Handing a completion with CloudKit - ios

Question

I have an app that has species and photos. I am adding cloudKit to the app. I have a working solution, but now I need to add a completion handler as if the user downloads new species that include images, this takes some time (of course depending on how many images). However, the app allows the user to work during most of this process as it runs in the background.
The issue is if an image is not yet fully downloaded and the user select that species the app crashes, naturally.
I need to input a completion handler (or if someone has a better idea) that will allow me to use an activity indicator until the full process is completed. I found a few examples, but they don't take into account multiple download processes, like my images and thumbnails.
Here is my code. Note that I have removed some of the irrelevant code to reduce the amount shown.
func moveSpeciesFromCloud() {
let predicate = NSPredicate(value: true)
let query = CKQuery(recordType: RemoteRecords.speciesRecord, predicate: predicate)
CKDbase.share.privateDB.perform(query, inZoneWith: nil) {
records, error in
if error != nil {
print(error!.localizedDescription)
} else {
guard let records = records else { return }
for record in records {
DispatchQueue.main.async {
self.remoteVersion = record[RemoteSpecies.remoteSpeciesVersion] as! Int
self.remoteSpeciesID = record[RemoteSpecies.remoteSpeciesID] as! Int
self.speciesDetail = AppDelegate.getUserDatabase().getSpeciesDetails(self.remoteSpeciesID)
self.localVersion = self.speciesDetail.version
// being sure that remote version is newer than local version
if self.localVersion >= self.remoteVersion {
print("Species version not newer")
} else {
self.commonNameLabel = record[RemoteSpecies.remoteCommonName] as! String
self.speciesLabel = record[RemoteSpecies.remoteSpeciesName] as! String
self.genusLabel = record[RemoteSpecies.remoteGenusName] as! String
self.groupLabel = record[RemoteSpecies.remoteGroupName] as! String
self.subGroupLabel = record[RemoteSpecies.remoteSubGroupName] as! String
self.speciesDetailsLabel = record[RemoteSpecies.remoteSpeciesDetails] as! String
// Here I sync records to SQLite, but removed code as not relevant.
// now syncing Photos, Thumbs, Groups, SubGroups and Favorties
self.syncPhotosFromCloud(self.remoteSpeciesID)
self.syncThumbsFromCloud(self.remoteSpeciesID)
}
}
}
}
}
}
Here is the code for the Thumbnails (Images are same process)
func syncThumbsFromCloud(_ id: Int) {
let predicate = NSPredicate(format: "thumbSpeciesID = \(id)")
let query = CKQuery(recordType: RemoteRecords.thumbsRecord, predicate: predicate)
CKDbase.share.privateDB!.perform(query, inZoneWith: nil)
{
records, error in
if error != nil {
print(error!.localizedDescription)
} else {
guard let records = records else { return }
for record in records {
DispatchQueue.main.async {
self.thumbName = (record.object(forKey: RemoteThumbs.remoteThumbName) as? String)!
self.thumbID = (record.object(forKey: RemoteThumbs.remoteThumbID) as? Int)!
if let asset = record[RemoteThumbs.remoteThumbFile] as? CKAsset,
let data = try? Data(contentsOf: (asset.fileURL)),
let image = UIImage(data: data)
{
let filemgr = FileManager.default
let dirPaths = filemgr.urls(for: .documentDirectory,
in: .userDomainMask)
let fileURL = dirPaths[0].appendingPathComponent(self.thumbName)
if let renderedJPEGData = image.jpegData(compressionQuality: 1.0) {
try! renderedJPEGData.write(to: fileURL)
}
}
// syncing records to SQLite
AppDelegate.getUserDatabase().syncThumbsFromCloudToSQLite(id: self.thumbID, name: self.thumbName, speciesID: id)
}
}
}
}
}
I call it here on SyncVC:
#IBAction func syncCloudToDevice(_ sender: Any) {
let cloudKit = CloudKit()
cloudKit.moveSpeciesFromCloud()
cloudKit.moveFavoritessFromCloud()
}
If I missed a detail, please let me know.
Any assistance would be greatly appreciated.

Answer 1

I'm kind of concerned that both the previous answers don't help answer your question.. One is asking you to restructure your database and the other is asking you to become dependent on a third-party library.
My suggestion would be to make your perform(_:inZoneWith:) into a synchronous operation so that you can easily perform one after another. For example:
func performSynchronously(query: CKQuery) throws -> [CKRecord] {
var errorResult: Error?
var recordsResult: [CKRecord]?
let semaphore = DispatchSemaphore(value: 0)
CKDbase.share.privateDB!.perform(query, inZoneWith: nil) { records, error in
recordsResult = records
errorResult = error
semaphore.signal()
}
// Block this thread until `semaphore.signal()` occurs
semaphore.wait()
if let error = errorResult {
throw error
} else {
return recordsResult ?? []
}
}
Ensure that you call this from a background thread so as to not block your UI thread! For example:
// ... start your activity indicator
DispatchQueue(label: "background").async {
do {
let records1 = try performSynchronously(query: CKQuery...)
// parse records1
let records2 = try performSynchronously(query: CKQuery...)
// parse records2
DispatchQueue.main.async {
// stop your activity indicator
}
} catch let e {
// The error e occurred, handle it and stop the activity indicator
}
}
Of course, please just use this code as inspiration on how to use a semaphore to convert your asynchronous operations into synchronous ones. Here's a good article that discusses semaphores in depth.

Answer 2

Well, in general that sort of things are easy to do with RxSwift. You set activity indicator to on/off in .onSubscribe() and .onTerminated(), respectively, and you get the end result in subscriber/observer when it is ready. Specifically for CloudKit, you can use RxCloudKit library.

Answer 3

Is there a reason why you made the pictures a separate record type? I would just add the thumbnail and the full photo to the Species record type:
thumbnail = Bytes data type (1MB max)
photo = Asset data type (virtually limitless)
That way when you do your initial Species query, you will instantly have your thumbnail available, and then you can access the CKAsset like you are currently doing and it will download in the background. No second query needed which will make your code simpler.