[SR-1770] DispatchQoS can't be used to initialize a DispatchQueue

[glessard]

Previous ID	SR-1770
Radar	rdar://problem/26879343
Original Reporter	@glessard
Type	Bug
Status	Closed
Resolution	Done

Environment

Xcode 8 (beta, 8S128d) with Swift 3.0 preview 1

Additional Detail from JIRA

Votes	0
Component/s	libdispatch
Labels	Bug
Assignee	mww (JIRA)
Priority	Medium

md5: b72edfc14216b508eb2396a9de513141

Issue Description:

DispatchQoS is used as a parameter on DispatchQueue.async and others, but can't be used as a parameter to initialize or obtain DispatchQueue instances. On the other hand, DispatchQueueAttributes contains some DispatchQoS information, but one cannot be initialized using a DispatchQoS instance.

DispatchQueue.global() and DispatchQueue.init() should have overloads with DispatchQoS parameters.
DispatchQueueAttributes.init() should have an overload that accepts a DispatchQoS parameter:

extension DispatchQueue {
  init(qos: DispatchQoS, attributes: DispatchQueueAttributes)
  class func global(qos: DispatchQoS, attributes: DispatchQueue.GlobalAttributes) -> DispatchQueue
}
extension DispatchQueueAttributes {
  // replacing dispatch_queue_attr_make_with_qos_class
  init(qos: DispatchQoS, attributes: DispatchQueueAttributes)
}

[swift-ci]

Comment by Matt Wright (JIRA)

Addressed in 8ac413a0b5a9f927f44438df3376a7757f607953.

[glessard]

I tried the new snapshot, and these work well. Thanks!

I understand why

public class func global(qos: DispatchQoS.QoSClass = default) -> DispatchQueue

takes a DispatchQoS.QoSClass, but it might be a simpler API to have a DispatchQoS parameter and extract the .qosClass value internally?

[swift-ci]

Comment by Matt Wright (JIRA)

It's better not to give the impression that the global(qos🙂 API does something useful with the ```relativePriority``` inside a DispatchQoS. Rather than present the API that's one less step but with increased risk of confusion (or surprise, when relativePriority is used and eventually doesn't work).

[glessard]

Fair enough. Thanks!

[glessard]

As previously stated, the resolution is fine. Perhaps more documentation would help.