NSAsynchronousFetchRequest test

coredata.swift

let asyncRequest = NSAsynchronousFetchRequest(fetchRequest: request) { fetchResult in
  // this may or may not right in the same thread as the context thread
  // except when the context is main thread, this will always run in main thread
  // TODO: can i access NSManagedObject safely if this is not the same thread?
  print("ttt: blk thread: \(Thread.current)")
  Thread.sleep(forTimeInterval: 100)
  guard let dataResult = fetchResult.finalResult else {
    return
  }
  print(dataResult.count)
  let summaries = aggregateSummary(result: dataResult, currencies: accountCurrencies)
  print("ttt: completed")
}

context.perform {
  print("ttt: perform thread: \(Thread.current)")
  do {
    // cannot just use asyncRequest.execute() due to this is iOS 10 and up
    let result = try context.fetch(currencyRequest)
    for data in result {
      if let id = data["id"] as? String,
         let currency = data["currency"] as? String {
        accountCurrencies[id] = currency
      }
    }
    // will finish right away
    try context.execute(asyncRequest)

  } catch let error {
    Crashlytics.crashlytics().record(error: CoreDataError.fetchError(#function, error))
  }
  print("ttt perform ended")
}