Dispatch group - 无法通知主线程

Question

在GCD上阅读Swift 3 evolution之后,我正在尝试创建调度组.问题是group.notify(queue:当我DispatchQueue.main作为队列传递时不通知,尽管它确实适用于后台队列.

此外,我不确定我的语法是否正确,因为我试图将代码从Swift 2转换为Swift 3.

typealias CallBack = (result: Bool) -> Void
func longCalculations (completion: CallBack) {

let backgroundQ = DispatchQueue.global(attributes: .qosBackground)

    let group = DispatchGroup()  
    var fill:[Int] = [] 
    for item in 0...200 {
        group.enter() 
        if item > 50 {
            fill.append(item)
        }
        group.leave() 
    }

//Below in the notify argument If I pass `backgroundQ`, it seems to work correctly but not when DispatchQueue.main is passed.

此代码不起作用

group.notify(queue: DispatchQueue.main, execute: {
    completion(result: true)
    })
 }

这工作正常

group.notify(queue: backgroundQ, execute: {
    completion(result: true)
    })
 }
_______________________________________________________

longCalculations() { print($0) }

Answer 1

在阅读了Matt建议的帖子之后,我发现我正在向主队列提交任务,当我要求在主线程本身上得到通知时,它陷入了僵局.

我已经修改了代码,现在它按预期工作了,

typealias CallBack = (result: [Int]) -> Void
func longCalculations (completion: CallBack) {
  let backgroundQ = DispatchQueue.global(attributes: .qosDefault)
  let group = DispatchGroup()

  var fill:[Int] = []
  for number in 0..<100 {
      group.enter()
      backgroundQ.async(group: group,  execute: {  
          if number > 50 {
            fill.append(number)
          }
          group.leave()

          })
      }

     group.notify(queue: DispatchQueue.main, execute: {
       print("All Done"); completion(result: fill)
     }) 
}

longCalculations(){print($0)}