Best way to do a massive data sync from HealthKit? - objective-c

Question

I am looking for a smart way to import ALL data for ALL time from HealthKit in a smart way for high volume users who log certain HealthKit values hundreds of times a day for months or years. I'm also looking to do this off the main thread and in a way that is robust to the fact that users can close the app whenever they feel like. Here's what my current implementation is:
- (void) fullHealthKitSyncFromEarliestDate {
dispatch_queue_t serialQueue = dispatch_queue_create("com.blah.queue", DISPATCH_QUEUE_SERIAL);
NSLog(#"fullHealthKitSyncFromEarliestDate");
NSDate *latestDate = [PFUser currentUser][#"earliestHKDate"];
if (!latestDate) latestDate = [NSDate date];
NSDate *earliestDate = [healthStore earliestPermittedSampleDate];
NSLog(#"earliest date %# and latest date %#", earliestDate, latestDate);
if ([earliestDate earlierDate:[[NSDate date] dateByAddingYears:-2]] == earliestDate) {
earliestDate = [[NSDate date] dateByAddingYears:-1];
}
__block NSDate *laterDate = latestDate;
__block NSDate *earlierDate = [latestDate dateByAddingMonths:-1];
int i = 0;
while ([earlierDate earlierDate:earliestDate] == earliestDate) {
// DISPATCH_QUEUE_PRIORITY_DEFAULT
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(i * 30 * NSEC_PER_SEC)), serialQueue, ^{
NSLog(#"calling sync from %# to %#", earlierDate, laterDate);
[self syncfromDate: earlierDate toDate:laterDate];
laterDate = [laterDate dateByAddingMonths:-1];
earlierDate = [laterDate dateByAddingMonths:-1];
earlierDate = [[earliestDate dateByAddingSeconds:1] laterDate:earlierDate];
});
}
}
I am having a few problems with the above code:
earlierDate is sometimes not getting updated for the next iteration. It's like there's some weird overlap that sometimes the same date range gets called twice or even more times.
Kernel memory allocation issues (as I retrieve objects I queue them for upload to a remote server)
Interface stops responding entirely even though this is all happening off the main queue. There may be something way down stream that gets called at the end of the entire query...but I'm still surprised the interface basically stops responding altogether. It usually does fine with similar syncing for short time periods. And here I'm just doing a month at a time in queries spaced 30 seconds apart
I realize without the details of additional code this may be too murky to address, but can people tell me what sort of solutions you have implemented for cases where you want to retrieve massive amounts of data from HealthKit. I realize I may get responses like 'that's what HealthKit is for...why are you duplicating all that data?'. Suffice it to say it's required and there's no way around it.
Also, am I just misunderstanding entirely how to use a serial queue?
Ideally I'd like to do the following: (1) Run only one query at a time in this for loop (2) and run them with some reasonable period of time between them, say 30 seconds apart, because at the end of each query I do need to do some interface updates on the main qeue (3) do all the queries on a background loop in such a way that it's not degrading interface performance and can be killed at any time the user decides he's gonna kill the app. That's what I thought I would achieve with the above...if folks can tell me where my threading is going wrong I'd really appreciate it. Thanks!