The dispatch framework does not guarantee that any given client has the last or only reference to a given object. Objects may be retained internally by the system.
When building with an Objective-C or Objective-C++ compiler, dispatch 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, dispatch objects are
- memory managed by the Objective-C runtime and explicit calls to the Fn dispatch_retain and Fn 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_map3 and dispatch_data_apply3.
- the Blocks runtime automatically retains and releases dispatch objects captured
- by blocks upon Fn Block_copy and Fn Block_release , e.g. as performed during asynchronous execution of a block via dispatch_async3.
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_cancel3 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 analyzer.
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 preprocessor 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 dispatch 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);
});
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_queue3, dispatch_source_set_event_handler3).
The dispatch framework always checks the suspension status before executing 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. Suspending an object causes it to be retained and resuming an object causes it to be released. Therefore it is important to balance calls to Fn dispatch_suspend and Fn dispatch_resume such that the dispatch object is fully resumed when the last reference is released. The result of releasing all references to a dispatch object while in an inactive or suspended state is undefined.