[swift-corelibs-dev] Cancelable DispatchQueue.concurrentPerform

[Brent Royal-Gordon]

> On Feb 23, 2017, at 8:35 AM, Karl Wagner via swift-corelibs-dev <swift-corelibs-dev at swift.org> wrote:
> 
> Would it be possible to make a cancelable version of DispatchQueue.concurrentPerform, allowing you to abort any yet-to-be-scheduled blocks?
> 
> The use-case is for when you’re concurrently performing a job and one of them sets a flag or fails in a way which already defines the result. Further execution wouldn’t affect the outcome, so it would be nice if an operation could flag back to concurrentPerform that it doesn’t have to schedule any more jobs.
> 
> For a real-world example, see this concurrent RandomAccessCollection wrapper I was building: https://gist.github.com/karwa/43ae838809cc68d317003f2885c71572

Your example is:


    var _error: Error?
    DispatchQueue.concurrentPerform(iterations: numericCast(count)) {
      do    { try body($0)   }
      catch { _error = error } // TODO: lock. Would be cool if we could cancel future iterations, too...
    }
    if let error = _error {
      try rescue(error)
    }

So how would cancelability be superior to saying this?

    var _error: Error?
    DispatchQueue.concurrentPerform(iterations: numericCast(count)) {
      guard _error == nil else { return }
      do    { try body($0)   }
      catch { _error = error } // TODO: lock. Would be cool if we could cancel future iterations, too...
    }
    if let error = _error {
      try rescue(error)
    }

-- 
Brent Royal-Gordon
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