How to make app more responsible? - ios

Question

When launching app first, app do prefill its local persistent store from backend. It happens on the DISPATCH_QUEUE_PRIORITY_LOW, but it tears when user scroll in tableView meantime. What else can I do? Heavy stuff are already on lowest priority.

Answer 1

DISPATCH_QUEUE_PRIORITY_BACKGROUND has even lower priority. If that does not help I think you could:
Pre-fill on a serial queue. Most iOS devices have more than 2 cores and 1 of those should be able to handle table scrolling.
Pre-fill slower. It may be that you fill the memory-bandwidth or flush the L2-cache during your pre-fill. This could be hard to solve. Maybe you can periodically reload all visible table cells to keep that code from going stale, but it may also interfere with user scrolling.

Answer 2

You could do a couple of things I believe
1) you could start with an empty dataset and when you're done getting all the data use ''' self.tableview.reloadData() ''' in your building block.
2) if that's not possible, then you could always present a loader so the rest of the UI is disabled while the data is being created, this one is pretty easy to use https://github.com/jdg/MBProgressHUD

Answer 3

I have one idea, but here i suggest use NSOperation (if you have enough time to refactor some of your code). You can organize your download process throw NSOperationQueue.
For example,
- (NSOperationQueue *)downloadQueue {
if (!_downloadQueue) {
_downloadQueue = [[NSOperationQueue alloc] init];
_downloadQueue.name = #"Download Queue";
_downloadQueue.maxConcurrentOperationCount = 1;
}
return _downloadQueue;
}
Subclass NSOperation add override main func, where you can write your donwload code
- (void)main {
// 4
#autoreleasepool {
// download code here
}
Next step - check when user start scrolling tableview (or whatever user interaction you want) and start/ stop operations executing on that event). for example, for uitableview it will look like:
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView {
// 1
[self.model suspendAllOperations];
}
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
// 2
[self.model resumeAllOperations];
}
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
[self.model resumeAllOperations];
}
Model here is a NSObject subclass, that handles all download process (independently of UI) where you can suspend/resume operations.
- (void)suspendAllOperations {
[_downloadQueue setSuspended:YES];
}
- (void)resumeAllOperations {
[_downloadQueue setSuspended:NO];
}
- (void)cancelAllOperations {
[_downloadQueue cancelAllOperations];
}
So, when you expect heavy operations in your UI, you can stop your background process and resume it when you need. Also you can change maxConcurrentOperationCount for your best performance (this param you can set after some testing / measurement )
Hope this helps.