How to write an extension for NSFetchedResultsController in Swift 4

Question

I\'m trying to write a simple extension to the NSFetchedResultsController class in Swift 4.

Here\'s my first attempt - which worked in Swift 3:

Answer 1

Subclass-ing is not good, composition is a better way to go. Here is my take on this:

public protocol FetchedDataProtocol {
    associatedtype T: NSFetchRequestResult
    var controller: NSFetchedResultsController { get }
    subscript(_ indexPath: IndexPath) -> T { get }
}

Doing it this way you avoid casting and mapping:

public extension FetchedDataProtocol {
    subscript(_ indexPath: IndexPath) -> T {
        return controller.object(at: indexPath)
    }
}

And then you use it as your own class that has subscript (and whatever you want):

public struct MainFetchedData: FetchedDataProtocol {
    public let controller: NSFetchedResultsController
    public init(_ controller: NSFetchedResultsController) {
        self.controller = controller
    }
}