Running the Run Loop

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 the CFRunLoopGetCurrent function. 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, until the run loop is stopped, or until after the next run loop source is processed.

Listing 1 contains a function, RunMyTimer, that uses the run loop with a timer to call a particular function every 5 seconds for 20 seconds. The function creates the run loop timer, myTimer, with the CFRunLoopTimerCreate function, initializing the timer’s callback function to MyTimerFunction, its initial firing time to 1 second in the future, and its periodicity to 5 seconds. Before the timer can fire, the function has to place the timer into a run loop mode, in this case a new mode named “MyCustomMode,” and then run the run loop in that mode by calling CFRunLoopRunInMode. The run loop will run for 20 seconds before control returns to the caller. During this time, the timer will fire and MyTimerFunction will be called 4 times. After the run loop exits, the function invalidates the timer to render it inoperable and to remove it from all run loop modes. Finally, because the function still holds a reference to the timer from the create function, the function releases the timer object.

Listing 1  Running a run loop with a timer

void MyTimerFunction( CFRunLoopTimerRef timer, void *info );
void RunMyTimer ()
    CFStringRef myCustomMode = CFSTR(“MyCustomMode”);
    CFRunLoopTimerRef myTimer;
    myTimer = CFRunLoopTimerCreate( NULL, CFAbsoluteTimeGetCurrent()+1.0,
                    5.0, 0, 0, MyTimerFunction, NULL );
    CFRunLoopAddTimer( CFRunLoopGetCurrent(), myTimer, myCustomMode );
    CFRunLoopRunInMode( myCustomMode, 20.0, false );
    CFRunLoopTimerInvalidate( myTimer );
    CFRelease( myTimer );

A run loop can only run if the requested mode has at least one source or timer to monitor. If a run loop mode is empty, the CFRunLoopRun and CFRunLoopRunInMode functions return immediately without doing anything.

Run loops can be run recursively. You can call CFRunLoopRun or CFRunLoopRunInMode from within any run loop callout and create nested run loop activations on the current thread’s call stack. You are not restricted in which modes you can run from within a callout. You can create another run loop activation running in any available run loop mode, including any modes already running higher in the call stack.