Swift 在静态方法中初始化 Self

Question

在 User.provideInstance 中无法让它工作时出现问题，我无法初始化 Self 并返回。有什么想法吗？

extension NSManagedObject {
    public convenience init(managedObjectContext: NSManagedObjectContext) {
        let entity = NSEntityDescription.entityForName(String(self.dynamicType), inManagedObjectContext: managedObjectContext)!
        self.init(entity: entity, insertIntoManagedObjectContext: managedObjectContext)
    }
}

public protocol Deserializable {
    static func provideInstance(json: [NSObject: AnyObject]) -> Self
}

@objc(User) public class User: NSManagedObject, Deserializable {

    public static func provideInstance(json: [NSObject: AnyObject]) -> Self {
       let context = DIContainer.instance.resolve(CoreDataStack.self).managedObjectContext
       let instance = self.init(managedObjectContext: context)
       return instance
    }
}

错误发生在let instance = self.init(managedObjectContext: context):

Constructing an object of class type Self with a metatype value must use a required initializer

Answer 1

为什么要为初始化器烦恼？

extension NSManagedObject {
    class func provide(managedObjectContext managedObjectContext: NSManagedObjectContext) -> Self {
        let entity = NSEntityDescription.entityForName(String(self.dynamicType), inManagedObjectContext: managedObjectContext)!
        return self.init(entity: entity, insertIntoManagedObjectContext: managedObjectContext)
    }
}

public protocol Deserializable {
    static func provideInstance(json: [NSObject: AnyObject]) -> Self
}

@objc(User) public class User: NSManagedObject, Deserializable {

    public static func provideInstance(json: [NSObject: AnyObject]) -> Self {
        let context = ...
        let instance = self.provide(managedObjectContext: context)
        return instance
    }
}

初始化器的问题在于它们并不总是被继承，除非它们是必需的。并且您不能在扩展中创建 required 初始值设定项。