NSFetchedResultsController 将所有实体返回，即使是 NSDiffableDataSourceSnapshotReference 中未更改的实体答案

【问题标题】：NSFetchedResultsController gives all entities back even not changed ones in NSDiffableDataSourceSnapshotReferenceNSFetchedResultsController 将所有实体返回，即使是 NSDiffableDataSourceSnapshotReference 中未更改的实体
【发布时间】：2021-01-30 17:54:45
【问题描述】：

我有一个 NSFetchedResultsController 来监控与其他实体耦合的实体。现在在 UICollectionView 中显示实体时，我使用 UICollectionViewDiffableDataSource。

问题在于，当更改 1 个实体时，快照会返回所有项目，并且不仅会刷新更新的实体，还会刷新所有实体。我可以做些什么来防止刷新没有变化的项目？

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
}

委托

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 可散列

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

【问题讨论】：

标签： ios swift core-data nsfetchedresultscontroller

【解决方案1】：

我目前正在处理正在运行的应用程序。在将新的运行会话结果获取到 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的数据获取到它之后简单地将旧数组的内容更改为新数组来解决它。

函数如下：

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", ascending: true), 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
    }

【讨论】：