swift 学习笔记二：Closures

Closure Expression Syntax（语法定义）

{ ( parameters) -> return type in

statements

}

看一个例子，对字符串数组排序：

var names = […]

var reversed = sorted(names,{(s1: String,s2: String) -> Bool in

return s1 > s2

})

swift还提供了一连串的简化方法：

1. Referring Type from Context （从上下文环境中推断参数和返回值的类型，就可以省略不写了）

var reversed = sorted(names,{s1,s2 in return s1 > s2})

2. Implicit Returns from Single-Expression Closures（如果只有一行语句，return 也可以省略）

var reversed = sorted(names,s2 in s1 > s2})

3. Shorthand Argument Names （命名参数也可以省略，用 $n 代替）

var reversed = sorted(names,{$0 > $1})

4. Operator Functions （直接用操作符）

var reversed = sorted(names,>)

. Trailing Closures （如果Closure是最后一个参数，可以写在参数括号的外面）

sample 1:

var reversed = sorted(names) { $0 > $1}

sample 2:

let digitNames = [
0 : "Zero" , 1 : "One" ,216)">2 : "Two" ,216)">3 : "Three" ,216)">4 : "Four" ,
5 : "Five" ,216)">6 : "Six" ,216)">7 : "Seven" ,216)">8 : "Eight" ,216)">9 : "Nine"
]
let numbers = [ 16 ,216)">58 ,216)">510 ]

numbers . map {
( var number) -> String in
var output = ""
while number > 0 {
output = digitNames [number% 10 ]! + output
number /= 10
}
return output

}

func makeIncrementer(forIncrement amount: Int ) -> () -> Int {

var runningTotal = 0
func incrementer() -> Int {
runningTotal += amount
return runningTotal
}

return incrementer
}

let incrementByTen = makeIncrementer (forIncrement: 10 )

incrementByTen ()10

incrementByTen ()20

let incrementBySeven = 7 )

incrementBySeven ()7

incrementBySeven ()14

let alsoIncrementByTen = incrementByTen