Blocks in Objective-C and Multithreds - asynchronous

Question

Intro:
On my Xcode project i’ve 3 datastore :SFGroupsItemStore, SFFeedsItemStore, SFQuestsItemStore. All of this class are subclass of 1 my custom class and all are singleton.
Each of this class i use at next controllers : SFGroupsListViewController SFFeedsListViewController, SFQuestsListViewController. All of this class has method: -downloadItems
Problem:
I want to create one controller SFHomeViewController, that will present table with all 3 datastore mixed.
For resolve this problem i want to create aggregator SFHomeItemStore, also singleton, and i want that this class has method -downloadItems.
So, i’v done it, there are the implemaemtation of -downloadIItems in SF HomeItemStore
1 step:
- (NSArray *)downloadItemsFrom:(int32_t)from withLimit:(int)limit andDirection:(NSString *)direction{
NSArray *groupsItems = [[SFGroupsItemStore shared] downloadItems];
NSArray *feedsItems = [[SFFeedsItemStore shared] downloadItems];
NSArray *questsItems = [[SFQuestsItemStore shared] downloadItems];
NSMutableArray *mixedArray = [NSMutableArray array];
[mixedArray addObjectsFromArray:groupsItems];
[mixedArray addObjectsFromArray:feedsItems];
[mixedArray addObjectsFromArray:questsItems];
NSArray *sortedArray = [mixedArray sortedArrayUsingDescriptors:[NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:#"createdAt" ascending:NO]]];
return sortedArray;
}
That work’s fine for me.
2 step:
But i do not like that call of method -downloadItems goes sequentaly, that why for call this method i need to spend more time
But if i add GCD i’ve a problem,
Now implementation method -downloadItems is
- (NSArray *)downloadItemsFrom:(int32_t)from withLimit:(int)limit andDirection:(NSString *)direction{
__block NSArray *sortedArray = nil;
__block NSArray *groupsItems;
__block NSArray *feedsItems;
__block NSArray *questsItems;
dispatch_group_t groupedTask = dispatch_group_create();
dispatch_queue_t globalQueue = dispatch_get_global_queue( 0, 0 );
dispatch_group_async( groupedTask, globalQueue, ^{
groupsItems = [[SFGroupsItemStore shared] downloadItems];
} );
dispatch_group_async( groupedTask, globalQueue, ^{
feedsItems = [[SFFeedsItemStore shared] downloadItems];
} );
dispatch_group_async( groupedTask, globalQueue, ^{
questsItems = [[SFQuestsItemStore shared] downloadItems];
} );
dispatch_group_notify( groupedTask, globalQueue, ^{
NSMutableArray *mixedArray = [NSMutableArray array];
[mixedArray addObjectsFromArray:groupsItems];
[mixedArray addObjectsFromArray:feedsItems];
[mixedArray addObjectsFromArray:questsItems];
sortedArray = [mixedArray sortedArrayUsingDescriptors:[NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:#"createdAt" ascending:NO]]];
} );
return sortedArray;
}
Problem is that method return result regardless of dispatch_group_notify and that why array sortedArray is empty
So, my question is:
How i can do that method will return result only after end of dispatch_group_notify function?

Answer 1

You have two options:
If it's ok for your method to block the current thread, then you can use dispatch_group_wait.
Use a completion handler to return your NSArray.
dispatch_group_wait
From the docs:
Waits synchronously for the previously submitted block objects to
complete; returns if the blocks do not complete before the specified
timeout period has elapsed.
Your method would look something like this:
- (NSArray *)downloadItemsFrom:(int32_t)from withLimit:(int)limit andDirection:(NSString *)direction{
// ...
dispatch_group_async( groupedTask, globalQueue, ^{
groupsItems = [[SFGroupsItemStore shared] downloadItems];
} );
// ...
dispatch_group_wait(groupedTask, DISPATCH_TIME_FOREVER);
NSMutableArray *mixedArray = [NSMutableArray array];
[mixedArray addObjectsFromArray:groupsItems];
[mixedArray addObjectsFromArray:feedsItems];
[mixedArray addObjectsFromArray:questsItems];
sortedArray = [mixedArray sortedArrayUsingDescriptors:[NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:#"createdAt" ascending:NO]]];
return sortedArray;
}
Using a completion handler
This is the method I prefer.
You'd need to change your method signature so that it takes a completion handler as a new parameter, and returns void instead of NSArray. With this method, you'd still be using dispatch_group_notify.
It would look something like this:
- (void)downloadItemsFrom:(int32_t)from
withLimit:(int)limit
direction:(NSString *)direction
andCompletionHandler:(void (^)(NSArray*))completionHandler
{
// ...
dispatch_group_async( groupedTask, globalQueue, ^{
groupsItems = [[SFGroupsItemStore shared] downloadItems];
} );
// ...
dispatch_group_notify( groupedTask, globalQueue, ^{
NSMutableArray *mixedArray = [NSMutableArray array];
[mixedArray addObjectsFromArray:groupsItems];
[mixedArray addObjectsFromArray:feedsItems];
[mixedArray addObjectsFromArray:questsItems];
sortedArray = [mixedArray sortedArrayUsingDescriptors:[NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:#"createdAt" ascending:NO]]];
completionHandler(sortedArray);
} );
}
Calling it would now look like:
[object downloadItemsFrom:...
withLimit:...
diretion:...
andCompletionHandler:^(NSArray *items)
{
}];