如何使 NSFetchedResultsController 更新与 UISearchController 一起工作？

Question

我有一个 NSFetchedResultsController 显示来自 Core Data 的一些数据，我正在尝试让搜索栏使用它。搜索有效，但 controller(didChangeObject:) 是使用来自完整表的索引路径调用的，而不是过滤的，因此更新要么修改错误的记录，要么因为索引路径超出范围而导致错误。尽管我知道这是错误的根源，但我不确定如何修复它。我的代码的主要部分如下：

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCellWithIdentifier("PlayerTableViewCell", forIndexPath: indexPath) as! PlayerTableViewCell
    let coreDataIndexPath: NSIndexPath
    if searchController.active && searchController.searchBar.text != "" {
        coreDataIndexPath = fetchedResultsController.indexPathForObject(filteredPlayers[indexPath.row])!
    } else {
        coreDataIndexPath = indexPath
    }
    configureCell(cell, atIndexPath: coreDataIndexPath)
    return cell
}

override func tableView(tableView: UITableView, commitEditingStyle editingStyle: UITableViewCellEditingStyle, forRowAtIndexPath indexPath: NSIndexPath) {
    if editingStyle == .Delete {
        // Fetch Record
        let record = fetchedResultsController.objectAtIndexPath(indexPath) as! NSManagedObject

        // Delete Record
        managedObjectContext.deleteObject(record)
    }
}


// MARK: Fetched Results Controller Delegate Methods
func controllerWillChangeContent(controller: NSFetchedResultsController) {
    tableView.beginUpdates()
}

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

func controller(controller: NSFetchedResultsController, didChangeObject anObject: AnyObject, atIndexPath indexPath: NSIndexPath?, forChangeType type: NSFetchedResultsChangeType, newIndexPath: NSIndexPath?) {
    switch (type) {
    case .Insert:
        if let indexPath = newIndexPath {
            tableView.insertRowsAtIndexPaths([indexPath], withRowAnimation: .Fade)
        }
    case .Delete:
        if let indexPath = indexPath {
            tableView.deleteRowsAtIndexPaths([indexPath], withRowAnimation: .Fade)
        }
    case .Update:
        if let indexPath = indexPath {
            let cell = tableView.cellForRowAtIndexPath(indexPath) as! PlayerTableViewCell
            configureCell(cell, atIndexPath: indexPath)
        }
    case .Move:
        if let indexPath = indexPath {
            tableView.deleteRowsAtIndexPaths([indexPath], withRowAnimation: .Fade)
        }

        if let newIndexPath = newIndexPath {
            tableView.insertRowsAtIndexPaths([newIndexPath], withRowAnimation: .Fade)
        }
    }
}

// MARK: UISearchResultsUpdating
func updateSearchResultsForSearchController(searchController: UISearchController) {
    if let searchText = searchController.searchBar.text {
        let predicate = NSPredicate(format: "name CONTAINS %@", searchText)
        if let fetchedObjects = fetchedResultsController.fetchedObjects as? [Player] {
            filteredPlayers = fetchedObjects.filter({return predicate.evaluateWithObject($0)})
        }
    }
    tableView.reloadData()

override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    if searchController.active && searchController.searchBar.text != "" {
        return filteredPlayers.count
    } else if let sections = fetchedResultsController.sections {
        let sectionInfo = sections[section]
        return sectionInfo.numberOfObjects
    }

    return 0
}

configureCell 只是将 indexPath 记录中的数据加载到单元格中。

我之前曾尝试通过更改fetchedResultsController.fetchRequest.predicate 进行搜索，但尽管我尽了最大努力，但当搜索栏被取消时，表格仍无法恢复正常。该代码如下：

// MARK: UISearchResultsUpdating
func updateSearchResultsForSearchController(searchController: UISearchController) {
    let searchText = searchController.searchBar.text!
    let predicate = NSPredicate(format: "%K CONTAINS[c] %@", argumentArray: ["name", searchText])
    fetchedResultsController.fetchRequest.predicate = predicate
    do {
        try self.fetchedResultsController.performFetch()
    } catch {
        let fetchError = error as NSError
        print("\(fetchError), \(fetchError.userInfo)")
    }
    tableView.reloadData()
}

