CoreData 和 Codable 类编译器错误：从初始值设定项返回之前，不会在所有路径上调用 'self.init'

Question

在以下说明之后：https：//stackoverflow.com/a/46917019/6047611

我不允许遇到编译器错误'self.init' isn't called on all paths before returning from initializer. super.init()。但是，调用self.init(entity: entity, insertInto: context)，然后以某种方式初始化类中的所有属性，并未完全初始化类。

我迷路了。我对Coredata非常陌生，并且对此不满意，所以我希望这是我无知的问题。关于如何解决此错误的任何想法？

import Foundation
import CoreData
class Product: NSManagedObject, Encodable, Decodable
{
    @NSManaged var mongoID:[String:String]
    @NSManaged var title:String
    @NSManaged var productID:Int
    @NSManaged var mpn:String
    @NSManaged var listPrice:Float
    @NSManaged var price:Float
    @NSManaged var uom:String
    @NSManaged var uomQty:Int
    @NSManaged var inventory:Float
    @NSManaged var minSaleQty:Int
    @NSManaged var desc:String
    @NSManaged var categories:[String]
    @NSManaged var imageURL:String
    @NSManaged var upc:String
    @NSManaged var quantity:Int
    @NSManaged var disc:Bool
    enum CodingKeys: String, CodingKey {
        case mongoID = "mongoID"
        case title = "title"
        case productID = "productID"
        case mpn = "mpn"
        case listPrice = "listPrice"
        case price = "price"
        case uom = "uom"
        case uomQty = "uomQty"
        case inventory = "inventory"
        case minSaleQty = "minSaleQty"
        case desc = "desc"
        case categories = "categories"
        case imageURL = "imageURL"
        case upc = "upc"
        case quantity = "quantity"
        case disc = "disc"
    }
    required convenience init(from decoder:Decoder) throws
    {
        guard let context = decoder.userInfo[CodingUserInfoKey.context!] as? NSManagedObjectContext else { print("failed context get"); return }
        guard let entity = NSEntityDescription.entity(forEntityName: "Product", in: context) else { print("failed entity init"); return }

        self.init(entity: entity, insertInto: context)
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.mongoID = try container.decodeIfPresent([String:String].self, forKey: .mongoID) ?? ["$id":"nil"]
        self.title = try container.decodeIfPresent(String.self, forKey: .title) ?? ""
        self.productID = try container.decodeIfPresent(Int.self, forKey: .productID) ?? 0
        self.mpn = try container.decodeIfPresent(String.self, forKey: .mpn) ?? ""
        self.listPrice = try container.decodeIfPresent(Float.self, forKey: .listPrice) ?? 0.0
        self.price = try container.decodeIfPresent(Float.self, forKey: .price) ?? 0.0
        self.uom = try container.decodeIfPresent(String.self, forKey: .uom) ?? ""
        self.uomQty = try container.decodeIfPresent(Int.self, forKey: .uomQty) ?? 0
        self.inventory = try container.decodeIfPresent(Float.self, forKey: .inventory) ?? 0.0
        self.minSaleQty = try container.decodeIfPresent(Int.self, forKey: .minSaleQty) ?? 0
        self.desc = try container.decodeIfPresent(String.self, forKey: .desc) ?? ""
        self.categories = try container.decodeIfPresent([String].self, forKey: .categories) ?? [""]
        self.imageURL = try container.decodeIfPresent(String.self, forKey: .imageURL) ?? ""
        self.upc = try container.decodeIfPresent(String.self, forKey: .upc) ?? ""
        self.quantity = try container.decodeIfPresent(Int.self, forKey: .quantity) ?? 0
        self.disc = try container.decodeIfPresent(Bool.self, forKey: .disc) ?? false
    }//'self.init' isn't called on all paths before returning from initializer
    public func encode(to encoder: Encoder) throws
    {
    }
}

extension CodingUserInfoKey {
    static let context = CodingUserInfoKey(rawValue: "context")
}

Answer 1

此编译器错误与核心数据无关。它是由两个guard语句引起的，该语句在调用self.init之前可以return。

在下面的声明中，如果context为nil，则else条件将打印"失败上下文获取"，然后return：

guard let context = decoder.userInfo[CodingUserInfoKey.context!] as? NSManagedObjectContext 
else { print("failed context get"); return }

您正在尝试在调用self.init之前返回。这是不允许的。您的便利性初始化器必须返回正确初始化的对象。

但是，如果无法满足guard语句，您可以出路：您可以throw例外。然后，以任何方式处理例外是有意义的。

要这样做，您需要创建一个符合Error协议的enum，例如：

enum ProductError: Error {
    case contextMissing
    case entityCreationFailed
}

然后，您可以重写这样的guard语句：

guard let context = decoder.userInfo[CodingUserInfoKey.context!] as? NSManagedObjectContext 
else { print("failed context get"); throw ProductError.contextMissing }

创建Product时，您可以执行此操作：

let product = try? Product(from: decoder)  
//product is an optional, might be nil

或以下：

if let product = try? Product(from: decoder) {
    //product is not an optional, cannot be nil
}