import UIKit


protocol EncodingProtocol {
    mutating func decodeFromStream()
}

class ViewController: UIViewController {

override func viewDidLoad() {

    super.viewDidLoad()

    let ip: EventIpRes = EventIpRes()
    ip.decodeFromStream()

    print("111 \(ip.m_ip)")

    }
}

struct IpInfo : EncodingProtocol { 

    var ip: Int = 0

    mutating func decodeFromStream() {
        ip = 110110110
    }
}

class EventIpRes : EncodingProtocol{// #​2. not impelement EncodingProtocol, everything is ok.

    var m_ip = IpInfo()

    func decodeFromStream(){
        let testIp = ProtoBufSerializer.decodeStruct(event: m_ip) as! IpInfo
        print("222 \(testIp)") // #​0. Problem: testIp is default value..

        m_ip = testIp
    }
}

class ProtoBufSerializer {

    class func decodeStruct(event: EncodingProtocol)-> EncodingProtocol{
        // print("33 \(event)")// #​1. uncomment this line, everything is ok.

        var new = event
        new.decodeFromStream()
        return new
    }

}