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- (void)doSomethingWithCompletion:(void (^)())completion
{
    dispatch_group_t myGroup = dispatch_group_create();
    
    for (id d in [self data])
    {
        dispatch_group_enter(myGroup);
        
        // Each iteration is done independently, in its own thread.
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
            
            // Here is where the data is being processed.
            [self process:d];
            
            // Here is where the UI is being updated.
            dispatch_async(dispatch_get_main_queue(), ^{
                // Do what you like with the UI...
                
                dispatch_group_leave(myGroup);
            });
        });
    }
    
    dispatch_group_notify(myGroup, dispatch_get_main_queue(),^{
        // All iterations have been completed.
        completion();
    });
}