Stop Reloading a UITableView Inside a Loop - ios

Question

I have the following code where I am calling self.tableView reloadData inside the loop which is a bad idea. If I put it outside the loop then it would not work as expected since it will be called before updating the noOfGroceryItems field.
How can I improve it?
-(void) populateShoppingLists {
[_groceriesService getAllShoppingLists:^(NSArray *results, NSError *error) {
if(error) {
NSLog(#"%#",error.localizedDescription);
}
for(CKRecord *record in results) {
ShoppingList *shoppingList = [[ShoppingList alloc] initWithRecord:record];
// get the number of grocery items in the shopping list
[_groceriesService getItemsByShoppingList:shoppingList result:^(NSArray *results, NSError *error) {
shoppingList.noOfGroceryItems = results.count;
[_shoppingLists addObject:shoppingList];
// THIS IS BAD IDEA
**dispatch_async(dispatch_get_main_queue(), ^{
_shoppingLists = [_shoppingLists sort:#"title" ascending:YES];
[self.tableView reloadData];
});**
}];
}
}];
}
Implementation for getItemsShoppingList method:
-(void) getItemsByShoppingList:(ShoppingList *)shoppingList result:(GetItemsByShoppingList) getItemsByShoppingList
{
CKQuery *query = [[CKQuery alloc] initWithRecordType:#"GroceryItems" predicate:[NSPredicate predicateWithFormat:#"ShoppingList == %#",shoppingList.record]];
[_privateDB performQuery:query inZoneWithID:nil completionHandler:^(NSArray *results, NSError *error) {
getItemsByShoppingList(results,error);
}];
}

Answer 1

You could add a conditional to reload the table if and only if the last shopping list has been fetched, ex:
[_groceriesService getAllShoppingLists:^(NSArray *results, NSError *error) {
if(error) {
NSLog(#"%#",error.localizedDescription);
}
// Variable to count the number
// of records processed
__block int recordsProcessed = 0;
for(CKRecord *record in results) {
ShoppingList *shoppingList = [[ShoppingList alloc] initWithRecord:record];
// (I've changed this second results variable from results
// to results2 in order to distinguish between the two
// "results" variables.)
// get the number of grocery items in the shopping list
[_groceriesService getItemsByShoppingList:shoppingList
result:^(NSArray *results2, NSError *error) {
shoppingList.noOfGroceryItems = results2.count;
[_shoppingLists addObject:shoppingList];
dispatch_async(dispatch_get_main_queue(), ^{
_shoppingLists = [_shoppingLists sort:#"title" ascending:YES];
// Increment recordsProcessed to indicate another
// record has been processed
recordsProcessed ++;
// If all the records have been processed,
// reload the table (using the outer block's
// results variable, not the inner block's result2
// variable).
if (recordsProcessed == results.count) {
[self.tableView reloadData];
}
});
}];
}
}];
Update: And a second solution. Assuming that _shoppingLists starts out empty, you can simply compare results with the number of elements put into _shoppingLists, ex:
[_groceriesService getAllShoppingLists:^(NSArray *results, NSError *error) {
if(error) {
NSLog(#"%#",error.localizedDescription);
}
for(CKRecord *record in results) {
ShoppingList *shoppingList = [[ShoppingList alloc] initWithRecord:record];
// (I've changed this second results variable from results
// to results2 in order to distinguish between the two
// "results" variables.)
// get the number of grocery items in the shopping list
[_groceriesService getItemsByShoppingList:shoppingList
result:^(NSArray *results2, NSError *error) {
shoppingList.noOfGroceryItems = results2.count;
[_shoppingLists addObject:shoppingList];
dispatch_async(dispatch_get_main_queue(), ^{
_shoppingLists = [_shoppingLists sort:#"title" ascending:YES];
// If the number of shopping lists stored
// equals the number of records processed,
// reload the table (using the outer block's
// results variable, not the inner block's result2
// variable).
if (_shoppingLists.count == results.count) {
[self.tableView reloadData];
}
});
}];
}
}];

Answer 2

If you put a dispatch_async after the for loop, you're guaranteed that the block will be dispatched on the main queue, after the loops completes (and your data is updated).
Why not just dispatch the reloadData call once after all your model are processed and updated?

Answer 3

Instead of fast enumerating the results array, you could enumerate with a block, that gives you the index of the current object. if the index is [results count] - 1, trigger the reload.
Of course you should not name two arrays results if the inner one does hide the outer, especially if you intend to call other peoples code «not a very good solution.»