ios - FRC 内容更改时 UITableView 异常

Question

我有一个 UITableView，它显示来自 NSFetchedResultsController 的数据。 FRC 中显示的数据量各不相同，从一次提取约 20 条记录到最大一次提取约 14k 条记录不等。我遇到的问题是，如果我在 tableView 在大提取上滚动时执行提取，则会出现异常。在 cellForRowAtIndexPath 被调用时，FRC 已经更新并且那里没有数据，导致异常。

我找到了 this post ，这听起来像我遇到的情况，但我无法通过该方法解决它。

  func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: cellReuseID, for: indexPath) as! CustomTableViewCell
    // Exception occurs here
    let myObject = fetchedResultsController.object(at: indexPath)
    cell.myObject = myObject

    return cell
  }

这是堆栈跟踪:

0   CoreFoundation                      0x0000000110b241e6 __exceptionPreprocess + 294
1   libobjc.A.dylib                     0x000000010b820031 objc_exception_throw + 48
2   CoreData                            0x000000010c2458fd -[NSFetchedResultsController objectAtIndexPath:] + 685
3   MyApp                         0x000000010aaf36c5 _T011MyApp0B14ViewControllerC05tableC0So07UITableC4CellCSo0fC0C_10Foundation9IndexPathV12cellForRowAttF + 437
4   MyApp                         0x000000010aaf39ec _T011MyApp0B14ViewControllerC05tableC0So07UITableC4CellCSo0fC0C_10Foundation9IndexPathV12cellForRowAttFTo + 92
5   UIKit                               0x000000010cf45567 -[UITableView _createPreparedCellForGlobalRow:withIndexPath:willDisplay:] + 783
6   UIKit                               0x000000010cf45ae4 -[UITableView _createPreparedCellForGlobalRow:willDisplay:] + 74
7   UIKit                               0x000000010cf0ceaa -[UITableView _updateVisibleCellsNow:isRecursive:] + 3168
8   UIKit                               0x000000010cf2d7e0 -[UITableView layoutSubviews] + 176
9   UIKit                               0x000000010ceb77a8 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1515
10  QuartzCore                          0x000000010cc21456 -[CALayer layoutSublayers] + 177
11  QuartzCore                          0x000000010cc25667 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 395
12  QuartzCore                          0x000000010cbac0fb _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 343
13  QuartzCore                          0x000000010cbd979c _ZN2CA11Transaction6commitEv + 568
14  UIKit                               0x000000010cde22ef _UIApplicationFlushRunLoopCATransactionIfTooLate + 167
15  UIKit                               0x000000010d747662 __handleEventQueueInternal + 6875
16  CoreFoundation                      0x0000000110ac6bb1 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
17  CoreFoundation                      0x0000000110aab4af __CFRunLoopDoSources0 + 271
18  CoreFoundation                      0x0000000110aaaa6f __CFRunLoopRun + 1263
19  CoreFoundation                      0x0000000110aaa30b CFRunLoopRunSpecific + 635
20  GraphicsServices                    0x0000000115c2fa73 GSEventRunModal + 62
21  UIKit                               0x000000010cde8057 UIApplicationMain + 159
22  MyApp                         0x000000010aacd427 main + 55
23  libdyld.dylib                       0x0000000111c5b955 start + 1

我欢迎提出解决此问题的建议。在 tableview 完成减速之前，我想避免做一些像禁用新获取这样的骇人听闻的事情。感谢您的阅读。

更新

作为对评论的回应，这是我对 UIFetchedResultsControllerDelegate 的实现。这是 Apple 在 documentation 中的代码:

extension ViewController: NSFetchedResultsControllerDelegate {
  func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    tableView.beginUpdates()
  }

  func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>,
                  didChange sectionInfo: NSFetchedResultsSectionInfo,
                  atSectionIndex sectionIndex: Int,
                  for type: NSFetchedResultsChangeType) {
    switch type {
    case .insert:
      tableView.insertSections(IndexSet(integer: sectionIndex), with: .fade)
    case .delete:
      tableView.deleteSections(IndexSet(integer: sectionIndex), with: .fade)
    case .move:
      break
    case .update:
      break
    }
  }

