Dispatch encapsulates quality of service classes.
- iOS 7.0+
- macOS 10.9+
- tvOS 9.0+
- watchOS 2.0+
- Xcode 8.0+
A quality of service (QoS) class categorizes work to be performed on a
Dispatch. By specifying a QoS to work, you indicate its importance, and the system prioritizes it and schedules it accordingly.
Because higher priority work is performed more quickly and with more resources than lower priority work, it typically requires more energy than lower priority work. Accurately specifying appropriate QoS classes for the work your app performs ensures that your app is responsive and energy efficient.