Swift_闭包
闭包优化
//闭包优化func testClosures() { //函数做参数 排序 let names = ["XuBaoAiChiYu", "1045214799", "iOS", "Swift", "OC"] func backwards(_ s1: String, _ s2: String) -> Bool { return s1 > s2 } var reversed = names.sorted(by: backwards) print(reversed) //闭包排序 reversed = names.sorted(by: { (s1: String, s2: String) -> Bool in return s1 > s2 }) print(reversed) //可以写为一行 reversed = names.sorted( by: { (s1: String, s2: String) -> Bool in return s1 > s2 } ) print(reversed) //闭包可以自动判断参数类型和返回属性 reversed = names.sorted( by: { s1, s2 in return s1 > s2 } ) print(reversed) //当只有一行时，可省略return写法。 reversed = names.sorted( by: { s1, s2 in s1 > s2 } ) print(reversed) //闭包中的参数可使用$去获得 第一个参数为$0 第二个为$1 reversed = names.sorted( by: { $0 > $1 } ) print(reversed) //当闭包中只有两个参数 做比较操作时 只需要写入符号 reversed = names.sorted(by: >) print("\(reversed)") /* print ["iOS", "XuBaoAiChiYu", "Swift", "OC", "1045214799"] ["iOS", "XuBaoAiChiYu", "Swift", "OC", "1045214799"] ["iOS", "XuBaoAiChiYu", "Swift", "OC", "1045214799"] ["iOS", "XuBaoAiChiYu", "Swift", "OC", "1045214799"] ["iOS", "XuBaoAiChiYu", "Swift", "OC", "1045214799"] ["iOS", "XuBaoAiChiYu", "Swift", "OC", "1045214799"] ["iOS", "XuBaoAiChiYu", "Swift", "OC", "1045214799"] */} 尾随闭包
//尾随闭包func testTrailingClosures() { let names = ["Chris", "Alex", "Ewa", "Barry", "Daniella"] //如果函数需要一个闭包作为参数，且这个参数是最后一个参数. //尾随闭包可以放在函数参数列表外，也就是括号外 var reversed = names.sorted() { $0 > $1 } print("\(reversed)") //如果一个闭包表达式作为一个唯一的参数，你又正在使用尾随闭包，可以省略() reversed = names.sorted { $0 > $1 } print("\(reversed)") /* print ["Ewa", "Daniella", "Chris", "Barry", "Alex"] ["Ewa", "Daniella", "Chris", "Barry", "Alex"] */} 捕获值
//捕获值func testCapturingValues() { /* 闭包可以根据环境上下文捕获到定义的常量和变量。闭包可以引用和修改这些捕获到的常量和变量， 就算在原来的范围内定义为常量或者变量已经不再存在（很牛逼）。 在Swift中闭包的最简单形式是嵌套函数。 */ 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())") print("\(incrementByTen())") print("\(incrementByTen())") let incrementBySeven = makeIncrementer(forIncrement: 7) print("\(incrementBySeven())") print("\(incrementByTen())") //闭包是引用类型 let alsoIncrementByTen = incrementByTen print(alsoIncrementByTen()) /* print 10 20 30 7 40 50 */} 避免内存泄露
//避免内存泄露var completionHandlers: [() -> Void] = []func someFunctionWithNoescapeClosure(_ closure: () -> Void) { closure()// completionHandlers.append(closure) //会报错 closure无法被保存}func someFunctionWithEscapingClosure(_ completionHandler: @escaping () -> Void) { completionHandler() completionHandlers.append(completionHandler)}class SomeClass { var x = 10 func doSomething() { //内存溢出 someFunctionWithEscapingClosure { self.x = 100 } someFunctionWithNoescapeClosure { x = 200 } }}func testNonescapingClosures() { //@noescape 保留环问题 闭包中布应使用self 避免内存泄露 let instance = SomeClass() instance.doSomething() print(instance.x) completionHandlers.first?() print(instance.x) /* print 200 100 */}