The purpose of a run loop is to keep your thread busy when there is work to do and put your thread to sleep when there is none.

A run loop receives events from two different types of sources. Input sources deliver asynchronous events, usually messages from another thread or from a different application. Timer sources deliver synchronous events, occurring at a scheduled time or repeating interval. Both types of source use an application-specific handler routine to process the event when it arrives.

In addition to handling sources of input, run loops also generate notifications about the run loop’s behavior. Registered run-loop observers can receive these notifications and use them to do additional processing on the thread. You use Core Foundation to install run-loop observers on your threads.

Listing 3-1 Creating a run loop observer

- (void)threadMain
{
    // The application uses garbage collection, so no autorelease pool is needed.
    NSRunLoop* myRunLoop = [NSRunLoop currentRunLoop];
 
    // Create a run loop observer and attach it to the run loop.
    CFRunLoopObserverContext  context = {0, self, NULL, NULL, NULL};
    CFRunLoopObserverRef    observer = CFRunLoopObserverCreate(kCFAllocatorDefault,
            kCFRunLoopAllActivities, YES, 0, &myRunLoopObserver, &context);
 
    if (observer)
    {
        CFRunLoopRef    cfLoop = [myRunLoop getCFRunLoop];
        CFRunLoopAddObserver(cfLoop, observer, kCFRunLoopDefaultMode);
    }
 
    // Create and schedule the timer.
    [NSTimer scheduledTimerWithTimeInterval:0.1 target:self
                selector:@selector(doFireTimer:) userInfo:nil repeats:YES];
 
    NSInteger    loopCount = 10;
    do
    {
        // Run the run loop 10 times to let the timer fire.
        [myRunLoop runUntilDate:[NSDate dateWithTimeIntervalSinceNow:1]];
        loopCount--;
    }
    while (loopCount);
}

Starting the Run Loop

Starting the run loop is necessary only for the secondary threads in your application. A run loop must have at least one input source or timer to monitor. If one is not attached, the run loop exits immediately.

There are several ways to start the run loop, including the following:

• Unconditionally
• With a set time limit
• In a particular mode

CFRunLoop Reference

There is exactly one run loop per thread. You neither create nor destroy a thread’s run loop. Core Foundation automatically creates it for you as needed. You obtain the current thread’s run loop with CFRunLoopGetCurrent. Call CFRunLoopRun to run the current thread’s run loop in the default mode until the run loop is stopped with CFRunLoopStop. You can also call CFRunLoopRunInMode to run the current thread’s run loop in a specified mode for a set period of time (or until the run loop is stopped). A run loop can only run if the requested mode has at least one source or timer to monitor.

参考：
1.https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Multithreading/RunLoopManagement/RunLoopManagement.html
2.https://developer.apple.com/library/ios/documentation/CoreFoundation/Reference/CFRunLoopRef/index.html#//apple_ref/doc/uid/20001441
3.https://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSRunLoop_Class/index.html#//apple_ref/doc/uid/TP40003725
4.https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Multithreading/RunLoopManagement/RunLoopManagement.html#//apple_ref/doc/uid/10000057i-CH16-131281
5.https://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSPort_Class/index.html#//apple_ref/occ/cl/NSPort