* 1>生成String类型的hh,如果为nil,则返回""

postfix operator ~~

postfix func ~~(
    v: String?
    ) -> String {
    return v == nil ? "" : v!
}
var foo : String?

var hh  = foo~~

*2> sequence基础 1

        for i in sequence(first: 1, next: {  $0 * 2}){
            // $0 代表 first  然后 在将 $0返回给first
            if i > 100 {
                break
            }
            print(i) // 1 2 4 8 16 32 64
        }

        //另一种写法
        for _ in sequence(first: 1, next: {
            //返回nil就是结束
            print($0)
            let number = $0 * 2
            return  number <= 100 ? $0 * 2 : nil
        }){}

• sequence基础 2

        let fibonacci = sequence(state: (0,1)) {(state: inout (Int, Int)) -> Int? in
            defer {state = (state.1, state.0 + state.1)}
            return state.0
        }
        //只取10 个数
        for number in fibonacci.prefix(10){
            print(number)
        }

• sequence基础 3 返回(0,0)~(5,2)的所有组合

    for i in cartesianSequence(xCount: 5, yCount: 2){
        print("x : \(i.x)    y : \(i.y)")
    }

   func cartesianSequence(xCount : Int , yCount : Int) -> UnfoldSequence<SeqPointType , Int> {
     assert(xCount > 0 && yCount > 0 ,  "必须是整数")
     return  sequence(state: 0, next: {
       (index : inout Int) -> SeqPointType? in
       guard index < xCount * yCount else {return nil}
       defer {index += 1}
       print(index)
       return (x: index % xCount, y : index / xCount)
        })
    }

3. <a href = "https://www.baidu.com/s?ie=utf-8&f=8&rsv_bp=0&rsv_idx=1&tn=baidu&wd=NS_REQUIRES_NIL_TERMINATION&rsv_pq=bde9f19c0000d37b&rsv_t=412deZWuo0iQXSD1Mczsoqd1eS%2B6uPz%2F09L4BL95LtUNUDeRUQE4UKElEAs&rqlang=cn&rsv_enter=1&rsv_sug3=1">NS_REQUIRES_NIL_TERMINATION</a>

4. <a href ="">NSAssert</a>

5.关键字 <a href = "#">@autoclosure </a>

    func add(l : Bool, _ r : @autoclosure ()->Bool) -> Bool {
        guard l else {
            return false
        }
        return r()
    }
    
    
    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        
        if add(l: true,  true ) {
            print("进行操作")
        }
    }

6. swift 锁

func synchronized(lock: AnyObject, closure: () -> ()) {
        objc_sync_enter(lock)
        closure()
        objc_sync_exit(lock)
    }

7.Stride函数

        let numbers = [1,2,3,4,5,6]
        
        let damp = stride(from: 0, through: numbers.count, by: 2)
        
        for item in damp {
            
            print(item) // 0 2 4
        }

8. 数学

<code>let num = exp(1.0)</code> e的1次方

9. split 根据字符串拆分 , <code>maxSplits</code> : 拆分次数

        let lipsum = "Lorem ipsum dolor sit amet, consectetur adipiscing elit."
        let words = lipsum.characters.split(separator: " ").map(String.init)
        let words2 = lipsum.characters.split(separator: " ", maxSplits: 1, omittingEmptySubsequences: true).map(String.init)

10. lazy

        let counts = words.lazy.map({ $0 })

        for i in counts {
            print(i)
        }

11.sorted : 排序

        let dict = ["k1":3,"k2":2]
        let keys = Array(dict.keys)
        let sortedKeys = keys.sorted(by: {
            let v1 = dict[$0]
            let v2 = dict[$1]
            return v1! < v2!
        })
        print(sortedKeys)  // k2 k1