当通过参数传递泛型类型时，Swift无法推断泛型类型

时间：2018-12-12 16:03:58

标签： swift generics core-data protocols associated-types

我正在为核心数据编写通用包装类。

这是我的一些基本类型。没什么特别的。

import datetime

def date_time_to_date(input_date_time):
   new_date = datetime.datetime.strptime(input_date_time, "%d-%M-%Y").date()
   return str(new_date.month) + "/"+ str(new_date.year)

udf_date_time_to_date = udf(date_time_to_date, StringType())
data.toDF(['id' , 'date']).withColumn('new_date', udf_date_time_to_date('date'))

我已经将我的coredata写入了协议中。如果您让我知道您对我要提出的抽象的意见，我将不胜感激。 但是在扩展程序中，我遇到了以下错误：

  

无法将类型“ NSFetchRequest”的值转换为   预期参数类型'NSFetchRequest <_>'

不确定我该如何解决。我尝试过各种更改代码的尝试，但没有成功... 

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)
}

另外，如果我更改行：

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)
        }
    }
}

至：

let fetchRequest : NSFetchRequest = storableClass.fetchRequest()

我收到以下错误：

  

使用未声明类型'storableClass'`

我的直觉告诉我，编译器无法映射“类型的参数”，即它不了解let fetchRequest : NSFetchRequest<storableClass> = storableClass.fetchRequest() 实际上是类型。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并将其编写为：

storableClass

但是下面的内容不是通用的，因为我使用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 } 进行了投射。基本上，这不是Vadian所说的Swifty：

as? ManagedObject

有什么办法解决吗？

2 个答案:

答案 0 :(得分：7)

我的建议有些不同。它使用静态方法

在loadFromDB子类上调用fetch和NSManagedObject。这样做的好处是，始终返回关联的类型，而无需进行任何其他类型的强制转换。

另一个变化是throw个错误。由于Core Data API广泛依赖于throw处理错误，因此我建议删除CoreDataResult<Value>。所有错误均已通过。成功返回对象，失败返回错误。

我省略了与id相关的代码和update方法。您可以添加一个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
    }
}

答案 1 :(得分：4)

问题在于NSManagedObject.fetchRequest()的返回类型为NSFetchRequest<NSFetchRequestResult>，这是非通用的。您需要更新fetch函数的定义以解决此问题。顺便说一句，协议扩展中默认实现的功能签名实际上与协议定义中的功能签名不匹配，因此也需要对其进行更新。

您还需要更改fetch(request:,from:)的实现，因为NSManagedObjectContext.fetch()返回类型为[Any]的值，因此您需要将其强制转换为[ManagedObject]以匹配类型您自己的fetch方法的签名。

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)
        }
    }
}
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