Swift 在通过参数传递泛型类型时无法推断泛型类型

Question

我正在为核心数据编写通用包装器 class。

这是我的一些基本类型。没什么特别的。

typealias CoreDataSuccessLoad = (_: NSManagedObject) -> Void
typealias CoreDataFailureLoad = (_: CoreDataResponseError?) -> Void
typealias ID = String


enum CoreDataResult<Value> {
    case success(Value)
    case failure(Error)
}

enum CoreDataResponseError : Error {
    typealias Minute = Int
    typealias Key = String
    case idDoesNotExist
    case keyDoesNotExist(key: Key)
    case fetch(entityName: String)
}

我已经在协议中抽象了我的核心数据写入。如果您让我知道您对我试图实现的抽象的评论，我将不胜感激。然而在扩展中我运行进入以下错误：

Cannot convert value of type 'NSFetchRequest' to expected argument type 'NSFetchRequest<_>'

不确定我该如何修复它。我尝试了多种更改代码的方法，但没有成功...

protocol CoreDataWriteManagerProtocol {
    associatedtype ManagedObject : NSManagedObject

    var persistentContainer : NSPersistentContainer {get}
    var idName : String {get}
    func loadFromDB(storableClass : ManagedObject.Type, id: ID) throws -> CoreDataResult<ManagedObject>
    func update(storableClass : ManagedObject.Type, id: ID, fields: [String : Any]) throws
    func fetch(request: NSFetchRequest<ManagedObject>, from context: NSManagedObjectContext)
    init(persistentContainer : NSPersistentContainer)
}

extension CoreDataWriteManagerProtocol {
    private func loadFromDB(storableClass : ManagedObject.Type, id: ID) -> CoreDataResult<ManagedObject>{
        let predicate = NSPredicate(format: "%@ == %@", idName, id)

        let fetchRequest : NSFetchRequest = storableClass.fetchRequest()
        fetchRequest.predicate = predicate

        // ERROR at below line!
        return fetch(request: fetchRequest, from: persistentContainer.viewContext) 
    }

    func fetch<ManagedObject: NSManagedObject>(request: NSFetchRequest<ManagedObject>, from context: NSManagedObjectContext) -> CoreDataResult<ManagedObject>{
        guard let results = try? context.fetch(request) else {
            return .failure(CoreDataResponseError.fetch(entityName: request.entityName ?? "Empty Entity Name")) // @TODO not sure if entityName gets passed or not.
        }
        if let result = results.first {
            return .success(result)
        }else{
            return .failure(CoreDataResponseError.idDoesNotExist)
        }
    }
}

此外，如果我更改行：

let fetchRequest : NSFetchRequest = storableClass.fetchRequest()

至：

let fetchRequest : NSFetchRequest<storableClass> = storableClass.fetchRequest()

我收到以下错误：

Use of undeclared type 'storableClass'`

我的直觉告诉我编译器无法映射 'parameters that are types' 即它不理解 storableClass 实际上是一个类型。相反，它只能映射泛型参数或实际类型。因此，这是行不通的。

编辑：

我使用静态方法 Vadian 并写了这个：

private func create(_ entityName: String, json : [String : Any]) throws -> ManagedObject {

    guard let entityDescription = NSEntityDescription.entity(forEntityName: entityName, in: Self.persistentContainer.viewContext) else {
        print("entityName: \(entityName) doesn't exist!")
        throw CoreDataError.entityNotDeclared(name: entityName)
    }

    let _ = entityDescription.relationships(forDestination: NSEntityDescription.entity(forEntityName: "CountryEntity", in: Self.persistentContainer.viewContext)!)
    let relationshipsByName = entityDescription.relationshipsByName

    let propertiesByName = entityDescription.propertiesByName

    guard let managedObj = NSEntityDescription.insertNewObject(forEntityName: entityName, into: Self.persistentContainer.viewContext) as? ManagedObject else {
        throw CoreDataError.entityNotDeclared(name: entityName)
    }

    for (propertyName,_) in propertiesByName {
        if let value = json[propertyName] {
            managedObj.setValue(value, forKey: propertyName)
        }
    }
    // set all the relationships
    guard !relationshipsByName.isEmpty else {
        return managedObj
    }

    for (relationshipName, _ ) in relationshipsByName {
        if let object = json[relationshipName], let objectDict = object as? [String : Any] {
            let entity = try create(relationshipName, json: objectDict)
            managedObj.setValue(entity, forKey: relationshipName)
        }
    }
    return managedObj
}

