NSFetchedResultsController在NSDiffableDataSourceSnapshotReference中将所有实体都退回，即使没有更改

如何解决NSFetchedResultsController在NSDiffableDataSourceSnapshotReference中将所有实体都退回，即使没有更改

我有一个NSFetchedResultsController，它监视与另一个实体耦合的实体。现在，当在UICollectionView中显示实体时，我使用UICollectionViewDiffableDataSource。

问题在于，当更改1个实体时，快照将返回所有项目，不仅刷新更新的实体，还刷新所有实体。如何防止刷新没有更改的项目？

FetchedResultsController

class func fetchResultsController(template: Template,context: NSManagedobjectContext) -> NSFetchedResultsController<Counters> {
    let fetchRequest: NSFetchRequest<Counters> = Counters.fetchRequest()
    fetchRequest.sortDescriptors = [
        NSSortDescriptor(key: #keyPath(Counters.name),ascending: true)
    ]
    fetchRequest.predicate = nspredicate(format: "%@ = %K",argumentArray: [template,#keyPath(Counters.template)])
    let fetchResultController = NSFetchedResultsController(fetchRequest: fetchRequest,managedobjectContext: context,sectionNameKeyPath: nil,cacheName: nil)
    return fetchResultController
}

代理

func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>,didChangeContentWith snapshot: NSDiffableDataSourceSnapshotReference) {
    var diff = NSDiffableDataSourceSnapshot<String,CounterCellviewmodel>()
    snapshot.sectionIdentifiers.forEach { section in
        diff.appendSections([section as! String])
        let items = snapshot.itemIdentifiersInSection(withIdentifier: section)
            .map { (objectId: Any) -> CounterCellviewmodel in
                let oid = objectId as! NSManagedobjectID
                let object = controller.managedobjectContext.object(with: oid) as! Counters
                return CounterCellviewmodel(counter: object)
        }
        diff.appendItems(items,toSection: section as? String)
    self.countersDataSource.apply(diff,animatingDifferences: false)
}

}

数据源

    private lazy var countersDataSource: UICollectionViewDiffableDataSource<String,CounterCellviewmodel> = {
    return UICollectionViewDiffableDataSource(collectionView: self.collectionView) { [uNowned self] (collectionView,indexPath,viewmodel)
        -> UICollectionViewCell? in
        let cell = collectionView.deo_dequeueReusableCell(ofType: CounterCell.self,for: indexPath)
        cell.viewmodel = viewmodel
        return cell
    }
}()

CounterCellviewmodel可哈希化

func hash(into hasher: inout Hasher) {
    hasher.combine(self.counter.id)
}

解决方法

我目前正在运行“正在运行的应用程序”。在将新的正在运行的会话结果获取到UITableView时，我曾经遇到过同样的问题。我通过将数据加载到新数组中并在将更改后的数据提取到其中后将旧数组的内容更改为新内容中来解决了该问题。

功能如下：

func loadSessions() -> [Run] {

        var _runs : [Run] = []
        let context = CoreDataStack.persistentContainer.viewContext
        let fetchRequest: NSFetchRequest<Run> = Run.fetchRequest()
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "timestamp",ascending: true),NSSortDescriptor(key: "distance",NSSortDescriptor(key: "duration",ascending: true)]
        
        do {
            let sessions = try context.fetch(fetchRequest)
            if(sessions.count > 0) {
                _runs = sessions
            }
        } catch {
            print("Something happened while trying to retrieve tasks...")
        }
        return _runs
    }