Question

I was just recently futzing about with a sample application trying to get my head completely wrapped around NSRunLoop. The sample I wrote created a simple secondary thread via NSOperation. The secondary thread does a few tasks such as process an NSTimer and also some rudimentary streaming with NSStream. Both of these input sources require a properly configured NSRunLoop in order to execute.

My question is this. Originally I had some code that looked like this in the secondary thread:

NSRunLoop* myRunLoop = [NSRunLoop currentRunLoop];
self.connectTimer = [NSTimer scheduledTimerWithTimeInterval:connectTimeout
                                                     target:self
                                                   selector:@selector(connectionConnectedCheck:)
                                                   userInfo:nil 
                                                    repeats:NO];

[myRunLoop addTimer:self.connectTimer forMode:NSDefaultRunLoopMode]; // added source here
[myRunLoop run];

[NSStream getStreamsToHostNamed:relayHost port:relayPort inputStream:&inputStream outputStream:&outputStream];
if ((inputStream != nil) && (outputStream != nil))
{
    sendState = kSKPSMTPConnecting;
    isSecure = NO;

    [inputStream retain];
    [outputStream retain];

    [inputStream setDelegate:self];
    [outputStream setDelegate:self];

    [inputStream scheduleInRunLoop: myRunLoop //[NSRunLoop currentRunLoop]  
                                    forMode:NSRunLoopCommonModes];
    [outputStream scheduleInRunLoop: myRunLoop //[NSRunLoop currentRunLoop] 
                                    forMode:NSRunLoopCommonModes];

    [inputStream open];
    [outputStream open];

    self.inputString = [NSMutableString string];



    return YES;
}

Now, with the code above, the events would never process. Not on currentRunLoop. I did something terrible afterwards, as this was just an educational exercise, and modified it to run under NSRunLoop mainRunLoop. Worked like magic. However, I’m almost certain that depending on my main threads run loop in a secondary thread is on 10 different levels of wrong.

So my question is in two parts, and I hope that’s OK.

1. What can possibly go wrong with the little ‘hack’ I applied in order to get the secondary thread to run and respond to events via the run loop?

2. What’s the proper way to configure the secondary thread to listen to all events/timer based sources so I don’t have to do step 1.

Thanks for the insight all.

Answer 1

Answering your questions in reverse order:

2. You’ve got two problems. The documentation for -[NSRunLoop run] says:

If no input sources or timers are
attached to the run loop, this method
exits immediately; otherwise, it runs
the receiver in the
NSDefaultRunLoopMode by repeatedly
invoking runMode:beforeDate:. In other
words, this method effectively begins
an infinite loop that processes data
from the run loop’s input sources and
timers.

So using your thread’s own run loop, there are probably not input sources defined for the run loop, so it is returning immediately. If there are, your run loop is looping infinitely, and the rest of your code after that point is never being executed.

In order to get things to work correctly, your run loop needs to first have some input sources, and then needs to run periodically to check for events. Note that you don’t want to use [NSRunLoop run], as you’ll never get control back. Instead, I would recommend setting up a loop right before return YES that continually runs the run loop until the thread has been cancelled, or until you’re done streaming data. That will allow the run loop to process data as it arrives. Something like this:

while (!done) {
    [myRunLoop runUntilDate:[NSDate dateWithTimeIntervalSinceNow:1]];
}

[NSRunLoop runUntilDate:] processes events until the specified date, and then returns control to your program so that you can do whatever you want.

1. It’s working because your main thread’s run loop is being run periodically, so events are being handled. If your main thread ever blocked, however, your data would stop arriving. This could be particularly bad if the main thread was waiting for data from your second thread. Also, NSRunLoop is not thread-safe:

Warning: The NSRunLoop class is
generally not considered to be
thread-safe and its methods should
only be called within the context of
the current thread. You should never
try to call the methods of an
NSRunLoop object running in a
different thread, as doing so might
cause unexpected results.
(from the NSRunLoop documentation.)

Apple’s Threading Programming Guide has a section entitled Run Loop Management, which explains all of this to some degree. It’s not the clearest document I’ve ever read, but if you’re working with run loops, it’s a good place to start.