fetchRecordCompletionBlock 和信号量 - 帮助理解执行顺序

Question

上下文：

• 包含 CloudKit 中所有数据的应用程序
• ViewController 调用查询来加载 tableview 的数据
• tableview 崩溃，因为 tableview 的数据数组没有 从CK回来
• 我研究了信号量，并且几乎 工作但似乎无法弄清楚在哪里放置 semaphore.signal() 以获得正确的行为

在viewDidLoad中，我调用了函数：

 Week.fetchWeeks(for: challenge!.weeks!) { weeks in
        self.weeks = weeks
    }

和功能：

static func fetchWeeks(for references: [CKRecord.Reference],
                       _ completion: @escaping ([Week]) -> Void) {
    let recordIDs = references.map { $0.recordID }
    let operation = CKFetchRecordsOperation(recordIDs: recordIDs)
    operation.qualityOfService = .utility
    let semaphore = DispatchSemaphore(value: 0)

    operation.fetchRecordsCompletionBlock = { records, error in
        let weeks = records?.values.map(Week.init) ?? []

        DispatchQueue.main.async {
            completion(weeks)
            //Option 1: putting semaphore.signal() here means it never completes
            // beyond initialization of the week records 
        }
        //Option 2: putting semaphore.signal() here means it completes after the
        // initialization of the Week items, but before completion(weeks) is done 
        // so the array isn't initialized in the view controller in time.  so the
        // VC tries to use weeks and unwraps a nil.
        semaphore.signal()
    }

    Model.currentModel.publicDB.add(operation)
    semaphore.wait() // blocking the thread until .signal is called        
}

注意：我已经测试了视图控制器中的 week 数组最终是否正确设置 - 所以它似乎纯粹是一个时间问题 :)

我已经测试了 .signal() 的位置，如果我将它放在“DispatchQueue.main.async”块中，它永远不会被触发 - 可能是因为该块本身正在等待信号。

但是，如果我将它放在其他任何地方，则视图控制器会在此时启动，并且不会及时调用完成（周）。

也许这很明显 - 但作为我第一次使用信号量 - 我正在努力弄清楚！

更新 1：它适用于 DispatchQueue(label: "background")

一旦我发现 semaphore.wait() 永远不会在主线程上被 semaphore.signal() 调用，我就能让它工作。

所以我把它从： DispatchQueue.main.async 到 DispatchQueue(label: "background").async 并在里面弹出 semaphore.signal() 就成功了

欢迎评论/批评！

    static func fetchWeeks(for references: [CKRecord.Reference],
                           _ completion: @escaping ([Week]) -> Void) {
        NSLog("inside fetchWeeks in Week ")
        let recordIDs = references.map { $0.recordID }
        let operation = CKFetchRecordsOperation(recordIDs: recordIDs)

        operation.qualityOfService = .utility
        let semaphore = DispatchSemaphore(value: 0)

        operation.fetchRecordsCompletionBlock = { records, error in

            if error != nil {
                print(error?.localizedDescription)
            }

            let weeks = records?.values.map(Week.init) ?? []

            DispatchQueue(label: "background").async {
                completion(weeks)
                semaphore.signal()
            }

        }
        Model.currentModel.publicDB.add(operation)
        semaphore.wait() // blocking the thread until .signal is called
    }

}

更新 2：尽量避免使用信号量

每个评论线程——我们不应该在 CloudKit 中使用信号量——所以我很可能在做一些愚蠢的事情 :)

将 fetchWeeks() 移动到 viewController 以尝试隔离问题...但它仍然会崩溃，因为 fetchWeeks() 在代码尝试执行之后的行并使用周数组之前尚未完成

我的视图控制器：

class ChallengeDetailViewController: UIViewController {

@IBOutlet weak var rideTableView: UITableView!
//set by the inbound segue
var challenge: Challenge?
// set in fetchWeeks based on the challenge
var weeks: [Week]?

override func viewDidLoad() {

    super.viewDidLoad()
    rideTableView.dataSource = self
    rideTableView.register(UINib(nibName: K.cellNibName, bundle: nil), forCellReuseIdentifier: K.cellIdentifier)
    rideTableView.delegate = self

    fetchWeeks(for: challenge!.weeks!) { weeks in
        self.weeks = weeks
    }
//This is where it blows up as weeks is nil
    weeks = weeks!.sorted(by: { $0.weekSequence < $1.weekSequence })   
 }

//moved this to the view controller
func fetchWeeks(for references: [CKRecord.Reference],
                       _ completion: @escaping ([Week]) -> Void) {

    let recordIDs = references.map { $0.recordID }
    let operation = CKFetchRecordsOperation(recordIDs: recordIDs)
    operation.qualityOfService = .utility

    operation.fetchRecordsCompletionBlock = { records, error in

        if error != nil {
            print(error?.localizedDescription)
        }

        let weeks = records?.values.map(Week.init) ?? []

        DispatchQueue.main.sync {
            completion(weeks)
        }
    }
    Model.currentModel.publicDB.add(operation)
}

Answer 1

再次重申：切勿在 CloudKit API 中使用信号量。

首先将数据源数组声明为非可选的空数组，以消除不必要的解包可选

var weeks = [Week]()

错误是你没有在正确的地方使用获取的数据。

由于闭包是异步的，你必须在 内部 闭包

fetchWeeks(for: challenge!.weeks!) { [weak self] weeks in
    self?.weeks = weeks
    self?.weeks = weeks.sorted(by: { $0.weekSequence < $1.weekSequence }) 
}

或更简单

fetchWeeks(for: challenge!.weeks!) { [weak self] weeks in
    self?.weeks = weeks.sorted{ $0.weekSequence < $1.weekSequence }
}

如果您需要重新加载表格视图，请在 内部 关闭

fetchWeeks(for: challenge!.weeks!) { [weak self] weeks in
    self?.weeks = weeks.sorted{ $0.weekSequence < $1.weekSequence }
    self?.rideTableView.reloadData()
}

为此，您必须在主线程上调用completion

DispatchQueue.main.async {
    completion(weeks)
}

最后删除丑陋的信号量！

罢工>

let semaphore = DispatchSemaphore(value: 0)
...
semaphore.signal()
...
semaphore.wait()