Iglistkit: Cannot find section controller for item (though it appears to be present)

[clayreimann]

New issue checklist

• [x] I have reviewed the README and documentation
• [x] I have searched existing issues and this is not a duplicate

General information

• IGListKit version: 3.0.0
• iOS version(s): 10/11
• CocoaPods/Carthage version: 1.2.1
• Xcode version: 9 beta 3
• Devices/Simulators affected: sim iPhone 7s/plus, device iPhone 6s
• Reproducible in the demo project? (Yes/No): Not trivially (have a demo that does not exhibit this issue 😢)
• Related issues:

I'm seeing a bizarre issue where IGListKit only finds a SectionController for one of my items when it is the only item returned by objects(for listAdapter:).

The class in question is pretty simple (code below). When I do some basic debugging it appear that there is a cached SectionController present but it doesn't get returned appropriately, which is surprising given the (apparent) simplicity of the functions involved. My only supposition is that because the LoadingItem class is a native swift class some internal assumption of the NSMapTable is violated. Though this does not appear to be born out by the demo project I ginned up which appears unaffected by this bug.

# debugging crash from IGListAdapter+UICollectionView collectionView:cellForItemAtIndexPath:
(lldb) po [self.sectionMap objectForSection:indexPath.section]
LoadingItem<BC747166-6B7C-4C48-85A8-294E883B8AF6>

(lldb) po self.sectionMap.objectToSectionControllerMap
NSMapTable {
[1] Text[id=70390 msgs=1] -> <texts_from_last_night.TextSectionController: 0x1c41164a0>
[2] Text[id=70387 msgs=1] -> <texts_from_last_night.TextSectionController: 0x1c4116530>
[3] Text[id=70378 msgs=2] -> <texts_from_last_night.TextSectionController: 0x1c4116a40>
[5] Text[id=70379 msgs=1] -> <texts_from_last_night.TextSectionController: 0x1c41169b0>
[6] Text[id=70376 msgs=1] -> <texts_from_last_night.TextSectionController: 0x1c4116b60>
[8] Text[id=70380 msgs=2] -> <texts_from_last_night.TextSectionController: 0x1c4116920>
[16] Text[id=70381 msgs=1] -> <texts_from_last_night.TextSectionController: 0x1c4116890>
[18] Text[id=70383 msgs=1] -> <texts_from_last_night.TextSectionController: 0x1c4116770>
[20] Text[id=70389 msgs=1] -> <texts_from_last_night.TextSectionController: 0x1c4116410>
[22] Text[id=70385 msgs=3] -> <texts_from_last_night.TextSectionController: 0x1c4116650>
[23] Text[id=70388 msgs=1] -> <texts_from_last_night.TextSectionController: 0x1c4115f00>
[25] Text[id=70384 msgs=1] -> <texts_from_last_night.TextSectionController: 0x1c41166e0>
[27] Text[id=70377 msgs=1] -> <texts_from_last_night.TextSectionController: 0x1c4116ad0>
[29] Text[id=70382 msgs=1] -> <texts_from_last_night.TextSectionController: 0x1c4116800>
[30] LoadingItem<BC747166-6B7C-4C48-85A8-294E883B8AF6> -> <texts_from_last_night.LoadingSectionController: 0x1c00f0b80>
[31] Text[id=70386 msgs=1] -> <texts_from_last_night.TextSectionController: 0x1c41165c0>
}
(lldb) po [self.sectionMap sectionControllerForSection:indexPath.section]
 nil

Class in question:

class LoadingItem {
    let uuid = UUID().uuidString
    var lastLoadDate = Date()
    var isLoading: Bool {
        return container.isLoading
    }

    var container: TextsVC

    init(container: TextsVC) {
        self.container = container
    }

    func load() {
        container.loadMore()
    }

    func didLoad() {
        lastLoadDate = Date()
    }
}

extension LoadingItem: ListDiffable {
    public func diffIdentifier() -> NSObjectProtocol {
        return lastLoadDate as NSObjectProtocol
    }

    public func isEqual(toDiffableObject object: ListDiffable?) -> Bool {
        guard let loader = object as? LoadingItem else { return false }
        return self.container == loader.container
    }
}

extension LoadingItem: CustomStringConvertible {
    var description: String {
        return "LoadingItem<\(uuid)>"
    }
}

