当泛型类型通过参数传递时,Swift无法推断泛型类型

Question

我正在为核心数据编写通用的包装类.

以下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coredata写入.如果你让我知道你对我试图完成的抽象的评论,我将不胜感激.然而在扩展中我遇到了以下错误:

无法将'NSFetchRequest'类型的值转换为预期的参数类型'NSFetchRequest <_>'

不知道我究竟如何解决它.我尝试过更改代码的变体,但没有找到成功......

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)
        }
    }
}

另外,如果我更改线路:

let fetchRequest : NSFetchRequest = storableClass.fetchRequest()

至:

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

我收到以下错误:

使用未申报的'storableClass'

我的直觉告诉我,编译器不能映射"类型的参数",即它不理解storableClass实际上是一种类型.相反,它只能映射泛型参数或实际类型.因此,这不起作用.

编辑:

我用静态方法Vadian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
}

但是下面的部分并不是通用的,因为我正在使用它as? ManagedObject.基本上它并不像Vadian所说的那样是Swifty:

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

那有什么办法吗？

Answer 1

我的建议有点不同.它使用静态方法

调用loadFromDB并fetch在NSManagedObject子类上.好处是始终返回关联的类型而不进行任何进一步的类型转换.

另一个变化是throw错误.由于Core Data API广泛依赖于throw错误,我的建议是放弃CoreDataResult<Value>.所有错误都通过.成功时返回对象,失败时会抛出错误.

我遗漏了id相关的代码和update方法.你可以添加一个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
    }
}