Question

I’m making server requests to a server on different threads. When they are done loading, I get a delegate callback which updates a tableView. At first, I noticed some strange behavior, and figured it might be because some UI elements weren’t performed on the main thread.

I added the code below, which solved the problem, but the code crashed on iOS 4 devices (Turns out DISPATCH_QUEUE_PRIORITY_BACKGROUND isn’t compatible with iOS 4)

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND,0), ^(void) {
  dispatch_async(dispatch_get_main_queue(), ^(void) {
    //Update view code etc...
  });
});

If I remove the first dispatch_async, and only call dispatch_get_main_queue, the code runs fine:

dispatch_async(dispatch_get_main_queue(), ^(void) {
  //Update view code etc...
});

Before sending in my update to Apple, I wanted to ask if this is okay to do? Will the code run correctly on iOS 4 devices?

Thanks in advance.

Answer 1

DISPATCH_QUEUE_PRIORITY_BACKGROUND is only available in iOS 5 and upper

From apple docs

DISPATCH_QUEUE_PRIORITY_BACKGROUND Items dispatched to the queue run
at background priority; the queue is scheduled for execution after all
high priority queues have been scheduled and the system runs items on
a thread whose priority is set for background status. Such a thread
has the lowest priority and any disk I/O is throttled to minimize the
impact on the system. Available in iOS 5.0 and later.

So if you are thinking of distributing your app for iOS 4 too, then you cant use it

Source apple docs

However if you remove the DISPATCH_QUEUE_PRIORITY_BACKGROUND, your current code will only run in the main ui thread, wich may lead to slow downs when you are doing your remote requests