swift - 循环中的异步 DispatchQueue

Question

So my goal here is to basically perform a query in a while loop and append results of the query to my array. When I run the code my "level" variable starts from zero and increments infinitely. I'm highly convinced that my problem is caused by fact that my code is running on 2 async queues but just can't figure out the exact cause.

func displayPathOf(参数:参数，threadTableView:UITableView){

    array.removeAll()
    threadTableView.reloadData()
    var level = argument.level!-1
    array.insert(argument, at: 0)
    var stop = false

    DispatchQueue.global(qos: .userInteractive).async {

        repeat {

            level += 1
            print(level)
            let query = Argument.query()?.whereKey("level", equalTo: level).addDescendingOrder("reach")
            query?.getFirstObjectInBackground(block: { (object, error) in

                if object != nil {

                    DispatchQueue.main.async {
                        array.append(object as! Argument)
                        print(array)
                        threadTableView.reloadData()}

                } else {

                    stop = true
                    print(error)

                }
            })
         } while stop == false
    }
}

Answer 1

您的代码归结为:

do-in-background {
    repeat {
        level += 1
        do-in-background { ... }
    } while stop == false
}

do-in-background(即async和getFirstObjectInBackground)立即返回，因此从这个循环的角度来看，它 block 中的内容并不重要。这相当于一个紧密循环，尽可能快地增加级别。

您似乎正在尝试序列化对 getFirstObjectInBackground 的调用。您可以通过以下两种方式之一执行此操作:

• 让完成 block 自行启动下一个搜索并删除repeat循环。
• 使用 DispatchGroup 等待完成 block 完成。

就您而言，我可能会推荐第一个。摆脱 stop 并使函数(模糊地)类似于:

func fetchObject(at level: Int) {
    let query = Argument.query()?.whereKey("level", equalTo: level).addDescendingOrder("reach")
    query?.getFirstObjectInBackground(block: { (object, error) in
        if let object = object {
            DispatchQueue.main.async {
            array.append(object as! Argument)
            print(array)
            threadTableView.reloadData()}

            // Schedule the next loop
            DispatchQueue.global(qos: .userInteractive).async { fetchObject(level + 1) }

         } else {
            print(error)
        }
    })
}