objective c - Realm migration race condition -

how can guarantee realm migrations finish before accessing realm? there seems race condition in migrating realm db , beginning read/write realm. here's issue:

currently setting realm configurations migration closure when user launches app. once realm configuration has been set rlmrealmconfiguration.setdefaultconfiguration(config), set rootviewcontroller , begin accessing realm data.

however, when migration needed, realm accessed before migrations can complete - causing crash (rlmexception: object has been deleted or invalidated). if migration async task no callback, how can guarantee finishes before accessing realm?

here's realm config code:

 class func setrealmconfigurationforuser(userid: string) {     let config = rlmrealmconfiguration.defaultconfiguration()      // migration setup     config.schemaversion = 10     config.migrationblock = { migration, oldschemaversion in         if oldschemaversion < 2 {             migration.enumerateobjects(contactuser.classname()) { oldobject, newobject in                 // change primary key phonenumber phone_baseid_key                 let phone = oldobject?["phonenumber"]                 let baseid = oldobject?["baseid"]                  newobject?["phone_baseid_key"] = string(phone) + "_" + string(baseid)             }         }         if oldschemaversion < 3 {             migration.enumerateobjects(digitscontact.classname()) { oldobject, newobject in                 newobject?["externalid"] = ""             }         }         if oldschemaversion < 9 {             migration.deletedataforclassname(digitscontact.classname())             migration.deletedataforclassname(address.classname())             migration.deletedataforclassname(email.classname())             migration.deletedataforclassname(phonenumber.classname())         }     }      // use default directory, replace filename userid     config.path = (((config.path! nsstring).stringbydeletinglastpathcomponent nsstring)                                             .stringbyappendingpathcomponent(userid) nsstring)                                             .stringbyappendingpathextension("realm")     rlmrealmconfiguration.setdefaultconfiguration(config) 

according realm's migrations docs, migrations applied when opening realm configuration on schema version , migration blocks set:

when creating realm configuration, migration block applied update realm given schema version if migration needed.

so if you're calling setrealmconfigurationforuser asynchronously, concurrent realm accesses use previous default configuration , not see migration block you're in process of defining.

performing migration happens synchronously, either implicitly creating realm migration block in configuration or explicitly calling rlmrealm.migraterealm(_:).

all other realm accesses (both reads & writes) blocked on migration completing. long migration block & configuration set before realm initialized, there should no race condition.

ps: you're doing lot of wasted work in migration block when oldschema < 3 because you're setting digitscontact.externalid properties "", delete digitscontact objects. removing oldschemaversion < 3 conditional , contents lead same result.

pps: there reason you're using realm objective-c swift rather realm swift? realm has dedicated api that's nicer use swift: https://realm.io/docs/swift/latest/