闭包

简介

内存管理

闭包类型

闭包语法

信息

闭包语法

{ (parameters) -> return type in
    statements
}


func backward(_ s1: String, _ s2: String) -> Bool {
    return s1 > s2
}
//If we didn't use closures, our code structure would be as follows.

var reversedNames = names.sorted(by: backward)

闭包语法

let names = ["Chris", "Alex", "Ewa", "Barry", "Daniella"]
let reversedNames = names.sorted(by: { (s1: String, s2: String) -> Bool in
    return s1 > s2
})

print(reversedNames)

///sorted method takes closure as parameter.

关键字

从上下文推断类型

示例

let names = ["Chris", "Alex", "Ewa", "Barry", "Daniella"]
let reversedNames = names.sorted(by: { (s1: String, s2: String) -> Bool in
    return s1 > s2
})

print(reversedNames)

简写语法

/*
In this example, the return keyword has been extracted.
It already has single expression. 
There is no need to express it as an extra with return.
*/

var names = ["Coddy", "Kit"]
let reversedNames = names.sorted(by: { s1, s2 in s1 > s2 } )
print(reversedNames)

示例

reversedNames = names.sorted(by: { s1, s2 in s1 > s2 } )

名称简写

let names = ["Coddy", "Kit"]
let reversedNames = names.sorted(by: { $0 > $1 } )
print(reversedNames)

// In this example $0 will shortcut the first parameter and $1 will shortcut the second parameter.

简写示例

var names = ["Coddy", "Kit"]

let reversedNames = names.sorted(by: >)
print(reversedNames)
// Greater than operator takes two parameters and the rotation type is squat.

TRAILING CLOSURE

最后一个参数

class Animator {
    class func animate(withDuration duration: Double, animations: () -> Void) {
        animations()
    }
}

class MyViewController {
    func start() {
        // With the traditional method, we can call this fund as follows.
        Animator.animate(withDuration: 1, animations: { [unowned self] in
            print("red")
        })
    }
}
MyViewController().start()

由于上面的代码可读性较差，可以写成如下形式

import Foundation
Thread.sleep(forTimeInterval: 1)
print("Background color changed to red")

示例

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

// Here's how you call this function without using a trailing closure:
someFunctionThatTakesAClosure(closure: {
    // closure's body goes here
})

// Here's how you call this function with a trailing closure instead:
someFunctionThatTakesAClosure() {
    // trailing closure's body goes here
}

语法

/*
In the example below, animations
are expressed as closure.
*/
public class func animate(withDuration: TimeInterval, animations: () -> Void)

// In traditional usage, we need to make a call like the following.
UIView.animate(withDuration: 1, animations: { [unowned self] in
    self.view.backgroundColor = UIColor.red
})

/*
In this way, both the readability of the code
increases and we avoid the use of extra brackets.
If it is as above and the last parameter of the method 
is closure, Swift allows to make a call like the one below.
*/

UIView.animate(withDuration: 1) { [unowned self] in
    self.view.backgroundColor = UIColor.red
}

我们可以使用下面的代码测试代码

func greetThenRunClosure(name: String, closure: () -> ()) {
    print("Hello, \(name)!")
    closure()
}

greetThenRunClosure(name: "Paul") {
    print("The closure was run")
}

闭包捕获值

补充

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

let incrementByTen = makeIncrementer(forIncrement: 10)

print(incrementByTen())
// returns a value of 10
print(incrementByTen())
// returns a value of 20
print(incrementByTen())
// returns a value of 30

续

警告

@escaping

@ESCAPING 关键字

nonEscaping

使用逃逸闭包

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