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

We ended up with subclass (Swift 4). Idea behind - pass type as argument. Rest of setup operates NSManagedObject type in order to work around compile errors.

public class FetchedResultsController: NSFetchedResultsController {

   public convenience init(fetchRequest: NSFetchRequest, context: NSManagedObjectContext, _: T.Type) {
      self.init(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
   }

   public var numberOfSections: Int {
      return sections?.count ?? 0
   }

   public var numberOfFetchedEntities: Int {
      return fetchedObjects?.count ?? 0
   }

   public func entity(at indexPath: IndexPath) -> T {
      if let value = object(at: indexPath) as? T {
         return value
      } else {
         fatalError()
      }
   }

   public func entity(at row: Int) -> T {
      let ip = IndexPath(item: row, section: 0)
      return entity(at: ip)
   }

   public var fetchedEntities: [T] {
      let result = (fetchedObjects ?? []).compactMap { $0 as? T }
      return result
   }
}

Extensions:

extension Requests {

   public static func all(_: T.Type) -> NSFetchRequest {
      let request = NSFetchRequest(entityName: T.entityName)
      return request
   }

   public static func ordered(by: String, ascending: Bool, _: T.Type) -> NSFetchRequest {
      let request = NSFetchRequest(entityName: T.entityName)
      request.sortDescriptors = [NSSortDescriptor(key: by, ascending: ascending)]
      return request
   }

}


extension NSManagedObject {

    public static var entityName: String {
        let className = NSStringFromClass(self) // As alternative can be used `self.description()` or `String(describing: self)`
        let entityName = className.components(separatedBy: ".").last!
        return entityName
    }

    public static var entityClassName: String {
        let className = NSStringFromClass(self)
        return className
    }

}

Usage:

extension ParentChildRelationshipsMainController {

   private func setupFetchedResultsController() -> FetchedResultsController {
      let request = DB.Requests.ordered(by: #keyPath(IssueList.title), ascending: true, IssueList.self)
      let result = FetchedResultsController(fetchRequest: request, context: PCDBStack.shared.mainContext, IssueList.self)
      return result
   }
}

extension ParentChildRelationshipsMainController: NSTableViewDataSource, NSTableViewDelegate {

   func numberOfRows(in tableView: NSTableView) -> Int {
      return frController.numberOfFetchedEntities
   }

   func tableView(_ tableView: NSTableView, viewFor tableColumn: NSTableColumn?, row: Int) -> NSView? {
      let rowID = NSUserInterfaceItemIdentifier("rid:generic")
      let item = frController.entity(at: row)
      // ...
   }
}