如何使用 NSFetchedResultsController 和多个获取的实体设置 collectionView（或 TableView）

Question

我在这里读到，使用 1 个 NSFetchedResultsController 获取多个实体的方法是使用父子继承模型。我有这样一个模型：

https://imgur.com/a/nckHzvr

如您所见，TextPost、VideoPost 和 ImagePost 都有 Skill 作为父实体。我正在尝试制作一个所有三个孩子都出现的collectionView。我对如何设置委托方法有点困惑......

这是视图控制器的代码

class Timeline2ViewController: UIViewController {

    @IBOutlet var postsCollectionView: UICollectionView!
    var skillName: String?

    fileprivate lazy var skillFetchedResultsController: NSFetchedResultsController<Skill> = {
        let appDelegate =
            UIApplication.shared.delegate as? AppDelegate
        let managedContext =
            appDelegate?.persistentContainer.viewContext
        let request: NSFetchRequest<Skill> = NSFetchRequest(entityName: "Skill")
        request.predicate = NSPredicate(format: "name == %@", self.skillName!)
        let timeSort = NSSortDescriptor(key: "timeStamp", ascending: true)
        request.sortDescriptors = [timeSort]

        let skillFetchedResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: managedContext!, sectionNameKeyPath: nil, cacheName: nil)
        return skillFetchedResultsController
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        do {
            try skillFetchedResultsController.performFetch()
        } catch let error as NSError {
            print("SkillFetchError")
        }
    }
}
extension Timeline2ViewController: UICollectionViewDelegate, UICollectionViewDataSource {
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        guard let sectionInfo = skillFetchedResultsController.sections?[section] else { return 0 }
        return sectionInfo.numberOfObjects
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        guard let cell = postsCollectionView.dequeueReusableCell(withReuseIdentifier: "pcell", for: indexPath) as? PostViewCell else { return fatalError("unexpected Index Path")
        }
        let post = skillFetchedResultsController[indexPath.row] /* This is the line im not sure about
        cell.background
        return cell
    }
}

由于实际上只返回了 1 个实体，我不确定如何访问特定索引路径中的元素。例如 SkillFetchedResultsController[indexPath.row] 我认为只有 1 个实体 - 技能本身。我真的很想访问它的孩子。我是否必须以某种方式继承 SkillFetchedResultsController 并只返回我感兴趣的孩子？

编辑：@pbasdf 建议 - 我有这个模型：

现在当我像这样创建一个实体时：

        guard let appDelegate =
         UIApplication.shared.delegate as? AppDelegate else {
             return
         }
         let managedContext = appDelegate.persistentContainer.viewContext
        let textPost = NSEntityDescription.insertNewObject(forEntityName: "TextPost", into: managedContext) as! TextPost
        textPost.text = "test text post"
        try! managedContext.save()

然后我设置我的获取结果控制器来查看“Post2”，如下所示：

    fileprivate lazy var skillFetchedResultsController: NSFetchedResultsController<Post2> = {
        let appDelegate =
            UIApplication.shared.delegate as? AppDelegate
        let managedContext =
            appDelegate?.persistentContainer.viewContext
        let request: NSFetchRequest<Post2> = NSFetchRequest(entityName: "Post2")
//        request.predicate = NSPredicate(format: "skill = %@", self.skill!)
        let timeSort = NSSortDescriptor(key: "timeStamp", ascending: true)
        request.sortDescriptors = [timeSort]
        let skillFetchedResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: managedContext!, sectionNameKeyPath: nil, cacheName: nil)
        return skillFetchedResultsController
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.

        do {
            try skillFetchedResultsController.performFetch()
        } catch _ as NSError {
            print("SkillFetchError")
        }

    }

我没有看到返回结果：

func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    guard let sectionInfo = skillFetchedResultsController.sections?[section] else { return 0 }
    return sectionInfo.numberOfObjects
}

我是否需要以某种方式将两者联系起来？就像同时创建 Post 对象和 TextPost 对象一样？当我尝试直接获取 TextPost 对象时，它可以工作。

Answer 1

我认为问题在于您的模型。您应该创建一个Post 实体，并使其成为TextPost、VideoPost 和ImagePost 的父实体。如果您的子实体有任何共同的属性，请将它们从子实体移动到Post 实体。然后建立从Skill到Post的一对多关系。

您的 FRC 应获取 Post 对象（默认情况下将包括所有子实体），如有必要，使用谓词将其限制为与您所需的 Skill 对象相关的 Post 对象，例如。

NSPredicate(format:"skill.name == %@",self.skillName!)