dispatch_object(3)       BSD Library Functions Manual       dispatch_object(3)

NAME
     dispatch_object -- General manipulation of dispatch objects

SYNOPSIS
     #include <dispatch/dispatch.h>

     void
     dispatch_retain(dispatch_object_t object);

     void
     dispatch_release(dispatch_object_t object);

     void
     dispatch_suspend(dispatch_object_t object);

     void
     dispatch_resume(dispatch_object_t object);

     void
     dispatch_activate(dispatch_object_t object);

     void *
     dispatch_get_context(dispatch_object_t object);

     void
     dispatch_set_context(dispatch_object_t object, void *context);

     void
     dispatch_set_finalizer_f(dispatch_object_t object,
         dispatch_function_t finalizer);

DESCRIPTION
     Dispatch objects share functions for coordinating memory management, sus-
     pension, cancellation and context pointers.

MEMORY MANAGEMENT
     Objects returned by creation functions in the dispatch framework may be
     uniformly retained and released with the functions dispatch_retain() and
     dispatch_release() respectively.

     The dispatch framework does not guarantee that any given client has the
     last or only reference to a given object. Objects may be retained inter-
     nally by the system.

   INTEGRATION WITH OBJECTIVE-C
           When  building  with an Objective-C or Objective-C++ compiler, dis-
           patch objects are declared as Objective-C types.  This  results  in
           the following differences compared to building as plain C/C++:

           -   if  Objective-C  Automated  Reference Counting is enabled, dis-
               patch objects are memory managed by the Objective-C runtime and
               explicit  calls to the dispatch_retain() and dispatch_release()
               functions will produce build errors.

               Note: when ARC is enabled, care needs to be taken with dispatch
               API returning an interior pointer that is only valid as long as
               an associated object has not been released. If that  object  is
               held  in  a  variable with automatic storage, it may need to be
               annotated with the objc_precise_lifetime attribute,  or  stored
               in  a  __strong  instance  variable instead, to ensure that the
               object is not prematurely  released.  The  functions  returning
               interior    pointers    are   dispatch_data_create_map(3)   and
               dispatch_data_apply(3).

           -   the Blocks runtime automatically retains and releases  dispatch
               objects    captured    by    blocks   upon   Block_copy()   and
               Block_release(), e.g. as performed during  asynchronous  execu-
               tion of a block via dispatch_async(3).

               Note:  retain  cycles  may  be  encountered  if dispatch source
               objects are captured by their handler blocks; these cycles  can
               be broken by declaring the captured object __weak or by calling
               dispatch_source_cancel(3) to cause its  handler  blocks  to  be
               released explicitly.

           -   dispatch  objects  can  be added directly to Cocoa collections,
               and their lifetime is tracked by the  Objective-C  static  ana-
               lyzer.

           Integration of dispatch objects with Objective-C requires targeting
           Mac OS X 10.8 or later, and  is  disabled  when  building  for  the
           legacy  Objective-C  runtime.   It can also be disabled manually by
           using compiler options to define the OS_OBJECT_USE_OBJC  preproces-
           sor macro to 0.

     Important: When building with a plain C/C++ compiler or when integration
     with Objective-C is disabled, dispatch objects are not automatically
     retained and released when captured by a block. Therefore, when a dis-
     patch object is captured by a block that will be executed asynchronously,
     the object must be manually retained and released:

           dispatch_retain(object);
           dispatch_async(queue, ^{
                   do_something_with_object(object);
                   dispatch_release(object);
           });

ACTIVATION
     Dispatch objects such as queues and sources may be created in an inactive
     state. Objects in this state must be activated before any blocks associ-
     ated with them will be invoked. Calling dispatch_activate() on an active
     object has no effect.

     Changing attributes such as the target queue or a source handler is no
     longer permitted once the object has been activated (see
     dispatch_set_target_queue(3), dispatch_source_set_event_handler(3) ).

SUSPENSION
     The invocation of blocks on dispatch queues or dispatch sources may be
     suspended or resumed with the functions dispatch_suspend() and
     dispatch_resume() respectively. Other dispatch objects do not support
     suspension.

     The dispatch framework always checks the suspension status before execut-
     ing a block, but such changes never affect a block during execution (non-
     preemptive).  Therefore the suspension of an object is asynchronous,
     unless it is performed from the context of the target queue for the given
     object.  The result of suspending or resuming an object that is not a
     dispatch queue or a dispatch source is undefined.

     Important: suspension applies to all aspects of the dispatch object life
     cycle, including the finalizer function and cancellation handler. Sus-
     pending an object causes it to be retained and resuming an object causes
     it to be released. Therefore it is important to balance calls to
     dispatch_suspend() and dispatch_resume() such that the dispatch object is
     fully resumed when the last reference is released. The result of releas-
     ing all references to a dispatch object while in an inactive or suspended
     state is undefined.

CONTEXT POINTERS
     Dispatch objects support supplemental context pointers. The value of the
     context pointer may be retrieved and updated with dispatch_get_context()
     and dispatch_set_context() respectively.  The dispatch_set_finalizer_f()
     specifies an optional per-object finalizer function that is invoked asyn-
     chronously if the context pointer is not NULL when the last reference to
     the object is released.  This gives the application an opportunity to
     free the context data associated with the object.  The finalizer will be
     run on the object's target queue.

SEE ALSO
     dispatch(3), dispatch_async(3), dispatch_group_create(3),
     dispatch_queue_create(3), dispatch_semaphore_create(3),
     dispatch_set_target_queue(3), dispatch_source_cancel(3),
     dispatch_source_create(3)

Darwin                           March 1, 2012                          Darwin