做过swift开发的肯定要用到闭包，swift中的闭包就相当于OC里面的block都是用于回调。

举个简单的例子，返回两个参数的和：

第一种 常规做法

        //返回两个数的和
        func sum(numberOne number1:Int,numberTwo number2:Int)->Int
        {
            return (number1 + number2)
        }
        
        let sumTwoNumber = sum(numberOne: 1,numberTwo: 2)

第二种 使用闭包

        var myCloure0:((Int,Int)->Int)?            //声明一个闭包函数
        typealias MyCloureType = (Int,Int) -> Int  //类型取一个别名
        var myCloure:MyCloureType?
        
       //参数列表和真正的函数体之间使用关键字in来分割
        myCloure0 = {
            (num1:Int,num2:Int) -> Int in
            return num1+num2
        }
        myCloure = {
            (num1:Int,num2:Int) -> Int in
            return num1+num2
        }
        //执行闭包函数
        let sumInt:Int =  myCloure0!(10,10)
        let sumInt1:Int =  myCloure!(20,20)
        print("sumInt = \(sumInt) sumInt1 = \(sumInt1)")

sumInt = 20 sumInt1 = 40

先定义两个控制器 FirstViewController SecondViewController

class FirstViewController: UIViewController {

    
    var fisrtBtn:UIButton = {
        let button:UIButton = UIButton()
        button.backgroundColor = UIColor.redColor()
        button.setTitle("Go SecondViewController",forState: .Normal)
        button.layer.masksToBounds = true
        return button
    }()
    
    var secondLabel:UILabel = {
        let label:UILabel = UILabel()
        label.backgroundColor = UIColor.greenColor()
        label.text = "显示Second中输入的值"
        label.font = UIFont.systemFontOfSize(13.0)
        return label
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()

        fisrtBtn.frame = CGRectMake(50,50,200,40)
        secondLabel.frame = CGRectMake(50,100,40)
        fisrtBtn.addTarget(self,action:#selector(FirstViewController.fisrtBtnClick(_:)),forControlEvents: .TouchUpInside)
        self.view.addSubview(fisrtBtn)
        self.view.addSubview(secondLabel)
    }

    func fisrtBtnClick(sender:UIButton)
    {
        let secondVc = SecondViewController()
        //实现回调，接收回调过来的值
        secondVc.setBackMyClosure { (inputText:String) in
            self.secondLabel.text = inputText
        }
        
    }
}

typealias InputClosureType = (String)->Void   //闭包类型的函数 参数为string 返回值void

class SecondViewController: UIViewController {

    var backBtn:UIButton = {
        let button:UIButton = UIButton()
        button.backgroundColor = UIColor.redColor()
        button.setTitle("back",forState: .Normal)
        button.layer.masksToBounds = true
        return button
    }()
    var backClosure:InputClosureType?        //声明闭包函数
    override func viewDidLoad() {
        super.viewDidLoad()
        backBtn.frame = CGRectMake(100,100)
        backBtn.addTarget(self,action: #selector(SecondViewController.tapBackButton(_:)),forControlEvents:.TouchUpOutside)
        self.view.addSubview(backBtn)
    }
    //闭包变量赋值
    func setBackMyClosure(tempClosure:InputClosureType)
    {
        backClosure = tempClosure
    }
    
    func tapBackButton(sender:UIButton)
    {
        if(backClosure != nil)
        {
            let tempString:String? = "peng xun xun"
            if(tempString != nil)
            {
                backClosure!(tempString!)  //执行闭包
            }
        }
        self.navigationController?.popViewControllerAnimated(true)
    }

}