如果我在 swift 的結構中使用協議類型，我的結構不符合協議“可解碼”/“可編碼”

Question

在這里，我試圖從 json 文件中讀取數據，並動態轉換它。 但是，如果我在結構中使用原型，則表明我does not conform to protocol 'Decodable' / 'Encodable'錯誤。 如果我在這里遺漏了什么，請告訴我。

struct ScreenData: Codable {
    var id: String
    var objectid : String
    var config : UIConfig
}

protocol UIConfig: class, Codable{
    var bgColor : String? { get set }
}

class LabelConfig : UIConfig {
    var bgColor: String?
    var label : String? = ""
}

class ButtonConfig : UIConfig {
    var bgColor: String?
    var btn_label : String = ""
    //var btn_text_color : UIColor = .black
}

在這里，我正在從 json 文件中讀取數據，並根據數據在堆棧視圖中添加組件

class ViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
//        create stack view to add components
        let stackView = UIStackView()
        stackView.axis = NSLayoutConstraint.Axis.vertical
        stackView.distribution = .fill
        stackView.alignment = .fill
        stackView.spacing = 10
        stackView.backgroundColor = .gray
        
        var screenData = [ScreenData]()
//        read components from json
        screenData =  loadScreen()
        //print("viewDidLoad screenData : \(screenData)")
        for data in screenData {
            let subView = loadScreenView(data: data, objectId: data.objectid)
            //add components in stack view
            stackView.addArrangedSubview(subView)
        }
        
        self.view.addSubview(stackView)
        stackView.translatesAutoresizingMaskIntoConstraints = false
        stackView.leadingAnchor.constraint(equalTo: self.view.leadingAnchor, constant: 10).isActive = true
        stackView.trailingAnchor.constraint(equalTo: self.view.trailingAnchor, constant: 10).isActive = true
        
        stackView.centerYAnchor.constraint(equalTo: self.view.centerYAnchor).isActive = true
        
        stackView.heightAnchor.constraint(equalToConstant: 200).isActive = true
    }
    
// function to laod data from json
    func loadScreen() -> [ScreenData] {
        var jsonData = [ScreenData]()
        if let fileLocation = Bundle.main.url(forResource: "screen_data", withExtension: "json"){
            do{
                let data = try Data(contentsOf: fileLocation)
                let jsonDecoder = JSONDecoder()
                let dataFromJson =  try jsonDecoder.decode([ScreenData].self, from: data)
                jsonData = dataFromJson
            }catch{
                print(error)
            }
        }
        //print("loadScreen screenData :: \(jsonData)")
        return jsonData
    }


Here I check the object type, and depending on that cast the config

    func loadScreenView(data : ScreenData,objectId : String) -> UIView {
        var view = UIView()
        if(objectId == "bd_label"){
            print("bd_label")
            let labelView = UILabel()
            //labelView.sizeToFit()
            let config = data.config as! LabelConfig
            labelView.text = config.label
            labelView.widthAnchor.constraint(equalToConstant: 300).isActive = true
            labelView.heightAnchor.constraint(equalToConstant: 35).isActive = true
            view = labelView
        }
        if(objectId.elementsEqual("bd_button")){
            print("bd_button")
            let buttonView = UIButton()
            let config = data.config as! ButtonConfig
            
            buttonView.setTitle(config.btn_label, for:.normal)
            buttonView.backgroundColor = .blue
            buttonView.widthAnchor.constraint(equalToConstant: 200).isActive = true
            buttonView.heightAnchor.constraint(equalToConstant: 35).isActive = true
            view = buttonView
        }
        if(objectId == "bd_input"){
            print("bd_input")
            let inputView = UITextView()
            let config = data.config as! InputConfig
            
            inputView.text = config.placeholder
            inputView.backgroundColor = .white
            inputView.widthAnchor.constraint(equalToConstant: 300).isActive = true
            inputView.heightAnchor.constraint(equalToConstant: 35).isActive = true
            view = inputView
        }
        
        return view
    }
    

}

Answer 1

JSONDecoder需要知道要將 JSON 解碼為的具體類型。 畢竟，一切都必須在運行時具有具體類型，您可以使用type(of:)獲得。 你不能告訴它只是“解碼協議”。 編碼器有點不同 - 它實際上不需要知道具體類型，並且有一種方法可以繞過它。

看起來UIConfig的類型取決於objectid ，所以我們可以檢查objectid並決定要解碼的UIConfig類型：

enum CodingKeys: CodingKey {
    case id, objectid, config
}

init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    id = try container.decode(String.self, forKey: .id)
    objectid = try container.decode(String.self, forKey: .objectid)
    if objectid == "bd_label" {
        config = try container.decode(LabelConfig.self, forKey: .config)
    } else if objectid == "bd_button" {
        config = try container.decode(ButtonConfig.self, forKey: .config)
    } 
    // other cases...
    else {
        throw DecodingError.dataCorruptedError(forKey: .config, in: container, debugDescription: "no suitable config type found for objectid \(objectid)!")
    } 
}

對於Encodable部分，您可以制作類似“類型橡皮擦”的東西：

struct AnyEncodable: Encodable {
    let encodeFunction: (Encoder) throws -> Void
    
    init(_ encodable: Encodable) {
        encodeFunction = encodable.encode(to:)
    }
    
    func encode(to encoder: Encoder) throws {
        try encodeFunction(encoder)
    }
}

並做：

func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    try container.encode(id, forKey: .id)
    try container.encode(objectid, forKey: .objectid)
    try container.encode(AnyEncodable(config), forKey: .config)
}

通過使用AnyEncodable ，我們基本上將協議包裝在一個具體的類型中，但不用擔心 - 這實際上不會在 JSON 中創建一對額外的花括號。