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//Start an activity indicator here
 
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
 
    //Call your function or whatever work that needs to be done
    //Code in this part is run on a background thread
 
    dispatch_async(dispatch_get_main_queue(), ^(void) {
 
        //Stop your activity indicator or anything else with the GUI

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NSString *predicateFormatString = [NSString stringWithFormat:@"(%@ = '%@' OR %@ = '%@') AND (%@ = '%@' OR %@ = '%@')",
                                kMessageUserSendIdKey, self.currentUser.objectId,
                                kMessageUserSendIdKey, self.userChat.objectId,
                                kMessageUserReceiveIdKey, self.currentUser.objectId,
                                kMessageUserReceiveIdKey, self.userChat.objectId];

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- (void)refreshData
{
    PFUser *currentUser = [PFUser currentUser];
    
    NSPredicate *predicate = [NSPredicate predicateWithFormat:[NSString stringWithFormat:@"%@ = '%@' OR %@ = '%@'",
                                                               kMessageUserSendIdKey, currentUser.objectId,
                                                               kMessageUserReceiveIdKey, currentUser.objectId]];
    
    PFQuery *query = [PFQuery queryWithClassName:kMessageClassKey predicate:predicate];
    [query addDescendingOrder:kMessageTimeCreatedKey];

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NSDate *start = [NSDate date];

NSDate *methodFinish = [NSDate date];
NSTimeInterval executionTime = [methodFinish timeIntervalSinceDate:start];
            
NSLog(@"Execution Time: %f", executionTime);