但下面的部分并不通用，因为我用 as? ManagedObject 铸造了它。基本上它不像 Vadian 所说的那样 Swifty：

guard let managedObj = NSEntityDescription.insertNewObject(forEntityName: entityName, into: Self.persistentContainer.viewContext) as? ManagedObject else {
    throw CoreDataError.entityNotDeclared(name: entityName)
}

有什么解决办法吗？

Answer 1

问题是 NSManagedObject.fetchRequest() 有一个 return 类型的 NSFetchRequest<NSFetchRequestResult>，它是非泛型的。您需要更新 fetch 函数的定义来解决这个问题。顺便说一句，协议扩展中默认实现的函数签名实际上与协议定义中的函数签名不匹配，所以这些也需要更新。

您还需要更改 fetch(request:,from:) 的实现，因为 NSManagedObjectContext.fetch() return 是 [Any] 类型的值，所以您需要将其转换为 [ManagedObject] 以匹配您自己的 fetch 方法的类型签名。

protocol CoreDataWriteManagerProtocol {
    associatedtype ManagedObject : NSManagedObject

    var persistentContainer : NSPersistentContainer {get}
    var idName : String {get}
    func loadFromDB(storableClass : ManagedObject.Type, id: ID) throws -> CoreDataResult<ManagedObject>
    func update(storableClass : ManagedObject.Type, id: ID, fields: [String : Any]) throws
    func fetch(request: NSFetchRequest<NSFetchRequestResult>, from: NSManagedObjectContext) -> CoreDataResult<ManagedObject>
    init(persistentContainer : NSPersistentContainer)
}

extension CoreDataWriteManagerProtocol {
    private func loadFromDB(storableClass : ManagedObject.Type, id: ID) -> CoreDataResult<ManagedObject>{
        let predicate = NSPredicate(format: "%@ == %@", idName, id)

        let fetchRequest = storableClass.fetchRequest()
        fetchRequest.predicate = predicate

        return fetch(request: fetchRequest, from: persistentContainer.viewContext)
    }

    func fetch(request: NSFetchRequest<NSFetchRequestResult>, from context: NSManagedObjectContext) -> CoreDataResult<ManagedObject> {
        guard let results = (try? context.fetch(request)) as? [ManagedObject] else {
            return .failure(CoreDataResponseError.fetch(entityName: request.entityName ?? "Empty Entity Name")) // @TODO not sure if entityName gets passed or not.
        }
        if let result = results.first {
            return .success(result)
        }else{
            return .failure(CoreDataResponseError.idDoesNotExist)
        }
    }
}

Answer 2

我的建议有点不同。它使用静态方法

在 NSManagedObject 子类上调用 loadFromDB 和 fetch。好处是总是返回关联的类型而无需任何进一步的类型转换。

另一个变化是 throwing 错误。由于核心数据 API 广泛依赖 throwing 错误，我的建议是删除 CoreDataResult<Value>。所有错误都通过。成功时返回对象，失败时抛出错误。

我遗漏了id相关代码和update方法。您可以添加 static func predicate(for id : ID)

protocol CoreDataWriteManagerProtocol {
    associatedtype ManagedObject : NSManagedObject = Self

    static var persistentContainer : NSPersistentContainer { get }
    static var entityName : String { get }
    static func loadFromDB(predicate: NSPredicate?) throws -> ManagedObject
    static func fetch(request: NSFetchRequest<ManagedObject>) throws -> ManagedObject
    static func insertNewObject() -> ManagedObject
}

extension CoreDataWriteManagerProtocol where Self : NSManagedObject {

    static var persistentContainer : NSPersistentContainer {
        return (UIApplication.delegate as! AppDelegate).persistentContainer
    }

    static var entityName : String {
        return String(describing:self)
    }

    static func loadFromDB(predicate: NSPredicate?) throws -> ManagedObject {
        let request = NSFetchRequest<ManagedObject>(entityName: entityName)
        request.predicate = predicate
        return try fetch(request: request)
    }

    static func fetch(request: NSFetchRequest<ManagedObject>) throws -> ManagedObject {
        guard let results = try? persistentContainer.viewContext.fetch(request) else {
            throw CoreDataResponseError.fetch(entityName: entityName)
        }
        if let result = results.first {
            return result
        } else {
            throw CoreDataResponseError.idDoesNotExist
        }
    }

    static func insertNewObject() -> ManagedObject {
        return NSEntityDescription.insertNewObject(forEntityName: entityName, into: persistentContainer.viewContext) as! ManagedObject
    }
}

Swift 在通过参数传递泛型类型时无法推断泛型类型

Swift can't infer generic type when generic type is being passed through a parameter

generics

protocols

core-data

associated-types

swift