成一个新数组。
http://www.infoq.com/cn/articles/swift-brain-gym-map-and-flatmap

let arr = [[1, 2, 3], [6, 5, 4]]
let brr = arr.flatMap {
    $0
}
// brr = [1, 2, 3, 6, 5, 4]

// 等价于：

let arr = [[1, 2, 3], [6, 5, 4]]
let crr = Array(arr.map{ $0 }.flatten())
// crr = [1, 2, 3, 6, 5, 4]

compactMap

Swift 4.1 引入。用以替代flatMap的特定实现Sequence.flatMap<U>(_: (Element) -> U?) -> [U]。

https://github.com/apple/swift-evolution/blob/master/proposals/0187-introduce-filtermap.md
'flatMap' is deprecated: Please use compactMap(:) for the case where closure returns an optional value. Use 'compactMap(:)' instead.

let names: [String?] = ["Tom", nil, "Peter", nil, "Harry"]
let valid = names.compactMap { $0 }
// ["Tom", "Peter", "Harry"]

let words = ["53", "nine", "hello","0"]
let values = words.compactMap { Int($0) }
// [53, 0]

Filter

let digits = [1, 4, 5, 10, 15]
let even = digits.filter { (number) -> Bool in
    return number % 2 == 0
}
// [4, 10]

Reduce

https://swift.gg/2015/12/10/reduce-all-the-things/
Reduce 的基础思想是将一个序列转换为一个不同类型的数据，期间通过一个累加器（Accumulator）来持续记录递增状态。为了实现这个方法，我们会向 reduce 方法中传入一个用于处理序列中每个元素的结合（Combinator）闭包 / 函数 / 方法。

求和范例：

// 初始值 initial 为 0，每次遍历数组元素，执行 + 操作
[0, 1, 2, 3, 4].reduce(0, combine: +)
// 10

反转数组：

// $0 指累加器（accumulator），$1 指遍历数组得到的一个元素
[1, 2, 3, 4, 5].reduce([Int](), combine: { [$1] + $0 })
// 5, 4, 3, 2, 1

划分（Partition）处理：

// 为元组定义个别名，此外 Acc 也是闭包传入的 accumulator 的类型
typealias Acc = (l: [Int], r: [Int])
func partition(lst: [Int], criteria: (Int) -> Bool) -> Acc {
   return lst.reduce((l: [Int](), r: [Int]()), combine: { (ac: Acc, o: Int) -> Acc in 
      if criteria(o) {
    return (l: ac.l + [o], r: ac.r)
      } else {
    return (r: ac.r + [o], l: ac.l)
      }
   })
}
partition([1, 2, 3, 4, 5, 6, 7, 8, 9], criteria: { $0 % 2 == 0 })
//: ([2, 4, 6, 8], [1, 3, 5, 7, 9])

Sort

• sorted()回传排序结果，不影响数组内容。
• sort()回传排序结果，并将结果存回数组。

var numbers = [2, 4, 6, 8, 1, 3, 5, 7]

numbers.sorted(<)   //return: [1, 2, 3, 4, 5, 6, 7, 8]
numbers             //return: [2, 4, 6, 8, 1, 3, 5, 7]

numbers.sort(>)
numbers             //return: [8, 7, 6, 5, 4, 3, 2, 1]

struct MyCustomStruct {
    var someSortableField: String
}

var customArray = [
    MyCustomStruct(someSortableField: "Jemima"),
    MyCustomStruct(someSortableField: "Peter"),
    MyCustomStruct(someSortableField: "David"),
    MyCustomStruct(someSortableField: "Kelly"),
    MyCustomStruct(someSortableField: "Isabella")
]

customArray.sort {
    $0.someSortableField < $1.someSortableField
}