Documentation for `DispatchQueue.concurrentPerform` confusingly references the 'target queue' over which callers have no direct control.

Question

jamie_sq OP

Created Apr ’24

Replies 4

Boosts 1

Participants 4

In the 'Discussion' section of the current documentation for Swift's DispatchQueue, it says:

If the target queue is a concurrent queue, the blocks run in parallel and must therefore be reentrant-safe.

However, unlike dispatch_apply (on which this API is built), this method provides no direct means of specifying a target queue, so this callout is somewhat more confusing than it ought to be. IMO, it's important to highlight the reentrancy considerations that apply in most (all?) cases, but the implicit reference to the implementation details should be removed or clarified.

Feedback filed as: FB13708750

Boost

Answer 1

DTS Engineer OP

Apple

Apr ’24

Thanks for filing FB13708750.

0

Answer 2

DTS Engineer OP

Apple

Apr ’24

Also see the last parameter in this initializer:

https://developer.apple.com/documentation/dispatch/dispatchqueue/2300059-init

and see this function:

https://developer.apple.com/documentation/dispatch/dispatchobject/1452989-settarget

The concept of "the target queue for this queue" is well-defined, but I agree the documentation could be clearer here. Note that the parameter to dispatch_apply is "just" a queue, not a target queue in this sense used above.

0

Answer 3

yetanotherme OP

Apr ’24

concurrentPerform is a class method that has no queue parameter, so the wording in the documentation about "If the target queue is a concurrent queue..." is bogus, not just unclear.

0

Answer 4

DTS Engineer OP

Apple

May ’24

Right. concurrentPerform(iterations:execute:) is the Swift projection of dispatch_apply. That method does take a queue parameter, which is what the docs are talking about. The Swift version has no queue parameter; it always acts like you supplied the “preferred value” of DISPATCH_APPLY_AUTO.

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