The default service level to apply to operations executed using the queue.
- iOS 8.0+
- macOS 10.10+
- UIKit for Mac 13.0+Beta
- tvOS 9.0+
- watchOS 2.0+
This property specifies the service level applied to operation objects added to the queue. If the operation object has an explicit service level set, that value is used instead. The default value of this property depends on how you created the queue. For queues you create yourself, the default value is NSOperationQualityOfServiceBackground. For the queue returned by the
main method, the default value is NSOperationQualityOfServiceUserInteractive and cannot be changed.
Service levels affect the priority with which operation objects are given access to system resources such as CPU time, network resources, disk resources, and so on. Operations with a higher quality of service level are given greater priority over system resources so that they may perform their task more quickly. You use service levels to ensure that operations responding to explicit user requests are given priority over less critical work.