Create serial queue for network calls generated by push notifications - ios

Question

I am receiving up to four push notifications for each event I am subscribed to. I have gone through everything related to my CloudKit subscriptions and notification registry and I am convinced this is an Apple problem. I have instead turned my attention toward correctly processing the notifications no matter how many I receive. Here is a simplified version of what I am doing:
func recievePrivatePush(_ pushInfo: [String:NSObject], completion: #escaping ()->Void) {
let notification = CKNotification(fromRemoteNotificationDictionary: pushInfo)
let alertBody = notification.alertBody
if let queryNotification = notification as? CKQueryNotification {
let recordID = queryNotification.recordID
guard let body = queryNotification.alertBody else {
return
}
if recordID != nil {
switch body {
case "Notification Type":
let id = queryNotification.recordID
switch queryNotification.queryNotificationReason {
case .recordCreated:
DataCoordinatorInterface.sharedInstance.fetchDataItem(id!.recordName, completion: {
//
})
break
default:
break
}
}
}
}
}
The fetching code looks something like this:
func fetchDataItem(_ id: String, completion: #escaping ()-> Void) {
if entityExistsInCoreData(id) {return}
let db = CKContainer.default().privateCloudDatabase
let recordID = CKRecordID(recordName: id)
db.fetch(withRecordID: recordID) { (record, error) in
if let topic = record {
//Here I create and save the object to core data.
}
completion()
}
}
All of my code works, the problem I am having is that when I receive multiple notifications, multiple fetch requests are started before the first core data entity is created, resulting in redundant core data objects.
What I would like to do is find a way to add the fetch requests to a serial queue so they are processed one at a time. I can put my request calls in a serial queue, but the callbacks always run asynchronously, so multiple fetch requests are still make before the first data object is persisted.
I have tried using semaphores and dispatch groups with a pattern that looks like this:
let semaphore = DispatchSemaphore(value: 1)
func recievePrivatePush(_ pushInfo: [String:NSObject], completion: #escaping ()->Void) {
_ = semaphore.wait(timeout: .distantFuture)
let notification = CKNotification(fromRemoteNotificationDictionary: pushInfo)
let alertBody = notification.alertBody
if let queryNotification = notification as? CKQueryNotification {
let recordID = queryNotification.recordID
guard let body = queryNotification.alertBody else {
return
}
if recordID != nil {
switch body {
case "Notification Type":
let id = queryNotification.recordID
switch queryNotification.queryNotificationReason {
case .recordCreated:
DataCoordinatorInterface.sharedInstance.fetchDataItem(id!.recordName, completion: {
semaphore.signal()
})
break
default:
break
}
}
}
}
}
Once the above function is called for the second time, and semaphore.wait is called, the execution of the first network request pauses, resulting in a frozen app.
Again, what I would like to accomplish it adding the asynchronous network requests to a queue so that they are made only one at a time i.e. the first network call is completed before the second request is started.

Answer 1

Carl,
Perhaps you'll find your solutions with dispatch groups, a few key expressions to look into.
let group = DispatchGroup()
group.enter()
... code ...
group.leave
group.wait()
I use them to limit the number of http requests I send out in a batch, to wait for the response. Perhaps you could use them together with the suggestion in my comment. Watch this video too, dispatch groups in here, I think more.
https://developer.apple.com/videos/play/wwdc2016/720/

Answer 2

These simple classes helped me solve the problem.
class PushQueue {
internal var pushArray: Array<String> = [String]()
internal let pushQueue = DispatchQueue(label: "com.example.pushNotifications")
public func addPush(_ push: Push) {
pushQueue.sync {
if pushArray.contains(push.id) {
return
} else {
pushArray.append(push.id)
processNotification(push: push)
}
}
}
internal func processNotification(push: Push) {
PushInterface.sharedInstance.recievePrivatePush(push.userInfo as! [String: NSObject])
}
}
class CKPush: Equatable {
init(userInfo: [AnyHashable: Any]) {
let ck = userInfo["ck"] as? NSDictionary
let id = ck?["nid"] as? String
self.id = id!
self.userInfo = userInfo
}
var id: String
var userInfo: [AnyHashable:Any]
public static func ==(lhs: CKPush, rhs: CKPush) -> Bool {
return lhs.id == rhs.id ? true : false
}
}
Please ignore the sloppy force unwraps. They need to be cleaned up.