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See: https://www.invasivecode.com/weblog/advanced-cloudkit-ii

01. In your project, create a new target and choose a Mac App. 
We need to enable CloudKit, as we did with our iOS App. 
Select the target that you have just created and follow the same steps, 
going to the Capabilities pane and switching on iCloud, and activating 
the CloudKit checkbox. This time we want to have access to the iCloud 
OS App and select its checkbox.

02. Now we are ready to create a CKRecord for each of our word definitions. 

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01 - 04. See: https://www.invasivecode.com/weblog/advanced-cloudkit-ii

05. See: https://www.whatmatrix.com/blog/a-guide-to-cloudkit-how-to-sync-user-data-across-ios-devices/


01. In the first method, we create a query with a predicate to fetch the records. 
We create a CKQueryOperation and pass it to the second method, along with the operation 
queue that we will use to execute it.

func updateWordDefinitionChanges() {

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See: https://www.invasivecode.com/weblog/advanced-cloudkit-i

01. Now when the App launches, instead of directly performing a fetch to CloudKit, what we are going to do first is to check if we have already subscribed to the record changes. If we are subscribed we do not need to do anything else, as we will be notified by CloudKit via remote push notifications. If we are still not subscribed (for example, the first time the user uses the App), before subscribing, we will need to perform a fetch of the record, to be sure that we have the most updated version of the data. Then, we will check if the user has an iCloud account available. In this case, we need an iCloud account to be available, because we are going to save the subscription on CloudKit, and therefore we need write permissions. If the iCloud account is available, we can create a subscription and save it on the public database. The subscription is setup to be fired anytime the record changes.

Let’s add this new method to our AppDelegate and call it w

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See: https://www.invasivecode.com/weblog/advanced-cloudkit-i

01. To access the schema of the database, click on the button CloudKit Dashboard, or 
go to https://icloud.developer.apple.com/dashboard. In the CloudKit dashboard, you 
can manage the schema of the database, create new Record Types, and add, edit and delete records.

02. Select Record Types and then click on the + button to create a new record type. 
Name the new record type WebServiceSettings. Then, add two fields of type string. 
Call them serviceURL and serviceAPIKey. By default, when you create new fields, 
indexes are added, to allow sorting, querying and searching on them. In this case, 

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See for example 1:https://www.invasivecode.com/weblog/advanced-cloudkit-i

See for example 2: https://stackoverflow.com/questions/44023936/cloudkit-full-and-complete-error-handling-example

01. Example 1

For example, if an operation cannot be performed because the service is unavailable, or you 
have exceeded the rate limits, there will be an entry in the dictionary for the key 
CKErrorRetryAfterKey. Its value is a NSNumber containing the number of seconds after 
you should retry the request:

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01. Save and Delete Records at the same time

let recordsToSave: [CKRecord] ...
let recordIDsToDelete: [CKRecordID] ...
let saveRecordsOperation = CKModifyRecordsOperation(
      recordsToSave: recordsToSave,
      recordIDsToDelete: recordIDsToDelete
)
saveRecordsOperation.modifyRecordsCompletionBlock = { savedRecords, deletedRecordIDs, error in
    // handle errors here

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01. Example of saving a single record

let privateDB = CKContainer.defaultContainer().privateCloudDatabase
let record = CKRecord(recordType: “Employee”)
record.setObject(“John”, forKey: “name”)
record.setObject(“Stephens Green West, Dublin 2, Dublin”, forKey: “address”)
privateDB.saveRecord(record) { savedRecord, error in
    // handle errors here
}

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01. Using a query with a predicate to get a list of records. 

let privateDB = CKContainer.defaultContainer().privateCloudDatabase
let predicate = NSPredicate(format: "firstName = %@", "John")
let query = CKQuery(recordType: "Artist", predicate: predicate)
privateDB.performQuery(query, inZoneWithID: nil) { records, error in
   guard let records = records else { return }
   //Use the records..
}

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CKContainer.defaultContainer().accountStatusWithCompletionHandler{ status, error in
   guard status == .Available else { return }
   //The user’s iCloud account is available..
}

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01. See:https://stackoverflow.com/questions/43553969/handling-cloudkit-errors
02. See: https://www.whatmatrix.com/blog/a-guide-to-cloudkit-how-to-sync-user-data-across-ios-devices/
03. See CloudKit error codes: https://developer.apple.com/documentation/cloudkit/ckerrorcode
04. https://stackoverflow.com/questions/44023936/cloudkit-full-and-complete-error-handling-example

01. Helper function to manage CloudKit Errors

import Foundation
import CloudKit