ios - 如何在后台执行计数获取请求

Question

我如何在后台执行计数请求，下面的代码严重阻塞了我的 UI。我尝试实现 NSAsynchronousFetchRequest 但无法将其组合在一起。任何帮助将不胜感激!

        let NewMessage: NSFetchRequest<Mesages> = Mesages.fetchRequest()

        NewMessage.predicate = NSPredicate(format: "timestamp = %@", date)

        do {
           //context.fetch blocking the main thread
            let checkMessages = try context.fetch(NewMessage)
            if checkMessages.count == 0 {

             print('no duplicates')

            }
        } catch  {
        }

Answer 1

对本地托管对象使用 PrivateQueueConcurrencyType 并使用 countForFetchRequest 方法代替 fetch :

let context: NSManagedObjectContext =   NSManagedObjectContext.init(concurrencyType: .PrivateQueueConcurrencyType)
// set context with parent child hierarchy.
let NewMessage: NSFetchRequest = NSFetchRequest.init(entityName: "Message")
var error:NSError? ;

let messagesCount = context.countForFetchRequest(NewMessage, error:&error)
print(messagesCount)