dispatch_group_leave crash in swift

问题

This happens very rarely. Here is the last line of the stack trace:

0  libdispatch.dylib              0x0000000197a85a9c dispatch_group_leave + 48

dispatch_group_leave is called in a complete closure which is invoked like this:

  dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
                { () -> Void in
                    let query = HKStatisticsCollectionQuery(quantityType: quantityType,
                        quantitySamplePredicate: nil,
                        options: statisticOptions,
                        anchorDate: anchorDate,
                        intervalComponents: interval)
                    query.initialResultsHandler = {

                        complete()

So we dispatch to a background thread, run a HKStatisticsCollectionQuery, and then call a function parameter closure called complete. Inside complete is where the dispatch_group_leave is called and the crash happens.

Any ideas are most appreciated! Thanks!

回答1:

If dispatch_group_leave call isn't balanced with dispatch_group_enter then crash may happen.

回答2:

In worst case if you want to chek the count of number of values enter You can do this by following patch

let count = self.groupExecuting.debugDescription.components(separatedBy: ",").filter({$0.contains("count")}).first!.components(separatedBy: CharacterSet.decimalDigits.inverted).filter({Int($0) != nil})

来源：https://stackoverflow.com/questions/30473158/dispatch-group-leave-crash-in-swift