How to use DispatchGroup to make asynchronous calls within a for loop

问题

In my example code below, I call complete(false) on failure. However, since I'm using a DispatchGroup object to make sure all asynchronous requests are complete, I cannot just call syncGroup.leave() on failure, as the notify will be called, which contains complete(true), making this function return true, when it should be returning false for failure.

Am I correct in not calling syncGroup.leave() on failure to complete my function correctly? Or should I be calling syncGroup.leave() and somehow trying to determine what the result is, so I can return false on failure?

let syncGroup = DispatchGroup()
syncGroup.enter()
for track in unsynced {
    register(time: time, withCompletion: { (success: Bool) -> () in

        if success {
            self.debug.log(tag: "SyncController", content: "Registered")
            syncGroup.leave()
        }
        else {
            complete(false)
        }
    })
}

//all requests complete
syncGroup.notify(queue: .main) {
    self.debug.log(tag: "SyncController", content: "Finished registering")
    complete(true)
}

回答1:

You have to enter the group within your for loop. You might want to introduce an additional error flag.

Example implementation:

    var fail = false
    let syncGroup = DispatchGroup()

    for track in unsynced {
        syncGroup.enter()
        register(time: time, withCompletion: { (success: Bool) -> () in

            if success {
                self.debug.log(tag: "SyncController", content: "Registered")
                syncGroup.leave()
            }
            else {
                fail = true
                syncGroup.leave()
            }
        })
    }

    //all requests complete
    syncGroup.notify(queue: .main) {
        if fail {
            complete(false)

        } else {
            self.debug.log(tag: "SyncController", content: "Finished registering")
            complete(true)
        }
    }

来源：https://stackoverflow.com/questions/40652869/how-to-use-dispatchgroup-to-make-asynchronous-calls-within-a-for-loop