Swift之where一般使用场景

使用where语句之前必须在where之前有一个主语变量名

• 协议约束

//基类A继承了SomeProtocol协议才能添加扩展 extension SomeProtocol where Self: A { func showParamA() { print(self.a) } } 

• for...in...遍历

let arr = [1, 2, 4, 5, 6] for (index, value) in arr.enumerated() where value > 4 { print(index, value) } 

• case 语句，相当于if判断

let arr = [1, 2, 4, 5, 6] for (index, value) in arr.enumerated() where value > 4 { switch value { case let a where a < 6: print(index) default: print(value) } } 

• if let 和 guard中，在swift4.0版本以后，使用逗号代替where

let str : String? = "hello" if let value = str, value.count == 5 { print(value) } guard let value = str, value.count == 5 else { return } print(value)