Swift 语言之闭包学习笔记

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闭包表达式

闭包表达式语法：

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let names = ["Chris", "Alex", "Ewa", "Barry", "Daniella"]

func backwards(s1: String, s2: String) -> Bool {
    return s1 > s2
}

var reversed = sorted(names, backwards)
// reversed is equal to ["Ewa", "Daniella", "Chris", "Barry", "Alex"]

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

根据上下文推断类型：

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

单行表达式闭包的隐式返回：

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

简写参数名：

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

运算符函数：

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reversed = sorted(names, >)

尾随闭包

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func someFunctionThatTakesAClosure(closure: () -> ()) {
    // function body goes here
}

// here's how you call this function without using a trailing closure:

someFunctionThatTakesAClosure({
    // closure's body goes here
})

// here's how you call this function with a trailing closure instead:

someFunctionThatTakesAClosure() {
    // trailing closure's body goes here
}

值捕获

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

闭包是引用类型

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let incrementByTen = makeIncrementer(forIncrement: 10)
incrementByTen()
// returns a value of 10
incrementByTen()
// returns a value of 20
incrementByTen()
// returns a value of 30
incrementByTen()
// returns a value of 40

let alsoIncrementByTen = incrementByTen
alsoIncrementByTen()
// returns a value of 50