// MARK: UISearchBarDelegate
func searchBarCancelButtonClicked(searchBar: UISearchBar) {
    fetchedResultsController.fetchRequest.predicate = nil
    do {
        try self.fetchedResultsController.performFetch()
    } catch {
        let fetchError = error as NSError
        print("\(fetchError), \(fetchError.userInfo)")
    }
    tableView.reloadData()
}

Answer 1

你需要在这里做两件事：

• 在重新加载表数据之前，为updateSearchResultsForSearchController 中的每个新搜索事件重置 NSFetchedResultsController；
• 返回您的cellForRowAtIndexPath 作为 FRC 中 indexPath 处的单元格。无需轮询 searchBar 的状态，NSFetchedResultsController 已经是最新的了。

我还不熟悉 Swift 语法，但这是 ObjC 中的样子：

-(void)updateSearchResultsForSearchController:(UISearchController *)searchController{
    self.fetchedResultsController = nil;
    [self.tableView reloadData];
}


-(NSFetchedResultsController *)fetchedResultsController{
if (_fetchedResultsController == nil) {
    NSManagedObjectContext *context = self.managedObjectContext; // Creating FRC with main MOC

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"MyEntity" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];

    NSString *searchString = [self.searchController.searchBar.text];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name CONTAINS[cd] %@ ", searchString];

    [fetchRequest setPredicate:predicate];

    // Specify how the fetched objects should be sorted
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"someField" ascending:YES];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObjects:sortDescriptor, nil]];

    _fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:context sectionNameKeyPath:nil cacheName:nil];

    NSError *error = nil;
    if (![_fetchedResultsController performFetch:&error])
        NSLog(@"Error initializing FRC for Users. Error %@", error.userInfo);

    _fetchedResultsController.delegate = self;
  }
return _fetchedResultsController;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
        cell = [tableView dequeueReusableCellWithIdentifier:@"MyCell" forIndexPath:indexPath];

        if (cell == nil)
                cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"MyCell"];
        MyEntity *myObject = [self.fetchedResultsController objectAtIndexPath:indexPath];

    /* Populate cell content here */
    return cell;
}

Answer 2

斯威夫特

这是你原来的谓词：

  var predicate: NSPredicate {
    return NSPredicate(format: "sampleCDAttribute == true")
  }

文本过滤谓词：

  var searchTextPredicate: NSPredicate {
    let searchText = searchController.searchBar.text ?? ""
    let andPredicate = NSPredicate(format: "text CONTAINS[cd] %@", searchText)
    return NSCompoundPredicate(type: .and, subpredicates: [predicate, andPredicate])
  }

NSFetchRequest 可能已包含在 FRC 工厂中，但为了可读性已被提取：

  var request: NSFetchRequest<ManagedWord> {
    let request: NSFetchRequest<YourCDObject> = YourCDObject.fetchRequest()
    request.predicate = predicate
    request.sortDescriptors = [NSSortDescriptor(key: "text", ascending: true)]
    return request
  }

您的 FRC 声明：

  //this is the container referencing from AppDelegate (customized)
  var container: NSPersistentContainer = AppDelegate.persistentContainer

  lazy var fetchedResultsController: NSFetchedResultsController<YourCDObject> = {
    let frc = NSFetchedResultsController<YourCDObject>(
      fetchRequest: request,
      managedObjectContext: container.viewContext,
      sectionNameKeyPath: "firstLetter",
      cacheName: nil)
    frc.delegate = self
    return frc
  }()

UISearchResultsUpdating 协议：

extension YourViewController: UISearchResultsUpdating {
  func updateSearchResults(for searchController: UISearchController) {
    guard let text = searchController.searchBar.text else { return }
    
    do {
      fetchedResultsController.fetchRequest.predicate = text.isEmpty ? predicate : searchTextPredicate
      try fetchedResultsController.performFetch()
      tableView.reloadData()
    } catch {
      print(error.localizedDescription)
    }
  }
}