How to make a serial queue with GCD - swift

Question

I tried to make a serial queue for network operations with GCD like this:
let mySerialQueue = dispatch_queue_create("com.myApp.mySerialQueue", dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, QOS_CLASS_USER_INITIATED, 0))
func myFunc() {
dispatch_async(mySerialQueue) {
do {
// Get object from the database if it exists
let query = PFQuery(className: aClass)
query.whereKey(user, equalTo: currentUser)
let result = try? query.getFirstObject()
// Use existing object or create a new one
let object = result ?? PFObject(className: aClass)
object.setObject(currentUser, forKey: user)
try object.save()
} catch {
print(error)
}
}
}
The code first looks for an existing object in the database.
If it finds one, it updates it. If it doesn't find one, it creates a new one. This is using the Parse SDK and only synchronous network functions (.getFirstObject, .save).
For some reason it seems that this is not executed serially, because a new object is sometimes written into the database, although one existed already that should have been updated only.
Am I missing something about the GCD?

Answer 1

From the documentation on dispatch_queue_attr_make_with_qos_class:
relative_priority: A negative offset from the maximum supported scheduler priority for the given quality-of-service class. This value must be less than 0 and greater than MIN_QOS_CLASS_PRIORITY
Therefore you should be passing in a value less than 0 for this.
However, if you have no need for a priority, you can simply pass DISPATCH_QUEUE_SERIAL into the attr argument when you create your queue. For example:
let mySerialQueue = dispatch_queue_create("com.myApp.mySerialQueue", DISPATCH_QUEUE_SERIAL)