NSFetchedResultsController - 用多个部分和分页反转 NSIndexPath [英] NSFetchedResultsController - reversed NSIndexPath with multiply sections and pagination

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问题描述

为了简化我的示例，我开发了 ChatViewController，我需要在其中显示分组为多个部分(一天一个部分)的消息.我可以通过向下滑动 UITableView 加载更多消息.我这里需要一个分页.消息从最旧(顶部)到最新(表格底部)显示.显而易见的是，我需要获取按 createdAt 属性 DESCENDING 排序的消息.

To simplify my example, I develop ChatViewController where I need to display messages grouped into sections (one section for one day). I can load more messages swiping UITableView down. I need a pagination here. Messages are displayed from the oldest (top) to the latest (bottom of the table). The obvious is that I need to fetch messages sorted by createdAt property DESCENDING.

这就是我在 viewDidLoad() 上设置我的 NSFetcheResultsController 的方法:

So this is how I setup my NSFetcheResultsController on viewDidLoad():

private func setupFetchedResultsController() {

    let context = NSManagedObjectContext.MR_defaultContext()
    let fetchedRequest = NSFetchRequest(entityName: "Message")
    let createdAtDescriptor = NSSortDescriptor(key: "createdAt", ascending: false)

    fetchedRequest.predicate = NSPredicate(format: "conversation.identifier = %lld", identifier)
    fetchedRequest.sortDescriptors = [createdAtDescriptor]

    fetchedResultsController = NSFetchedResultsController(fetchRequest: fetchedRequest, managedObjectContext: context, sectionNameKeyPath: "normalizedCreatedAt", cacheName: nil)
    fetchedResultsController.delegate = self

    try! fetchedResultsController.performFetch()
    tableView.reloadData()

    debugFRC()
}

假设...这是我从 NSFetchedResultsController 获取的非常简单的示例:

Assuming... this is pretty simple example what I fetch from NSFetchedResultsController:

Section 0
    0 - 0 - message A, createdAt: '21-07-2016 08:42'
    0 - 1 - message B, createdAt: '21-07-2016 08:40'

Section 1
    1 - 0 - message C, createdAt: '20-07-2016 08:42'
    1 - 1 - message D, createdAt: '20-07-2016 08:40'
    1 - 2 - message E, createdAt: '20-07-2016 08:38'
    1 - 3 - message F, createdAt: '20-07-2016 08:36'
    1 - 4 - message G, createdAt: '20-07-2016 08:34'
    1 - 5 - message H, createdAt: '20-07-2016 08:32'

Section 2
    2 - 0 - message I, createdAt: '19-07-2016 08:42'
    2 - 1 - message J, createdAt: '19-07-2016 08:40'
    2 - 2 - message K, createdAt: '19-07-2016 08:38'

但是我需要在我的 UITableView 中像这样显示它:

But I need to display it like this in my UITableView:

Section 0
    0 - 0 - message K, createdAt: '19-07-2016 08:38'
    0 - 1 - message J, createdAt: '19-07-2016 08:40'
    0 - 2 - message I, createdAt: '19-07-2016 08:42'

Section 1
    1 - 0 - message H, createdAt: '20-07-2016 08:32'
    1 - 1 - message G, createdAt: '20-07-2016 08:34'
    1 - 2 - message F, createdAt: '20-07-2016 08:36'
    1 - 3 - message E, createdAt: '20-07-2016 08:38'
    1 - 4 - message D, createdAt: '20-07-2016 08:40'
    1 - 5 - message C, createdAt: '20-07-2016 08:42'

Section 2
    0 - 0 - message B, createdAt: '21-07-2016 08:40'
    0 - 1 - message A, createdAt: '21-07-2016 08:42'

所以，我创建了三个辅助私有函数:

So, I have created three helper private functions:

//MARK: - Private

private func debugFRC() {

    if let sections = fetchedResultsController.sections {

        for (index, section) in sections.enumerate() {

            print("FRC: \(index) ::: \(section.objects!.count)")

            for (index, message) in (section.objects as! [Message]).enumerate() {
                print("FRC MESSAGE: \(index) --->>> \(message.createdAt)")
            }
        }
    }
}

private func reversedIndexPathForIndexPath(indexPath: NSIndexPath) -> NSIndexPath {

    let section = reversedSectionForSection(indexPath.section)
    let row = fetchedResultsController.sections![section].objects!.count - indexPath.row - 1

    return NSIndexPath(forRow: row, inSection: section)
}

private func reversedSectionForSection(section: Int) -> Int {
    return fetchedResultsController!.sections!.count - section - 1
}

