swift - 类型安全 swift NSEntityDescriptor insertNewObjectForEntityForName(_ :inManagedObjectContext:)

Question

NSEntityDescriptor的insertNewObjectForEntityForName(_:inManagedObjectContext:)返回AnyObject 。如果我将我的实体命名为与它们的 NSManagedObject 相同的名称子类，有没有办法在 NSManagedObject 上创建通用扩展方法返回正确的类型，这样消费者就不需要向下转换？

Answer 1

public extension NSManagedObject {
    public class func entityName() -> String {
        let moduledName = self.description()
        if let firstDotRange = moduledName.rangeOfString(".") {
            return moduledName.substringFromIndex(firstDotRange.endIndex)
        } else {
            return moduledName
        }
    }

    public class func insert(#managedObjectContext: NSManagedObjectContext) -> Self {
        return _insert(managedObjectContext: managedObjectContext)
    }

    private class func _insert<SelfType>(#managedObjectContext: NSManagedObjectContext) -> SelfType {
        return NSEntityDescription.insertNewObjectForEntityForName(entityName(),
            inManagedObjectContext: managedObjectContext) as SelfType
    }
}

理想情况下，我们只需编写insert(managementObjectContext:)，如下所示:

public class func insert(#managedObjectContext: NSManagedObjectContext) -> Self {
    return NSEntityDescription.insertNewObjectForEntityForName(entityName(),
        inManagedObjectContext: managedObjectContext) as Self
}

但这失败了:

'Self' is only available in a protocol or as the result of a class method; did you mean 'NSManagedObject'?
            inManagedObjectContext: managedObjectContext) as Self
                                                             ^~~~
                                                             NSManagedObject

因此，我们只需将其保留为原始 insert(managementObjectContext:) 方法的结果，并让 swift 的泛型类型推断使 SelfType 成为适当的类型。