Swift - guard关键字(守护)

guard expression else { 
    //语句
    //必须包含一个控制语句：return，break，continue或throw。
}

• 这里，expression是一个布尔表达式（返回true或者false）。
• 如果对表达式求值false，guard则执行代码块内的语句。
• 如果对表达式求值true，guard则从执行中跳过代码块内的语句

func someFunction() {

    guard false else {
        print("Condition not met")
        return
    }
    print("Condition met")
}

someFunction()
print("Hello after function call")

func changeOptionalStringToUpperCase() {

    var name:String?
    guard let temp = name else {
        print("Name is nil. Cannot process")
        return
    }
    print("Uppercased:\(temp.uppercased())")
}

changeOptionalStringToUpperCase()

func changeOptionalStringToUpperCase() {
	var name:String? = ""
	guard let temp = name , temp.count > 0  else {
		print("Name is nil or an empty string. Cannot process")
		return
	}
	print("Uppercased:\(temp.uppercased())")
}

changeOptionalStringToUpperCase()

1 //guard----控制语句执行 即守护
 2  2 let oAge: Int? = 20
 3  3 //在swift3.0中，if-let和guard语句中的where都用“,”替代了
 4  4 if let age = oAge , age > 18 {
 5  5     print(age)
 6  6 }
 7  7 func guardTest(){
 8  8     let bAge:Int? = 30
 9  9     guard let age = bAge , age > 18 else {
10 10         print("年龄小于18岁")
11 11         return
12 12     }
13 13     print(age)
14 14 }
15 15 var aa: Int?? = 20
16 16 guardTest()