ARC开启的情况下是否需要对GCD对象dispatch_release ?

If your deployment target is iOS 6.0 or Mac OS X 10.8 or later

ARC will manage your queue for you. You do not need to (and cannot) use dispatch_retain or dispatch_release if ARC is enabled.

Starting in the iOS 6.0 SDK and the Mac OS X 10.8 SDK, every dispatch object (including a dispatch_queue_t) is also an Objective-C object. This is documented in the <os/object.h> header file:

 * By default, libSystem objects such as GCD and XPC objects are declared as
 * Objective-C types when building with an Objective-C compiler. This allows
 * them to participate in ARC, in RR management by the Blocks runtime and in
 * leaks checking by the static analyzer, and enables them to be added to Cocoa
 * collections.
 *
 * NOTE: this requires explicit cancellation of dispatch sources and xpc
 *       connections whose handler blocks capture the source/connection object,
 *       resp. ensuring that such captures do not form retain cycles (e.g. by
 *       declaring the source as __weak).
 *
 * To opt-out of this default behavior, add -DOS_OBJECT_USE_OBJC=0 to your
 * compiler flags.
 *
 * This mode requires a platform with the modern Objective-C runtime, the
 * Objective-C GC compiler option to be disabled, and at least a Mac OS X 10.8
 * or iOS 6.0 deployment target.

在dealloc方法中应该加上
#if !OS_OBJECT_USE_OBJC   //这个宏是在sdk6.0之后才有的,如果是之前的,则OS_OBJECT_USE_OBJC为0
    dispatch_release(_barrierQueue);
#endif
当然了,也可以使用
#if __IPHONE_OS_VERSION_MIN_REQUIRED < 60000 // 6.0sdk之前
    dispatch_release(_barrierQueue);
#endif
这里的宏  __IPHONE_OS_VERSION_MIN_REQUIRED  就是我们在工程的设置项里设置的最低部署sdk版本

原因就是  对于最低sdk版本>=ios6.0来说,GCD对象已经纳入了ARC的管理范围,我们就不需要再手工调用 dispatch_release了,否则的话,在sdk<6.0的时候,即使我们开启了ARC,这个宏OS_OBJECT_USE_OBJC 也是没有的,也就是说这个时候,GCD对象还必须得自己管理

如果你部署的最低目标低于 iOS 6.0 or Mac OS X 10.8
你应该自己管理GCD对象,使用(dispatch_retain,dispatch_release),ARC并不会去管理它们

如果你部署的最低目标是 iOS 6.0 or Mac OS X 10.8 或者更高的
ARC已经能够管理GCD对象了,这时候,GCD对象就如同普通的OC对象一样,不应该使用dispatch_retain ordispatch_release