Debug information

# Please include debug logs using the following lldb command:
(lldb) po [IGListDebugger dump]
IGListAdapter 0x1c0342310:
  Updater type: IGListAdapterUpdater
  Data source: <texts_from_last_night.TextsVC: 0x111d27250>
  Collection view delegate: (null)
  Scroll view delegate: (null)
  Is in update block: No
  View controller: <texts_from_last_night.TextsVC: 0x111d27250>
  Is prefetching enabled: No
  Registered cell classes:
  {(
    texts_from_last_night.DateCell,
    texts_from_last_night.MessageCell,
    texts_from_last_night.LoadingCell
)}
  IGListAdapterUpdater instance 0x1c00da010:
    Moves as deletes+inserts: No
    Allows background reloading: Yes
    Has queued reload data: No
    Queued update is animated: Yes
    State: Idle
  Section map details:
    Object and section controller at section: 0:
      Text[id=70390 msgs=1]
      <texts_from_last_night.TextSectionController: 0x1c41164a0>
    Object and section controller at section: 1:
      Text[id=70389 msgs=1]
      <texts_from_last_night.TextSectionController: 0x1c4116410>
    Object and section controller at section: 2:
      Text[id=70388 msgs=1]
      <texts_from_last_night.TextSectionController: 0x1c4115f00>
    Object and section controller at section: 3:
      Text[id=70387 msgs=1]
      <texts_from_last_night.TextSectionController: 0x1c4116530>
    Object and section controller at section: 4:
      Text[id=70386 msgs=1]
      <texts_from_last_night.TextSectionController: 0x1c41165c0>
    Object and section controller at section: 5:
      Text[id=70385 msgs=3]
      <texts_from_last_night.TextSectionController: 0x1c4116650>
    Object and section controller at section: 6:
      Text[id=70384 msgs=1]
      <texts_from_last_night.TextSectionController: 0x1c41166e0>
    Object and section controller at section: 7:
      Text[id=70383 msgs=1]
      <texts_from_last_night.TextSectionController: 0x1c4116770>
    Object and section controller at section: 8:
      Text[id=70382 msgs=1]
      <texts_from_last_night.TextSectionController: 0x1c4116800>
    Object and section controller at section: 9:
      Text[id=70381 msgs=1]
      <texts_from_last_night.TextSectionController: 0x1c4116890>
    Object and section controller at section: 10:
      Text[id=70380 msgs=2]
      <texts_from_last_night.TextSectionController: 0x1c4116920>
    Object and section controller at section: 11:
      Text[id=70379 msgs=1]
      <texts_from_last_night.TextSectionController: 0x1c41169b0>
    Object and section controller at section: 12:
      Text[id=70378 msgs=2]
      <texts_from_last_night.TextSectionController: 0x1c4116a40>
    Object and section controller at section: 13:
      Text[id=70377 msgs=1]
      <texts_from_last_night.TextSectionController: 0x1c4116ad0>
    Object and section controller at section: 14:
      Text[id=70376 msgs=1]
      <texts_from_last_night.TextSectionController: 0x1c4116b60>
    Object and section controller at section: 15:
      LoadingItem<BC747166-6B7C-4C48-85A8-294E883B8AF6>
      (null)
  Collection view details:
    Class: UICollectionView, instance: 0x11282d600
    Data source: <IGListAdapter: 0x1c0342310>
    Delegate: <IGListAdapter: 0x1c0342310>
    Layout: <UICollectionViewFlowLayout: 0x111e4df80>
    Frame: {{0, 0}, {375, 667}}, bounds: {{0, 2521}, {375, 667}}
    Number of sections: 16
      2 items in section 0
      2 items in section 1
      2 items in section 2
      2 items in section 3
      2 items in section 4
      4 items in section 5
      2 items in section 6
      2 items in section 7
      2 items in section 8
      2 items in section 9
      3 items in section 10
      2 items in section 11
      3 items in section 12
      2 items in section 13
      2 items in section 14
      1 items in section 15
    Visible cell details:
      Visible cell at section 12, item 0:
      <texts_from_last_night.MessageCell: 0x111e779a0; baseClass = UICollectionViewCell; frame = (0 2490; 375 192); layer = <CALayer: 0x1c423cbc0>>
      Visible cell at section 12, item 1:
      <texts_from_last_night.MessageCell: 0x111d865e0; baseClass = UICollectionViewCell; frame = (0 2687; 375 60); layer = <CALayer: 0x1c4425fe0>>
      Visible cell at section 12, item 2:
      <texts_from_last_night.DateCell: 0x111e83030; baseClass = UICollectionViewCell; frame = (0 2752; 375 54); layer = <CALayer: 0x1c0236100>>
      Visible cell at section 13, item 0:
      <texts_from_last_night.MessageCell: 0x111d87190; baseClass = UICollectionViewCell; frame = (0 2821; 375 82); layer = <CALayer: 0x1c0237e80>>
      Visible cell at section 13, item 1:
      <texts_from_last_night.DateCell: 0x111d67f80; baseClass = UICollectionViewCell; frame = (0 2908; 375 54); layer = <CALayer: 0x1c4423de0>>
      Visible cell at section 14, item 0:
      <texts_from_last_night.MessageCell: 0x111e7e160; baseClass = UICollectionViewCell; frame = (0 2977; 375 104); layer = <CALayer: 0x1c0235f20>>
      Visible cell at section 14, item 1:
      <texts_from_last_night.DateCell: 0x111d84890; baseClass = UICollectionViewCell; frame = (0 3086; 375 54); layer = <CALayer: 0x1c44211a0>>

[rnystrom]

@clayreimann I see that lastLoadDate is defined as a var. Is it being mutated anywhere? If so, that'll cause any lookups to fail. W/e you use for your diffIdentifier should never change.

[clayreimann]

@rnystrom That was exactly it. I've opened a PR to update the Getting Started guide. I spent several days trying to chase this down and I'd like to save the next person that trouble (or at least try to).