ios - Core Data private context performAndWait causes debug to crash -

i have problem debugging core data concurrency issues.

in scheme enabled:

-com.apple.coredata.logging.stderr 1 -com.apple.coredata.concurrencydebug 1 

i fetch records core data in background thread.

so in uiviewcontroller create dispatchqueue.global(qos: .background).async call core data fetch records.

i using parent/child:

    func performstationsfetch(filter: string = "") -> [stations] {     let privatemoc = nsmanagedobjectcontext(concurrencytype: .privatequeueconcurrencytype)     let parentmoc = (uiapplication.shared.delegate as! appdelegate).managedobjectcontext      privatemoc.parent = parentmoc      var fetchresults: [stations] = []      privatemoc.performandwait {         let fetchrequest: nsfetchrequest<stations> = stations.fetchrequest()         fetchrequest.predicate = nspredicate(format: "name contains[c] %@", filter)          {             fetchresults = try privatemoc.fetch(fetchrequest)         } catch {             print("fetch error")         }     }      return fetchresults } 

the problem if call privatemoc.perform code works fine.

but if call privatemoc.performandwait code crashes , gives me +[nsmanagedobjectcontext __multithreading_violation_allthatislefttousishonor__]

does have suggestion can problem?

the multi-threading violation occurs when try access results of function. if use perform, results empty - there no problem. if use performandwait, returning managedobjects being accessed on wrong thread.

what worse if can bad-access-error if try use objects. managedobjects not normal nsobjects. managedobject objectid , pointer context. whenever needs access properties forwards request context. if context gets removed, objects cannot work. in code context backing results removed memory when function ends. (note: context not removed. in debug environment context may kept around little while. in production released @ end of function).

so accessing results of function illegal 2 reasons - 1) on wrong thread , 2) not having backing context.

there few solutions. 1 copy values need managedobject , return array or strings, or custom object (a nsobject subclass).

another solution fetch on main context around lifetime of application.

alternately can pass context used function. , responsibility of caller ensure context kept around long managedobjects needed.