guard.swift

// MARK: guard

typealias JSON = [String: Any?]

struct Account {
    let udid: String
    let token: String
}

extension Account {
    
    init?(json: JSON) {
        guard let udid = json["udid"] as? String,
            let token = json["token"] as? String else {
            return nil
        }
        
        self.udid = udid
        self.token = token
    }
    
}

let json: JSON = ["udid": "1234-adfsdfg-12312f-asfsa", "token": nil]
let account = Account(json: json)
/*
 That example demostrate how to work 'guard' construction.
 'guard' has inverse logic from if statement. store variables in context to use it after condition.
 Be attentive with guard. It's require more time for compile step.
 */