然后我实现了我的NSFetchedResultsControllerDelegate:

//MARK: - NSFetchedResultsControllerDelegate

func controllerWillChangeContent(controller: NSFetchedResultsController) {
    tableView.beginUpdates()
}

func controller(controller: NSFetchedResultsController, didChangeSection sectionInfo: NSFetchedResultsSectionInfo, atIndex sectionIndex: Int, forChangeType type: NSFetchedResultsChangeType) {

    let indexSet = NSIndexSet(index: reversedSectionForSection(sectionIndex))

    switch type {
    case .Insert:

        tableView.insertSections(indexSet, withRowAnimation: .Fade)

    case .Delete:

        tableView.deleteSections(indexSet, withRowAnimation: .Fade)

    case .Update:

        fallthrough

    case .Move:

        tableView.reloadSections(indexSet, withRowAnimation: .Fade)
    }
}

func controller(controller: NSFetchedResultsController, didChangeObject anObject: AnyObject, atIndexPath indexPath: NSIndexPath?, forChangeType type: NSFetchedResultsChangeType, newIndexPath: NSIndexPath?) {

    switch type {
    case .Insert:

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

    case .Delete:

        if let indexPath = indexPath {
            tableView.deleteRowsAtIndexPaths([reversedIndexPathForIndexPath(indexPath)], withRowAnimation: .Fade)
        }

    case .Update:

        if let indexPath = indexPath {
            tableView.reloadRowsAtIndexPaths([reversedIndexPathForIndexPath(indexPath)], withRowAnimation: .Fade)
        }

    case .Move:

        if let indexPath = indexPath, let newIndexPath = newIndexPath {

            tableView.deleteRowsAtIndexPaths([reversedIndexPathForIndexPath(indexPath)], withRowAnimation: .Fade)
            tableView.insertRowsAtIndexPaths([reversedIndexPathForIndexPath(newIndexPath)], withRowAnimation: .Fade)
        }
    }
}

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

和 UITableViewDataSource 委托:

//MARK: - UITableViewDataSource

func numberOfSectionsInTableView(tableView: UITableView) -> Int {
    return fetchedResultsController!.sections!.count
}

func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    return fetchedResultsController.sections![reversedSectionForSection(section)].objects!.count
}

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {

    let message = fetchedResultsController?.objectAtIndexPath(reversedIndexPathForIndexPath(indexPath)) as! Message
    let cell = tableView.dequeueReusableCellWithIdentifier("cellIdentifier", forIndexPath: reversedIndexPathForIndexPath(indexPath)) as! TableViewCell

    cell.cellTitleLabel?.text = message.content

    return cell
}

debugFRC() 的输出如下:

