Wait not working for synchronous call

Have you put the whole in a user thread ?

See a good tutorial here: h ttps://www.raywenderlich.com/148515/grand-central-dispatch-tutorial-swift-3-part-2

DispatchQueue.global(qos: .userInitiated).async {
     let dispatchGroup = DispatchGroup()

     dispatchGroup.enter()
     characteristic.readValue(completionHandler:
      { (error) in
        if error != nil
          {
          print(error!.localizedDescription)
          }
         else
            {
              powerState = characteristic.value as! Int
              print("CLightBulb: -->Read the power state.  It is \(powerState) at " + Date().description(with: Locale.current))
            }
      })
dispatchGroup.leave()
dispatchGroup.wait()
  DispatchQueue.main.async { 
     print("CLightBulb: (After wait) The power value is \(powerState) at " + Date().description(with: Locale.current))
  }
}

Thanks... I did read that and several other explainations.

I tried what you suggested, and the results are the same:

CLightBulb: (After wait) The power value is 1

CLightBulb: -->Read the power state. It is 0

CLightBulb: (After wait) The power value is 1

CLightBulb: -->Read the power state. It is 0

Why do you get it twice ? (do you actionate the read twice ?)

Why don't you get de Date() in the printed log ?

You seem to have a nesting problem. Here’s your code, in a code block, with extraneous stuff cut away:

let dispatchGroup = DispatchGroup()

dispatchGroup.enter()
characteristic.readValue(completionHandler: { (error) in
    …
})
dispatchGroup.leave()
dispatchGroup.wait()

The problem is that you’re calling

leave()

1. Create group
2. Enter
3. Start async
4. Leave
5. Wait, which completes immediately because there’s no pending Enter
6. [some time later] Complete async

Instead you want this:

let dispatchGroup = DispatchGroup()

dispatchGroup.enter()
characteristic.readValue(completionHandler: { (error) in
    …
    dispatchGroup.leave()
})
dispatchGroup.wait()

Now the sequence is:

1. Create group
2. Enter
3. Start async
4. Wait, which blocks
5. [some time later] Complete async
6. Leave
7. Wait unblocks

Having said that, converting async calls to sync calls like this is generally not recommended, especially with a dispatch group which doesn’t provide any sort of lock ownership. WWDC 2017 Session 706 Modernizing Grand Central Dispatch Usage talks about this in detail.

In my experience, when dealing with I/O like this you inevitably end up creating a state machine, and it’s better to address that concept directly rather than trying to force an inherently async situation into a synchronous box.

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