let downloadGroup = dispatch_group_create()
var images = [UIImage]()
var errors = [NSError]()
dispatch_apply(UInt(urls.count), dispatch_get_global_qu
found out the reason seconds after i posted. it seems not me being stupid, but apple's document
QOS_CLASS_USER_INTERACTIVE, QOS_CLASS_USER_INITIATED, QOS_CLASS_UTILITY, or QOS_CLASS_BACKGROUND
can not be used on ios7, though
https://developer.apple.com/library/ios/documentation/Performance/Reference/GCD_libdispatch_Ref/#//apple_ref/c/func/dispatch_get_global_queue
doesn't bother to metion any of it
instead use
DISPATCH_QUEUE_PRIORITY_HIGH, DISPATCH_QUEUE_PRIORITY_DEFAULT, DISPATCH_QUEUE_PRIORITY_LOW, DISPATCH_QUEUE_PRIORITY_BACKGROUND