The dispatch queue used to execute operations.
- iOS 8.0+
- macOS 10.10+
- UIKit for Mac 13.0+Beta
- tvOS 9.0+
- watchOS 2.0+
The default value of this property is
nil. You can set the value of this property to an existing dispatch queue to have enqueued operations interspersed with blocks submitted to that dispatch queue.
The value of this property should only be set if there are no operations in the queue; setting the value of this property when
operation is not equal to
0 raises an
invalid. The value of this property must not be the value returned by
dispatch. The quality-of-service level set for the underlying dispatch queue overrides any value set for the operation queue's