如何在多个ViewController中使用NSFetchedResultsControllerDelegate而不重复控制器代码？

Question

I am making an app that uses Core Data, it happens that in this app I have three tableViews populated with different data and all of them are in different controllers, to avoid repeating the code (since it is exactly the same) I would like to have a solution for this.

I have tried through protocols and with a MainController that inherits from UIViewController, which in turn had the extension of NSFetchedResultsControllerDelegate. Im almost positive that doing a super class where I can put this code is the solution, but since I'm new to swift and to programming and I'm also positive that there is something that im not doing as it was supposed.

In the parent class I've made something like this:

class MainController: UIViewController, NSFetchedResultsController {

     var activeTableView: UITableView?
}

And in the child:

class SecdController: MainController {

   @IBOutlet weak var tableView: UITableView!

   override var activeTableView: UITableView? {
     get {return tableView}
     set {}
  }
}

Below is the code I want to be called three controllers.

extension SomeController: NSFetchedResultsControllerDelegate {

    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {

        switch type {

        case .insert:
            tableView.insertRows(at: [newIndexPath!], with: .top)
            break
        case .delete:
            tableView.deleteRows(at: [indexPath!], with: .fade)
            break
        case .update:
            tableView.reloadRows(at: [indexPath!], with: .top)
        case .move:
            tableView.reloadRows(at: [indexPath!], with: .top)
        default:
            fatalError("feature not yet implemented")
        }
    }

    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange sectionInfo: NSFetchedResultsSectionInfo, atSectionIndex sectionIndex: Int, for type: NSFetchedResultsChangeType) {

        let indexSet = IndexSet(integer: sectionIndex)

        switch type {

        case .insert:
            tableView.insertSections(indexSet, with: .top)
        case .delete:
            tableView.deleteSections(indexSet, with: .top)
        case .update, .move:
            fatalError("Invalid change.")
        default:
            fatalError("feature not yet implemented")
        }
    }

    func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {

        tableView.beginUpdates()
    }
    func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {

        tableView.endUpdates()
    }
}

Thanks in advance :)

Answer 1

Your MainController should handle NSFetchedResultsControllerDelegate instead of inherit NSFetchedResultsController

class MainController: UIViewController {

    @IBOutlet weak var tableView: UITableView!
    var fetchedResultsController: NSFetchedResultsController<NSManagedObject>?

override func viewDidLoad() {
        super.viewDidLoad()

        setupFetchedResultsController()

       fetchedResultsController?.delegate = self
       // perform fetch
    }

func setupFetchedResultsController() {
        preconditionFailure("The method must be overridden and initialize the fetch result controller.")
    }
}

extension MainController: NSFetchedResultsControllerDelegate {

    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {

// Do not force unwrap
        switch type {

        case .insert:
            tableView.insertRows(at: [newIndexPath!], with: .top)
            break
        case .delete:
            tableView.deleteRows(at: [indexPath!], with: .fade)
            break
        case .update:
            tableView.reloadRows(at: [indexPath!], with: .top)
        case .move:
            tableView.reloadRows(at: [indexPath!], with: .top)
        default:
            fatalError("feature not yet implemented")
        }
    }

    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange sectionInfo: NSFetchedResultsSectionInfo, atSectionIndex sectionIndex: Int, for type: NSFetchedResultsChangeType) {

        let indexSet = IndexSet(integer: sectionIndex)

        switch type {

        case .insert:
            tableView.insertSections(indexSet, with: .top)
        case .delete:
            tableView.deleteSections(indexSet, with: .top)
        case .update, .move:
            fatalError("Invalid change.")
        default:
            fatalError("feature not yet implemented")
        }
    }

    func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {

        tableView.beginUpdates()
    }
    func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {

        tableView.endUpdates()
    }
}

And then in the other view controller just override setupFetchedResultsController() and setup the fetch results controller:

class SecondController: MainController {

   override func createFetchResultController() {

        let fetchRequest: NSFetchRequest<...> = ...
        let sortDescriptor = NSSortDescriptor(key: "name", ascending: true)
        fetchRequest.sortDescriptors = [sortDescriptor]

        fetchedResultsController = ...
    }
}

Answer 2

This is a tricky one because the normal solutions of refactoring the delegates to its own class don't work because the class needs to handle both the table delegate (ie be a UITableViewController) and also be a fetch controller delegate. Thankfully there is a simple solution which is to use a technique called composition. Put the duplicated table and fetch in a UITableViewController subclass that implements NSFetchedResultsControllerDelegate and make a fetchedResultsController property (pictured on right in image below).

For the code that is different put that in several UIViewController subclasses (pictured on left in image below) for each specific case.

Use an embed segue to make this view controller contain the table controller. In the viewDidLoad create the fetch controller and set it to the table controller. eg

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    if([segue.identifier isEqualToString:@"embed"]){
        self.fetchedTableViewController = segue.destinationViewController;
    }
}

- (void)viewDidLoad {
    [super viewDidLoad];

    self.fetchedTableViewController.fetchedResultsController = self.fetchedResultsController;

    // optional for more customization
    self.fetchedTableViewController.tableView.delegate = self;
    self.fetchedTableViewController.tableView.dataSource = self; 
}

If you want to handle some specific table or fetch delegate methods then a nifty way is to set the view controller as the delegates, and then forward any of the method calls on to the embedded table controller. eg

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    [self.fetchedTableViewController controllerDidChangeContent:controller];

In that case you should also implement the forwardingTargetForSelector pattern to make sure everything else is forwarded on too.

- (id)forwardingTargetForSelector:(SEL)aSelector{
    if(MHFProtocolHasInstanceMethod(@protocol(UITableViewDelegate), aSelector)){
        if([self.fetchedTableViewController respondsToSelector:aSelector]){
            return self.fetchedTableViewController;
        }
    }
    else if(MHFProtocolHasInstanceMethod(@protocol(UITableViewDataSource), aSelector)){
        if([self.fetchedTableViewController respondsToSelector:aSelector]){
            return self.fetchedTableViewController;
        }
    }
    else if(MHFProtocolHasInstanceMethod(@protocol(NSFetchedResultsControllerDelegate), aSelector)){
        if([self.fetchedTableViewController respondsToSelector:aSelector]){
            return self.fetchedTableViewController;
        }
    }
    else if(MHFProtocolHasInstanceMethod(@protocol(UIDataSourceModelAssociation), aSelector)){
        if([self.fetchedTableViewController respondsToSelector:aSelector]){
            return self.fetchedTableViewController;
        }
    }
    return [super forwardingTargetForSelector:aSelector];
}

- (BOOL)respondsToSelector:(SEL)aSelector{
    if([super respondsToSelector:aSelector]){
        return YES;
    }
    else if(MHFProtocolHasInstanceMethod(@protocol(UITableViewDelegate), aSelector)){
        if([self.fetchedTableViewController respondsToSelector:aSelector]){
            return YES;
        }
    }
    else if(MHFProtocolHasInstanceMethod(@protocol(UITableViewDataSource), aSelector)){
        if([self.fetchedTableViewController respondsToSelector:aSelector]){
            return YES;
        }
    }
    else if(MHFProtocolHasInstanceMethod(@protocol(NSFetchedResultsControllerDelegate), aSelector)){
        if([self.fetchedTableViewController respondsToSelector:aSelector]){
            return YES;
        }
    }
    else if(MHFProtocolHasInstanceMethod(@protocol(UIDataSourceModelAssociation), aSelector)){
        if([self.fetchedTableViewController respondsToSelector:aSelector]){
            return YES;
        }
    }
    return NO;
}