Performing selector at beginning / end of run loop

问题

All events and methods in iOS are processed using NSRunLoop: user events, method calls, rotations, timers, connections, etc.

My question is:

How can I perform a selector in a precise moment of the run loop as the beginning and the end?

回答1:

You can create a CFRunLoopObserver which will call a block on loop entry and exit. You use CFRunLoopAddObserver to add your observer to the run loop, and CFRunLoopGetMain to obtain the run loop to add to.

Here is a rather pointless example using these:

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
   CFRunLoopObserverRef observer = CFRunLoopObserverCreateWithHandler(NULL, (kCFRunLoopEntry | kCFRunLoopExit), YES, 0, ^(CFRunLoopObserverRef observer, CFRunLoopActivity activity)
   {
      static unsigned long count = 0;
      NSLog(@"activity %lu: %@", ++count, (activity & kCFRunLoopEntry ? @"Enter" : @"Exit"));
   });
   CFRunLoopAddObserver(CFRunLoopGetMain(), observer, kCFRunLoopCommonModes);
}

This simply installs an observer which logs every entry & exit to the run loop. You can run it as a complete application in Xcode and see how many times the run loop goes around.

Note that CFRunLoopObserverCreateWithHandler returns a reference you own, if you remove the observer you are responsible for deallocation.

来源：https://stackoverflow.com/questions/16789342/performing-selector-at-beginning-end-of-run-loop