FRC: 0 ::: 35
FRC MESSAGE: 0 --->>> 2016-08-23 13:02:32 +0000
FRC MESSAGE: 1 --->>> 2016-08-23 12:59:25 +0000
FRC MESSAGE: 2 --->>> 2016-08-23 11:49:07 +0000
FRC MESSAGE: 3 --->>> 2016-08-23 11:48:21 +0000
FRC MESSAGE: 4 --->>> 2016-08-23 11:32:44 +0000
FRC MESSAGE: 5 --->>> 2016-08-23 11:30:19 +0000
FRC MESSAGE: 6 --->>> 2016-08-23 11:30:16 +0000
FRC MESSAGE: 7 --->>> 2016-08-23 11:27:48 +0000
FRC MESSAGE: 8 --->>> 2016-08-23 11:23:58 +0000
FRC MESSAGE: 9 --->>> 2016-08-23 11:23:32 +0000
FRC MESSAGE: 10 --->>> 2016-08-23 11:23:21 +0000
FRC MESSAGE: 11 --->>> 2016-08-23 11:18:51 +0000
FRC MESSAGE: 12 --->>> 2016-08-23 11:18:45 +0000
FRC MESSAGE: 13 --->>> 2016-08-23 11:18:36 +0000
FRC MESSAGE: 14 --->>> 2016-08-23 11:16:23 +0000
FRC MESSAGE: 15 --->>> 2016-08-23 11:16:09 +0000
FRC MESSAGE: 16 --->>> 2016-08-23 11:15:49 +0000
FRC MESSAGE: 17 --->>> 2016-08-23 11:15:38 +0000
FRC MESSAGE: 18 --->>> 2016-08-23 11:15:29 +0000
FRC MESSAGE: 19 --->>> 2016-08-23 10:57:44 +0000
FRC MESSAGE: 20 --->>> 2016-08-23 10:57:44 +0000
FRC MESSAGE: 21 --->>> 2016-08-23 10:57:43 +0000
FRC MESSAGE: 22 --->>> 2016-08-23 10:57:43 +0000
FRC MESSAGE: 23 --->>> 2016-08-23 10:57:42 +0000
FRC MESSAGE: 24 --->>> 2016-08-23 10:57:42 +0000
FRC MESSAGE: 25 --->>> 2016-08-23 10:57:40 +0000
FRC MESSAGE: 26 --->>> 2016-08-23 10:56:31 +0000
FRC MESSAGE: 27 --->>> 2016-08-23 10:55:49 +0000
FRC MESSAGE: 28 --->>> 2016-08-23 10:55:08 +0000
FRC MESSAGE: 29 --->>> 2016-08-23 10:53:45 +0000
FRC MESSAGE: 30 --->>> 2016-08-23 10:48:59 +0000
FRC MESSAGE: 31 --->>> 2016-08-23 10:48:40 +0000
FRC MESSAGE: 32 --->>> 2016-08-23 10:47:02 +0000
FRC MESSAGE: 33 --->>> 2016-08-23 10:45:06 +0000
FRC MESSAGE: 34 --->>> 2016-08-23 10:45:01 +0000
FRC: 1 ::: 2
FRC MESSAGE: 0 --->>> 2016-08-16 09:16:38 +0000
FRC MESSAGE: 1 --->>> 2016-08-16 09:16:22 +0000

但它不起作用.问题如下:

But it doesn't work. The problem is the following:

CoreData:错误:严重的应用程序错误.在调用 -controllerDidChangeContent: 期间，从 NSFetchedResultsController 的委托中捕获了异常.尝试从第 0 节中删除第 34 行，该行在更新前仅包含 2 行 userInfo (null)

CoreData: error: Serious application error. An exception was caught from the delegate of NSFetchedResultsController during a call to -controllerDidChangeContent:. attempt to delete row 34 from section 0 which only contains 2 rows before the update with userInfo (null)

我很确定我错过了一些东西，但我不知道是什么.有什么想法吗?

I am pretty sure I missed something, but I do not know what. Any ideas?

推荐答案

你能查一下 这个答案?它做你想做的同样的事情.

Can you check this answer? It does the same thing you want to do.

关键是创建 ** 瞬态字段** 作为检查标记数据模型中特定属性的瞬态字段(例如 sectionTitle).

The key is creating ** transient field** as Check mark the transient field in your data model for particular attribute (e.g. sectionTitle).

并将其计算为:

Message{
@NSManaged var body: String?
@NSManaged var seen: NSNumber?
@NSManaged var time: NSDate?
@NSManaged var uid: String?
@NSManaged var conversation: Conversation?

var sectionTitle: String? {
    //this is **transient** property
    //to set it as transient, check mark the box with same name in data model
    return time!.getTimeStrWithDayPrecision()
}
}

然后将 NSFetchedResultsController 初始化为:

let request = NSFetchRequest(entityName: "Message")
let sortDiscriptor = NSSortDescriptor(key: "time", ascending: true)
request.sortDescriptors = [sortDiscriptor]

let pred = NSPredicate(format: "conversation.contact.uid == %@", _conversation!.contact!.uid!)
request.predicate = pred

let mainThreadMOC = DatabaseInterface.sharedInstance.mainThreadManagedObjectContext()
fetchedResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: mainThreadMOC, sectionNameKeyPath: "sectionTitle", cacheName: nil)
fetchedResultsController.delegate = self

do {
    try fetchedResultsController.performFetch()
} catch {
    fatalError("Failed to initialize FetchedResultsController: \(error)")
}

这篇关于NSFetchedResultsController - 用多个部分和分页反转 NSIndexPath的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！

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