NSFetchedResultsController没有更新UI（Swift）

Question

我已经被困在这几天了，似乎无法弄明白。 我正在使用带有CoreDate的NSFetchedResultsController，并希望在更新核心数据对象时更新行的UI。在我的情况下，当用户完成练习时，状态的值更改为1.有趣的是，我可以打印出正确的值，但无法更新UI。谢谢！

class CoreDataTableViewController: UITableViewController, NSFetchedResultsControllerDelegate {

var fetchedResultsController: NSFetchedResultsController? {
    didSet {
        do {
            if let frc = fetchedResultsController {
                frc.delegate = self
                try frc.performFetch()
            }
            tableView.reloadData()
        } catch let error {
            print("NSFetchResultsController.performFetch() failed: \(error)")
        }
    }
}

// Mark: UITableViewDataSource (Part 1)

override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
    return fetchedResultsController?.sections?.count ?? 1
}

override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    if let sections = fetchedResultsController?.sections where sections.count > 0 {
        return sections[section].numberOfObjects
    } else {
        return 0
    }
}

override func tableView(tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
    if let sections = fetchedResultsController?.sections where sections.count > 0 {
        return sections[section].name
    } else {
        return nil
    }
}

override func sectionIndexTitlesForTableView(tableView: UITableView) -> [String]? {
    return fetchedResultsController?.sectionIndexTitles
}

override func tableView(tableView: UITableView, sectionForSectionIndexTitle title: String, atIndex index: Int) -> Int {
    return fetchedResultsController?.sectionForSectionIndexTitle(title, atIndex: index) ?? 0
}


override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCellWithIdentifier("ExerciseCell", forIndexPath: indexPath)

    configureCell(cell, atIndexPath: indexPath)


    return cell
}

func configureCell(cell: UITableViewCell, atIndexPath indexPath: NSIndexPath) {
    let exerciseObject = fetchedResultsController!.objectAtIndexPath(indexPath) as! CoreExerciseForPlan
    let cell = tableView.dequeueReusableCellWithIdentifier("ExerciseCell", forIndexPath: indexPath)        
    cell.detailTextLabel?.text = exerciseObject.exercise?.name
    cell.textLabel?.text = String(exerciseObject.status)
}


// Mark: NSFetchedResultsControllerDelegate (Part 2)

func controllerWillChangeContent(controller: NSFetchedResultsController) {
    tableView.beginUpdates()
}

func controller(controller: NSFetchedResultsController, didChangeSection sectionInfo: NSFetchedResultsSectionInfo, atIndex sectionIndex: Int, forChangeType type: NSFetchedResultsChangeType) {
    switch type {
    case .Insert: tableView.insertSections(NSIndexSet(index: sectionIndex), withRowAnimation: .Fade)
    case .Update: tableView.insertSections(NSIndexSet(index: sectionIndex), withRowAnimation: .Fade)
    case .Delete: tableView.deleteSections(NSIndexSet(index: sectionIndex), withRowAnimation: .Fade)
    default: break
    }
}

func controller(controller: NSFetchedResultsController, didChangeObject anObject: AnyObject, atIndexPath indexPath: NSIndexPath?, forChangeType type: NSFetchedResultsChangeType, newIndexPath: NSIndexPath?) {


    let exerciseObject = fetchedResultsController!.objectAtIndexPath(indexPath!) as! CoreExerciseForPlan

    switch type {

    case .Update:
        print("indexPath: \(indexPath!.row)")
        print("object: \(exerciseObject.exercise!.name) with status \(exerciseObject.status)")
    **// These print the correct values for status!!!!**

        configureCell(tableView.cellForRowAtIndexPath(indexPath!)!, atIndexPath: indexPath!)
    case .Insert:
        tableView.insertRowsAtIndexPaths([newIndexPath!], withRowAnimation: .Fade)
    case .Delete:
        tableView.deleteRowsAtIndexPaths([newIndexPath!], withRowAnimation: .Fade)
    case .Move:
        tableView.deleteRowsAtIndexPaths([newIndexPath!], withRowAnimation: .Fade)
        tableView.insertRowsAtIndexPaths([newIndexPath!], withRowAnimation: .Fade)
    }
  }

  func controllerDidChangeContent(controller: NSFetchedResultsController) {
    tableView.endUpdates()
  } 
}