GCD and Core Data - ios

Question

I'm trying to solve a problem where fetching data from CoreData executes earlier than retrieving data from Parse and saving it to CoreData. How do I implement a queue to perform fetching data From Parse earlier? Now when I launch the app the database is updated, but it is not shown in the tableView.
Retrieving data from CoreData:
func fetchFromCoreData() {
do {
let results = try context.executeFetchRequest(fetchRequest)
medicines = results as! [Medicine]
print("FetchFromCoreData")
tableViewMedicines.reloadData()
} catch let error as NSError {
print("Could not fetch \(error), \(error.userInfo)")
}
}
Fetching data From Parse:
func fetchFromParse() {
let entity = NSEntityDescription.entityForName("Medicine", inManagedObjectContext: context)
let query = PFQuery(className: PFUser.currentUser()!.username!)
query.findObjectsInBackgroundWithBlock { (objects, error) -> Void in
if error == nil {
for object in objects! {
if let name = object["medicineName"] as? String,
amount = object["amountQuantity"] as? String,
time = object["time"] as? String
{
let predicate = NSPredicate(format: "name = %#", name)
self.fetchRequest.predicate = predicate
do{
let fetchedEntities = try self.context.executeFetchRequest(self.fetchRequest) as! [Medicine]
//save to Core Data
if fetchedEntities.count <= 0 {
let medicine = NSManagedObject(entity: entity!, insertIntoManagedObjectContext: self.context)
medicine.setValue(name, forKey: "name")
medicine.setValue(amount, forKey: "amount")
medicine.setValue(time, forKey: "time")
}
} catch let error as NSError{
print(error)
}
}
}
}
do {
try self.context.save()
print("Context.save")
} catch let error as NSError {
print("Could not save \(error), \(error.userInfo)")
}
self.fetchFromCoreData()
}
}
I'm calling them in viewWillAppear:
override func viewWillAppear(animated: Bool) {
if Reachability.isConnectedToNetwork() {
//fetching data from Parse
fetchFromParse()
fetchFromCoreData()
tableViewMedicines.reloadData()
} else {
//fetching data from Core data
fetchFromCoreData()
logOutButton.enabled = false
}
}

Answer 1

This is happening because findObjectsInBackgroundWithBlock makes a network call and only executes its block later on, asynchronously, once the network call completes. Your code executes in this order:
Call fetchFromParse()
In fetchFromParse(), call Parse. Parse goes off and talks to the network in the background...
Call fetchFromCoreData()
At some point later on, Parse gets a network response, and your block executes.
You should expect that async network calls will always take a long time, and that callbacks from network activity will always be delayed to some unknown time in the future. Meaning, you can't somehow move step 4 above earlier in the list.
GCD is not the answer here. You have to wait until the network activity finishes before you can process results from Parse. In the meantime, your app needs to be responsive, so you can't just sit around waiting for that to happen.
What you need to do is re-do the Core Data fetch after the Parse call completes. Reloading the table view isn't enough if you're still displaying the results of the earlier fetch.