objective c - Best way to make NSRunLoop wait for a flag to be set?

Question

In the Apple documentation for NSRunLoop there is sample code demonstrating suspending execution while waiting for a flag to be set by something else.

BOOL shouldKeepRunning = YES;        // global
NSRunLoop *theRL = [NSRunLoop currentRunLoop];
while (shouldKeepRunning && [theRL runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]]);

I have been using this and it works but in investigating a performance issue I tracked it down to this piece of code. I use almost exactly the same piece of code (just the name of the flag is different :) and if I put a NSLog on the line after the flag is being set (in another method) and then a line after the while() there is a seemingly random wait between the two log statements of several seconds.

The delay does not seem to be different on slower or faster machines but does vary from run to run being at least a couple of seconds and up to 10 seconds.

I have worked around this issue with the following code but it does not seem right that the original code doesn't work.

NSDate *loopUntil = [NSDate dateWithTimeIntervalSinceNow:0.1];
while (webViewIsLoading && [[NSRunLoop currentRunLoop] runMode: NSDefaultRunLoopMode beforeDate:loopUntil])
  loopUntil = [NSDate dateWithTimeIntervalSinceNow:0.1];

using this code, the log statements when setting the flag and after the while loop are now consistently less than 0.1 seconds apart.

Anyone any ideas why the original code exhibits this behaviour?

Answer 1

Runloops can be a bit of a magic box where stuff just happens.

Basically you're telling the runloop to go process some events and then return. OR return if it doesn't process any events before the timeout is hit.

With 0.1 second timeout, you're htting the timeout more often than not. The runloop fires, doesn't process any events and returns in 0.1 of second. Occasionally it'll get a chance to process an event.

With your distantFuture timeout, the runloop will wait foreever until it processes an event. So when it returns to you, it has just processed an event of some kind.

A short timeout value will consume considerably more CPU than the infinite timeout but there are good reasons for using a short timeout, for example if you want to terminate the process/thread the runloop is running in. You'll probably want the runloop to notice that a flag has changed and that it needs to bail out ASAP.

You might want to play around with runloop observers so you can see exactly what the runloop is doing.

See this Apple doc for more information.