Swift之where一般使用场景

//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)
    }
}

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

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)
    }
}

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