Swift：異步方法進入while循環

Question

我想在while循環中使用異步函數，但是函數沒有足夠的時間來完成，而while循環再次啟動並且永遠不會結束。 我應該用增量變量來實現這個問題，但是解決方案是什么？ 非常感謝。
輸入循環“進入重復” - “進入功能”

var condition = true
var userId = Int.random(1...1000)
repeat {
     print("Into repeat")
     checkId(userId, completionHandler: { (success:Bool) -> () in
     if success {
           condition = false
     } else {
           userId = Int.random(1...1000)
       }
}) } while condition


func checkId(userId:Int,completionHandler: (success:Bool) -> ()) -> () {
        print("Into function")
        let query = PFUser.query()
        query!.whereKey("userId", equalTo: userId)
        query!.findObjectsInBackgroundWithBlock({ (object:[PFObject]?, error:NSError?) -> Void in
            if object!.isEmpty {
                completionHandler(success:false)
            } else {
                completionHandler(success:true)
            }
        })
    }

Answer 1

您可以使用遞歸函數執行此操作。 我沒有測試過這段代碼，但我覺得它看起來有點像這樣

func asyncRepeater(userId:Int, foundIdCompletion: (userId:Int)->()){
    checkId(userId, completionHandler: { (success:Bool) -> () in
        if success {
            foundIdCompletion(userId:userId)
        } else {
            asyncRepeater(userId:Int.random(1...1000), completionHandler:  completionHandler)
        }
    })
}

Answer 2

你應該使用dispatch_group

repeat {
     // define a dispatch_group
     let dispatchGroup = dispatch_group_create()
     dispatch_group_enter(dispatchGroup) // enter group
     print("Into repeat")
     checkId(userId, completionHandler: { (success:Bool) -> () in
         if success {
           condition = false
         } else {
           userId = Int.random(1...1000)
       }
    // leave group
    dispatch_group_leave(dispatchGroup)
     }) 
    // this line block while loop until the async task above completed
    dispatch_group_wait(dispatchGroup, DISPATCH_TIME_FOREVER)
} while condition

請參閱Apple文檔中的更多內容