How to initialize a struct from a json object

edit/update: Swift 4 or later you can use Codable Protocol:

struct Root: Codable {
    let user: User
    let address: Address
    let deliveryInstruction, deliveryMethod: String
}

struct Address: Codable {
    let house, street, addressId, fullAddress: String
    let town: Town
    let city: City
}

struct City: Codable {
    let cityId, cityName: String
}

struct Town: Codable {
    let townId, townName: String
}

struct User: Codable {
    let name, email, phoneNo: String
}

extension Decodable {
    init(data: Data, using decoder: JSONDecoder = .init()) throws {
        self = try decoder.decode(Self.self, from: data)
    }
    init(json: String, using decoder: JSONDecoder = .init()) throws {
        try self.init(data: Data(json.utf8), using: decoder)
    }
}

Just don't forget to set the JSONDecoder property keyDecodingStrategy to .convertFromSnakeCase:

let json = """
{"user": {"name": "crst","email": "[email protected]","phoneNo":"018833455"},"address": {"house": "100","street": "B","town":{"town_id": "1","town_name": "Galway city center"},"city":{"city_id": "10","city_name": "Galway"},"address_id":"200", "full_address":"100, B, Galway city center,Galway" },"delivery_instruction": "no call","delivery_method": "1" }
"""

do {
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    let root = try decoder.decode(Root.self, from: Data(json.utf8))
    print(root)
} catch {
    print(error)
}

or simply:

do {
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    let root = try Root(json: json, using: decoder)  // or  Root(data: data, using: decoder) 
    print(root)
} catch {
    print(error)
}

This will print

Root(user: __lldb_expr_112.User(name: "cruskuaka", email: "[email protected]", phoneNo: "018833455"), address: __lldb_expr_112.Address(house: "100", street: "B", addressId: "200", fullAddress: "100, B, Galway city center,Galway", town: __lldb_expr_112.Town(townId: "1", townName: "Galway city center"), city: __lldb_expr_112.City(cityId: "10", cityName: "Galway")), deliveryInstruction: "no call", deliveryMethod: "1")

Original Answer (before Codable protocol)

Swift 3

You have more than one error in your code, but you are in the right path. You are using the wrong key when initializing your user, city and town structs. I have also created two more initializers so you can initialize your struct with a dictionary, the json string or just its data:

struct Contact: CustomStringConvertible  {
    let user: User
    let address: Address
    let deliveryInstruction: String
    let deliveryMethod: String
    // customize the description to your needs
    var description: String { return "\(user.name) \(deliveryInstruction) \(deliveryMethod)" }
    init(dictionary: [String: Any]) {
        self.deliveryInstruction = dictionary["delivery_instruction"] as? String ?? ""
        self.deliveryMethod = dictionary["delivery_method"] as? String ?? ""
        self.address = Address(dictionary: dictionary["address"] as? [String: Any] ?? [:])
        self.user =  User(dictionary: dictionary["user"] as? [String: Any] ?? [:])
    }
    init?(data: Data) {
        guard let json = (try? JSONSerialization.jsonObject(with: data)) as? [String: Any] else { return nil }
        self.init(dictionary: json)
    }
    init?(json: String) {
        self.init(data: Data(json.utf8))
    }

}

struct User: CustomStringConvertible {
    let name: String
    let email: String
    let phone: String
    let description: String
    init(dictionary: [String: Any]) {
        self.name = dictionary["name"] as? String ?? ""
        self.email = dictionary["email"] as? String ?? ""
        self.phone = dictionary["phoneNo"] as? String ?? ""
        self.description = "name: \(name) - email: \(email) - phone: \(phone)"
    }
}

struct Address: CustomStringConvertible  {
    let id: String
    let street: String
    let house: String
    let city: City
    let town: Town
    let description: String
    init(dictionary: [String: Any] ) {
        self.id = dictionary["address_id"] as? String ?? ""
        self.description = dictionary["full_address"] as? String ?? ""
        self.house = dictionary["house"] as? String ?? ""
        self.street = dictionary["street"] as? String ?? ""
        self.city = City(dictionary: dictionary["city"] as? [String: Any] ?? [:])
        self.town = Town(dictionary: dictionary["town"] as? [String: Any] ?? [:])
    }
}

struct City: CustomStringConvertible {
    let id: String
    let name: String
    // customize the description to your needs
    var description: String { return name }
    init(dictionary: [String: Any] ) {
        self.id = dictionary["city_id"] as? String ?? ""
        self.name = dictionary["city_name"] as? String ?? ""
    }
    
}

struct Town: CustomStringConvertible {
    let id: String
    let name: String
    // customize the description to your needs
    var description: String { return name }
    init(dictionary: [String: Any]) {
        self.id = dictionary["town_id"] as? String ?? ""
        self.name = dictionary["town_name"] as? String ?? ""
    }
}

Testing the initialization from JSON:

let contact = Contact(json: json)   // crst no call 1

contact               // crst no call 1
contact?.user         // name: crst - email: [email protected] - phone: 018833455
contact?.user.name    //  "crst"
contact?.user.email   //  "[email protected]"
contact?.user.phone   //  "018833455"
contact?.address      //  100, B, Galway city center,Galway
contact?.address.id           //  200
contact?.address.street       //  B
contact?.address.town         // Galway city center
contact?.address.city         //  Galway
contact?.deliveryInstruction  // "no call"
contact?.deliveryMethod       //    1