How to update a property of a ViewController from data retrieved asynchronously? - iphone

Question

I would like to update a property of my ViewController self.matchedUsers, which takes data from a query that I run asynchronously through a block.
Then somewhere later When I retrieve the count via [self.matchedUsers count], I still get 0, despite knowing that multiple objects was added to my property. My question is, how do I ensure that my property gets updated even when I am retrieving data asynchronously through a block? Thanks!
Update:
For context, here is the block:
//Way earlier in an initializer:
self.matchedUsers = [[NSMutableArray alloc] init];
//In a method much later
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
if (!error){
//Code that updates self.matchedUsers with the NSArray objects
dispatch_async(dispatch_get_main_queue(), ^{
[self.matchedUsers addObjectsFromArray: objects];
});
//Question relates to ensure how property could be updated
}
}];

Answer 1

This should work provided you didn't forget to initialize matchedUsers, you check for its value after it's been changed and array does not lose its elements between the time you schedule and execute the block.
However, I would prefer to write a method that can be called from any thread, say
- (void)addUser ...
#synchronized(self.usersToAdd) {
[self.usersToAdd addObjectsFromArray: array];
Enqueue on main thread {
NSArray *addingNow;
#synchronized(self.usersToAdd) {
addingNow = [self.usersToAdd copy];
[self.usersToAdd removeObjects...
}
if (addingNow.count) {
[self.users addObjectsFromArray: addingNow;
[self.tableView insertRowsWithIndexPaths...
}
}
}
}

Answer 2

As others have written the problem could be missing initialization of matchedUsers but...
...the problem could also be due to your main thread being blocked. You write that you "somewhere later retrieve the count". If that is within the same method as the one that made the first dispatch you will be in trouble. Consider this code
NSMutableArray *collection = [[NSMutableArray alloc] init];
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSArray *array = [[NSArray alloc] initWithObjects:#"1", nil];
dispatch_async(dispatch_get_main_queue(), ^{
for (NSString *item in array){
[collection addObject:item];
}
NSLog(#"A");
});
});
[NSThread sleepForTimeInterval:5];
NSLog(#"B");
If this is running on the main thread it will output first B on then A (no matter the sleep time), because the block is not run until the method finishes executing. If you on the other hand dispatch to another global queue instead of the main queue it will be A and then B.