performSelector:OnThread:waitUntilDone not executing the selector all the time

Question

I have an app where the network activity is done in its separate thread (and the network thread continuously gets data from the server and updates the display - the display calls are made back on the main thread). When the user logs out, the main thread calls a disconnect method on the network thread as follows:

[self performSelector:@selector(disconnectWithErrorOnNetworkThread:) onThread:nThread withObject:e waitUntilDone:YES];

This selector gets called most of the time and everything works fine. However, there are times (maybe 2 out of ten times) that this call never returns (in other words the selector never gets executed) and the thread and the app just hang. Anyone know why performSelector is behaving erratically?

Please note that I need to wait until the call gets executed, that's why waitUntilDone is YES, so changing that to NO is not an option for me. Also the network thread has its run loop running (I explicitly start it when the thread is created).

Please also note that due to the continuous nature of the data transfer, I need to explicitly use NSThreads and not GCD or Operations queues.

Answer 1

That'll hang if:

• it is attempting to perform a selector on the same thread the method was called from

• the call to perform the selector is to a thread from which a synchronous call was made that triggered the perform selector

When your program is hung, have a look at the backtraces of all threads.

Note that when implementing any kind of networking concurrency, it is generally really bad to have synchronous calls from the networking code into the UI layers or onto other threads. The networking thread needs to be very responsive and, thus, just like blocking the main thread is bad, anything that can block the networking thread is a bad, too.

Note also that some APIs with callbacks don't necessarily guarantee which thread the callback will be delivered on. This can lead to intermittent lockups, as described.

Finally, don't do any active polling. Your networking thread should be fully quiescent unless some event arrives. Any kind of looped polling is bad for battery life and responsiveness.