Save array while it is being enumerated - ios

Question

I want to save an array to the NSUserDefaults every time a user interacts with the rows in a tableview (deletes them, for instance)
However, I'm encountering the following issue:
Imagine the following use case:
User deletes row -> row is deleted from datasource, datasource is saved to disk
User deletes another row -> row is deleted from datasource, app crashes because datasource of the first action is still being enumerated
How do I solve this problem? It's important that the app has live feedback to the user, so any checks that the user has to wait for are not preferred.
This is the code that I use to save the datasource to the disk:
if(allowSavePaymentArray){
dispatch_async(kAsyncQueue, ^{
allowSavePaymentArray = NO;
__block int loopCount = 0;
NSMutableArray *archiveArray = [NSMutableArray arrayWithCapacity:dataSourceArray.count];
for (NSMutableDictionary *object in dataSourceArray) {
// PFFile isn't easy to encode, but UIImage is, so whenever we encounter a PFFile, we convert it to UIImage
id imageFile = [payment objectForKey:#"img"];
if([imageFile isKindOfClass:[PFFile class]]){
[imageFile getDataInBackgroundWithBlock:^(NSData *imageData, NSError *error) {
if (!error) {
UIImage *image = [UIImage imageWithData:imageData];
[payment setObject:image forKey:#"img"]; // the PFFile is now replaced for an UIImage
NSData *paymentEncodedObject = [NSKeyedArchiver archivedDataWithRootObject:payment];
[archiveArray addObject:paymentEncodedObject];
loopCount++;
if(loopCount == [paymentArray count]){ // when done looping, save it all
NSUserDefaults *userData = [NSUserDefaults standardUserDefaults];
[userData setObject:archiveArray forKey:#"payments"];
allowSavePaymentArray = YES;
}
}
}];
} else {
loopCount++;
NSData *paymentEncodedObject = [NSKeyedArchiver archivedDataWithRootObject:payment];
[archiveArray addObject:paymentEncodedObject];
if(loopCount == [paymentArray count]){ // when done looping
NSUserDefaults *userData = [NSUserDefaults standardUserDefaults];
[userData setObject:archiveArray forKey:#"payments"];
allowSavePaymentArray =YES;
}
}
}
});
}

Answer 1

Your issue is that you are using fast enumeration on a background thread and you have the likely case that the array being enumerated on the background thread will be modified on the main (or some other) thread.
The fix is simple enough. Create a copy of the array:
__block int loopCount = 0;
NSMutableArray *archiveArray = [NSMutableArray arrayWithCapacity:dataSourceArray.count];
NSArray *tempArray = [dataSourceArray copy]; // make a copy
for (NSMutableDictionary *object in tempArray) {

Answer 2

You may use a serial queue, this will execute one process at the time.
dispatch_queue_t kAsyncQueue;
kAsyncQueue = dispatch_queue_create("com.example.MyQueue", NULL);
Here the documentation from apple:
https://developer.apple.com/library/mac/documentation/Performance/Reference/GCD_libdispatch_Ref/index.html#//apple_ref/c/func/dispatch_queue_create

Answer 3

When you finished working with your array on your background queue, spin off the block on main thread to make sure that you have everything going in certain order.