guard let的意思與if let都是針對(duì)于可選類型進(jìn)行解包，使用guard let可以是代碼更加清晰易讀

let dictionary: Dictionary<String, Any>? = ["data": ["book" : ["id" : "1"]]]
if let dict = dictionary {
    if let data = dict["data"] as? Dictionary<String, Any> {
        if let book = data["book"] as? Dictionary<String, Any> {
            if let id = book["id"] as? String {
                print(id)
            }
        }
    }
}

使用guard let與if的條件是一樣的，只是需要加上else return，這樣使用便不會(huì)嵌套很多if層，看起來(lái)更加直觀。

let dictionary: Dictionary<String, Any>? = ["data": ["book" : ["id" : "1"]]]
guard let dict = dictionary else { return }
guard let data = dict["data"] as? Dictionary<String, Any> else { return }
guard let book = data["book"] as? Dictionary<String, Any> else { return }
guard let id = book["id"] as?String else { return }
print(id)