Waiting on API response before deleting from SQLite DB - ios

Question

I am having trouble coming up with an efficient way of deleting information from a SQLite DB after a JSON response.
Basically how what I am trying to do is:
I have a DB with 100 records.
I read the first record
Build a JSON package and send it to my API.
WAIT for the API to respond Ok or Fail
The delete the record from the DB on OK.
Move to the next record.
There is some time between waiting on a response before I can delete and go on.
A simple FOR doesn't seem to work as it can't wait for the 'OK' from the API.
I looked into dispatch_async(dispatch_get_main_queue()); but if the OK comes and the DB is processing it will lock the DB while it is being accessed and the logic fails because the db is locked.
I need a mixture of the two. Does anyone know of a tutorial or where to start with something like this?

Answer 1

Use an operation queue and your own custom operation. Setup the operation queue to be serial or set it to allow some small number of concurrent operations.
Loop through each record creating an operation for each record. Put each operation on the queue.
The implementation of the operation will be to make the API call and then delete the one record if that API call indicates it should be deleted. Ensure that the operation's main doesn't reach its end until the API call is complete.

Answer 2

Create an array where you will hold temporarily your data to delete
NSMutableArray* itemsToDelete = [[NSMutableArray alloc] init]
Once you received a response with success you append the items to be deleted to this array. do the same process in all calls with success. once your done with uploading your data at your last response of success or failure check your itemsToDelete array inside a for loop and delete everything from your DB

Answer 3

I suggest using blocks to receive the response from async calls. In that case, you can perform the DB operation within the block.
dispatch_async(dispatch_get_main_queue()); is not for the async calls. It is mostly used to update the UI.
All the async calls needs to be done on background thread.
Coming to your problem, you can do something like this:
-(void)checkToDeleteWithCompletion:(MyCompletion)completion{
__block NSMutableArray *responseArray = [NSMutableArray array];
// Create a dispatch group
dispatch_group_t group = dispatch_group_create();
for (int i = 0; i < yourArray.count; i++) {
// Enter the group for each request we create
dispatch_group_enter(group);
[self ..methodToPErformAsynccallWithCOmpleton success:^(BOOL success, id response) {
dispatch_group_leave(group);
[responseArray addObject:response];
} failure:^(NSError *error, NSInteger statusCode) {
dispatch_group_leave(group);
}];
}
// Here we wait for all the requests to finish
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
// Do whatever you need to do when all requests are finished
completion(YES,[responseArray copy]);
});
}
And finally you can use a for loop on responseArray, to delete the unwanted objects.
You can also limit the number of concurrent calls by using :
dispatch_semaphore_create(maxConcurrencyValue)
Or you can use NSOperation as suggested by rmaddy!