swift闭包

闭包就是一段代码块，可以捕获其上下文的变量。

闭包的定义

{ (parameters) -> return type in
    statements
}

可以用一个变量保存闭包

let clou1 = {(str1:String, str2:String) -> String in
    print(str1)
    return str1 + str2
}
clou1("12", "34") // "1234"

//指定闭包类型
let clou2:(String, String) -> String = {(str1, str2) in
    return str1 + str2
}
clou2("hello ", "world")

闭包作为函数的最后一个参数

func walk(clou:() -> Void) {
    clou()
    print("sss")
}

//调用时的形式有以下2种
walk(clou: {
    print("222")
})

walk() {
    print("222")

}

值捕获

定义在函数内的闭包可以捕获函数中的变量和常量以及参数值

func makeIncrementer(amount: Int) -> () -> Int {
    var runningTotal = 0
    return {() -> Int in
        runningTotal += amount
        return runningTotal
    }
}
let incrementByTen = makeIncrementer(amount: 10)

incrementByTen() //10
incrementByTen() //20
incrementByTen() //30

逃逸闭包(Escaping Closures)

A closure is said to escape a function when the closure is passed as an argument to the function, but is called after the function returns.

闭包需要在函数执行结束后才执行,需要在闭包参数类型前加@escaping

func escapClosures(testClou:@escaping () -> Void) {
    DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 5) {
        testClou()
    }
}
escapClosures() {
    print("sfsdfs")
}

逃逸闭包中如果引用到实例变量，需要显示调用self，非逃逸闭包则不需要。

func testEscapingClosures(myCloures: @escaping () -> Void) {
        print("ssss")
        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 2) {
            myCloures()
        }
    }
    
func testNoEscapingClosures(myCloures: () -> Void) {
    myCloures()
}

testEscapingClosures() {
       //逃逸闭包中引用实例变量，要显示指明self
       print("now num is \(self.num)")
 }
testNoEscapingClosures {
       //非逃逸闭包，不用显示指明self
       print("no escaping, num is \(num)")
}