Swift：闭包(Closures)

闭包能够引用和改动这些捕获到的常量和变量。在Swift中闭包的最简单形式是嵌套函数。

func increment(#amount: Int) -> (() -> Int) {  
  var total = 0  
  func incrementAmount() -> Int {  
    total += amount // total是外部函数体内的变量。这里是能够捕获到的  
    return total  
  }  
  return incrementAmount // 返回的是一个嵌套函数（闭包）  
}

let incrementByTen = increment(amount: 10)   
incrementByTen() // return 10,incrementByTen是一个闭包  
// 这里是没有改变对increment的引用，所以会保存之前的值  
incrementByTen() // return 20     
incrementByTen() // return 30     
  
let incrementByOne = increment(amount: 1)  
incrementByOne() // return 1  
incrementByOne() // return 2      
incrementByTen() // return 40   
incrementByOne() // return 3  

__weak typeof(self) wSelf = self;

方式一： 使用 unowned keyword

class HTMLElement {
    var name: String
    var text: String?
    
    init(name: String, text: String?) {
        self.name = name
        self.text = text
    }
    
    lazy var asHtml: () -> String = {
        [unowned self] in
        if let text = self.text {
            return "<\(self.name)>\(self.text)</\(self.name)>"
        } else {
            return "<\(self.name)>"
        }
    }
}


var html = HTMLElement(name: "Node", text: "Jack")
html.asHtml()

上面的代码中。self对asHtml这个闭包属性有强引用，而asHtml内部又有对self的强引用，所以我们能够使用[unowned self] 将self “弱化”。从而解除了循环引用。

注：  unowned keyword就相当于oc中的__unsafe_unretained,当不安全指针指向的对象销毁时。指针依旧指向曾经指向的内存地址(野指针)

方式二： 使用 weak keyword

lazy var asHtml: () -> String = {
        [weak self] in
        if let text = self.text {
            return "<\(self.name)>\(self.text)</\(self.name)>"
        } else {
            return "<\(self.name)>"
        }
    }

注： weakkeyword相当于oc中的__weak, 当弱指针指向的对象销毁时。指针自己主动指向nil

weak var weakSelf = self
lazy var asHtml: () -> String = {
        if let text = weakSelf.text {
            return "<\(weakSelf.name)>\(weakSelf.text)</\(weakSelf.name)>"
        } else {
            return "<\(weakSelf.name)>"
        }
    }

注意：当中另外一种和第三种方法使用比較常见。

class TestView: UIView {


    private weak var textField1: UITextField!
    
    var testClosure: ((str1: String) -> Void)?
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        var size = UIScreen.mainScreen().bounds.size
        self.frame = CGRectMake(0, 0, size.width, kViewHeight)
        self.backgroundColor = UIColor.grayColor()
        
        var textField1 = UITextField(frame: CGRectMake(10, 30, 150, 30))
        textField1.backgroundColor = UIColor.whiteColor()
        self.addSubview(textField1)
        self.textField1 = textField1
        
        var btn = UIButton(frame: CGRectMake(300, 30, 50, 30))
        btn.setTitle("GO", forState: UIControlState.Normal)
        btn.addTarget(self, action: "showResult", forControlEvents: UIControlEvents.TouchDown)
        self.addSubview(btn)
    }
    
    func showResult() {
        self.testClosure?
(str1: self.textField1.text)
    }
    
    required init(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

var testClosure: ((str1: String) -> Void)?

self.testClosure?(str1: self.textField1.text)

class ViewController: UIViewController {
    
    @IBOutlet weak var testLabel: UILabel!


    override func viewDidLoad() {
        super.viewDidLoad()
        
        var tab = TestView()
        tab.testClosure = {
            [unowned self] // 去除循环引用
            (str1: String) -> Void in
            self.testLabel.text = "First Record:\(str1)"
        }
        self.view.addSubview(tab)
    }


    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}