  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: .fade)
    case .delete:
      tableView.deleteRows(at: [indexPath!], with: .fade)
    case .update:
      tableView.reloadRows(at: [indexPath!], with: .fade)
    case .move:
      tableView.moveRow(at: indexPath!, to: newIndexPath!)
    }
  }

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

新尝试

为了回应评论，我添加了我的 NSFetchedResultsControllerDelegate 实现，这是 Apple 在文档中列出的内容。我注意到有强制解包选项，所以我加入了一些 guard 语句，并关闭了动画以备不时之需。我在做同样的事情时在同一个地方遇到了同样的崩溃。

这是更新后的委托(delegate)方法，添加了保护语句并关闭了动画:

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

    guard let indexPath = indexPath else { return }

    switch type {
    case .insert:
      guard let newIndexPath = newIndexPath else { return }
      tableView.insertRows(at: [newIndexPath], with: .none)
    case .delete:
      tableView.deleteRows(at: [indexPath], with: .none)
    case .update:
      tableView.reloadRows(at: [indexPath], with: .none)
    case .move:
      guard let newIndexPath = newIndexPath else { return }
      tableView.moveRow(at: indexPath, to: newIndexPath)
    }
  }

更新 2

作为对评论的回应，这是我用来更新谓词的代码。我有一个 startDate 和 endDate 调用 updatePredicate() 的观察者。 startDate 和 endDate 在按下 segmentedControl 时更新。

  // This is used to set values for searching via segmentedControl
  @objc dynamic var startDate: Date?
  @objc dynamic var endDate: Date?

  func updatePredicate() {
    // you need a start date or stop executing
    guard let startDate = startDate else { return }

    var predicateArray: [NSPredicate] = []

    if let endDate = endDate {
      let startEndPredicate = NSPredicate(format: "time >= %@ AND time <= %@", argumentArray: [startDate, endDate])
      predicateArray.append(startEndPredicate)
    } else {
      // use the startDate's end of day if endDate is nil
      let startPredicate = NSPredicate(format: "time >= %@ AND time <= %@", argumentArray: [startDate, startDate.endOfDay!])
      predicateArray.append(startPredicate)
    }

    let sliderPredicate = NSPredicate(format: "magnitude >= %f", argumentArray: [magSlider.value])
    predicateArray.append(sliderPredicate)

    currentPredicate = NSCompoundPredicate(andPredicateWithSubpredicates: predicateArray)

    fetchResults()
  }

这里是 fetchResults():

  func fetchResults() {
    do {
      fetchedResultsController.fetchRequest.fetchBatchSize = 35
      fetchedResultsController.fetchRequest.fetchLimit = 1_000
      try fetchedResultsController.performFetch()
    } catch let error {
      print("\(error) \(error.localizedDescription)")
    }
    DispatchQueue.main.async {
      self.tableView.reloadData()
      self.tableView.setContentOffset(.zero, animated: true)
      self.updateLabelsForResults()
    }
  }

更新 3

为回应另一条评论，以下是 FRC 声明:

  private lazy var fetchedResultsController: NSFetchedResultsController<EarthquakeEntity> = {
    let managedObjectContext = appDelegate.persistentContainer.viewContext
    let fetchRequest: NSFetchRequest<EarthquakeEntity> = EarthquakeEntity.fetchRequest()
    let dateSortDescriptor = NSSortDescriptor(key: #keyPath(EarthquakeEntity.time), ascending: false)
    fetchRequest.sortDescriptors = [dateSortDescriptor]
    let frc = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: managedObjectContext, sectionNameKeyPath: frcKeyPath, cacheName: nil)
    frc.delegate = self

    return frc
  }()

Answer 1

我最终使用了 fetchLimit，这似乎解决了我的问题。

当我执行提取时，我按如下方式使用它:

fetchedResultsController.fetchRequest.fetchLimit = 2_500

2,500 是我在不使应用程序崩溃的情况下所能达到的最大大小。