RxCocoa/RxSwift crash when accessing TableViewDataSourceNotSet tableView(_:cellForRowAt:)

Question

I'm using RxCocoa and RxSwift to render a UITableView against an array provided by a BehaviorRelay. The code to bind the data is below:

// MyViewModel:
var itemList = BehaviorRelay(value: [MyItem]())
...
func loadData() {
    var items = [...] // load items
    itemList.accept(items)
}
// ==========

// MyView:
var viewModel = MyViewModel()
func bindData() {
    viewModel.itemList.bind(to: tableView.rx.items) { table, index, item in
        if item.type == ... {
            ... // pick cell type based on the item metadata
        } else {
            let cell = table.dequeueReusableCell(withIdentifier: "MyItemCell") as? MyItemCell {
            cell.bindItem(item)
            return cell
        }
        return new UITableViewCell()
    }).disposed(by: viewModel.disposeBag)
}
// ==========

In most cases it works just fine but in some rare cases which I cannot reproduce the app crashes with the following stacktrace, reported by a few Firebase uses:

Crashed: com.apple.main-thread
0  libswiftCore.dylib             0x37d7c _assertionFailure(_:_:file:line:flags:) + 312
1  mycoolapp                      0x8aa968 @objc TableViewDataSourceNotSet.tableView(_:cellForRowAt:) + 84 (RxCocoa.swift:84)
2  mycoolapp                      0x8aae30 @objc RxTableViewDataSourceProxy.tableView(_:cellForRowAt:) + 64 (RxTableViewDataSourceProxy.swift:64)
3  UIKitCore                      0x14b75c -[UITableView _createPreparedCellForGlobalRow:withIndexPath:willDisplay:] + 808
4  UIKitCore                      0x219c9c -[UITableView _createPreparedCellForRowAtIndexPath:willDisplay:] + 68
5  UIKitCore                      0x219884 -[UITableView _heightForRowAtIndexPath:] + 124
6  UIKitCore                      0x219720 -[UISectionRowData heightForRow:inSection:canGuess:] + 176
7  UIKitCore                      0x44c328 -[UITableViewRowData heightForRow:inSection:canGuess:adjustForReorderedRow:] + 228
8  UIKitCore                      0x5952c -[UITableViewRowData rectForRow:inSection:heightCanBeGuessed:] + 304
9  UIKitCore                      0x44b8b0 -[UITableViewRowData rectForGlobalRow:heightCanBeGuessed:] + 112
10 UIKitCore                      0x14b378 -[UITableView _prefetchCellAtGlobalRow:aboveVisibleRange:] + 240
11 UIKitCore                      0x14b264 __48-[UITableView _configureCellPrefetchingHandlers]_block_invoke + 52
12 UIKitCore                      0x2186a8 -[_UITableViewPrefetchContext updateVisibleIndexRange:withContentOffset:] + 2100
13 UIKitCore                      0x81be8 -[UITableView _updateCycleIdleUntil:] + 168
14 UIKitCore                      0x811ac ___UIUpdateCycleNotifyIdle_block_invoke + 612
15 libdispatch.dylib              0x2460 _dispatch_call_block_and_release + 32
16 libdispatch.dylib              0x3f88 _dispatch_client_callout + 20
17 libdispatch.dylib              0x127f4 _dispatch_main_queue_drain + 928
18 libdispatch.dylib              0x12444 _dispatch_main_queue_callback_4CF + 44
19 CoreFoundation                 0x9a6c8 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 16
20 CoreFoundation                 0x7c02c __CFRunLoopRun + 2036
21 CoreFoundation                 0x80eb0 CFRunLoopRunSpecific + 612
22 GraphicsServices               0x1368 GSEventRunModal + 164
23 UIKitCore                      0x3a1668 -[UIApplication _run] + 888
24 UIKitCore                      0x3a12cc UIApplicationMain + 340
25 mycoolapp.                     0x1d8564 main + 23 (AppDelegate.swift:23)
26 ???                            0x1b3d9c960 (Missing)

The crash is hard to reproduce (I wasn't able to) but it's affects multiple users. The bindData method is called once at viewDidLoad, and I call loadData on every page appearance (willAppear, return back from another page or app activation deactivation) but it's not clear why it crashes and how to reproduce/fix it.

The crash is coming from TableViewDataSourceNotSet which seems to be set by the RxCocoa/RxSwift framework and crashes when cell for an item is requested. I'm not setting that custom dataSource explicitly anywhere. Any ideas how I can prevent this from happening?

score 1 · Answer 1 · answered Apr 06 '23 at 21:32

1

The bindData method is called on every page appearance (willAppear) but it's not clear why it crashes and how to reproduce/fix it.

The above is a red flag. You should not be binding on every page appearance. Best case that is causing an unbinding and then rebinding. It could be that this is tripping up the system in some places where the table view is calling numberOfRowsInSection from the previously installed data source, and then calling cellForRowAt on the TableViewDataSourceNotSet object.

Your binding should only happen once. Usually in the viewDidLoad.

answered Apr 06 '23 at 21:32

Daniel T.

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I have re-checked the code and confirmed that I described it incorrectly in my questions. I'm calling `loadData` on every `willDisplay` but the binding happens only once on `viewDidLoad` as you described. Thanks for pointing out the possible red flag. I'm updating my question RN to remove the confusion – Mando Apr 07 '23 at 02:43
1

The answer is still the same. When `numberOfRowsInSection` was called by the table view, there was a delegate that returns a positive number. When `cellForRowAt` was called, the delegate was gone. Why that is happening is not clear from the code you posted. – Daniel T. Apr 07 '23 at 15:52
as of now, no solution, but will post it here if found – Mando Apr 10 '23 at 17:35

RxCocoa/RxSwift crash when accessing TableViewDataSourceNotSet tableView(_:cellForRowAt:)

1 Answers1