我可能建议不要使用信号量之类的东西来阻塞线程，以便仅出于DispatchWorkItem的原因就可以使异步任务同步运行.

I might advise against using semaphores or the like to block threads so that you can make asynchronous tasks behave synchronously, solely for the sake of DispatchWorkItem.

当我想在异步任务之间建立依赖关系时，我一直使用Operation而不是DispatchWorkItem. (不可否认，在iOS 13及更高版本中，我们可能会考虑合并的未来/承诺，但目前操作仍在进行中.)操作旨在支持与DispatchWorkItem相比，异步过程的包装要优雅得多.因此，您可以使用maxConcurrentOperationCount为1的队列，如下所示:

When I want to establish dependencies between asynchronous tasks, I have historically used Operation rather than DispatchWorkItem. (Admittedly, in iOS 13 and later, we might contemplate Combine’s Future/Promise, but for now operations are the way to go.) Operations have been designed to support wrapping of asynchronous processes much more elegantly than DispatchWorkItem. So you can use a queue whose maxConcurrentOperationCount is 1, like so:

let networkQueue = OperationQueue()
networkQueue.maxConcurrentOperationCount = 1

let completionOperation = BlockOperation {
    print("all done")
}

for url in urls {
    let operation = NetworkOperation(url: url) { result in
        switch result {
        case .failure(let error):
            ...

        case .success(let data):
            ...
        }
    }
    completionOperation.addDependency(operation)
    networkQueue.addOperation(operation)
}

OperationQueue.main.addOperation(completionOperation)

或者您可以使用更合理的maxConcurrentOperationCount并仅在需要此顺序行为的那些操作之间使用依赖项:

Or you can use a more reasonable maxConcurrentOperationCount and use dependencies only between those operations where you need this sequential behavior:

let networkQueue = OperationQueue()
networkQueue.maxConcurrentOperationCount = 4

let completionOperation = BlockOperation {
    print("all done")
}

var previousOperation: Operation?

for url in urls {
    let operation = NetworkOperation(url: url) { result in
        switch result {
        case .failure(let error):
            ...

        case .success(let data):
            ...
        }
    }
    if let previousOperation = previousOperation {
        operation.addDependency(previousOperation)
    }
    completionOperation.addDependency(operation)
    networkQueue.addOperation(operation)
    previousOperation = operation
}

OperationQueue.main.addOperation(completionOperation)

NetworkOperation可能是这样的:

class NetworkOperation: AsynchronousOperation {
    typealias NetworkCompletion = (Result<Data, Error>) -> Void

    enum NetworkError: Error {
        case invalidResponse(Data, URLResponse?)
    }

    private var networkCompletion: NetworkCompletion?
    private var task: URLSessionTask!

    init(request: URLRequest, completion: @escaping NetworkCompletion) {
        super.init()

        task = URLSession.shared.dataTask(with: request) { data, response, error in
            defer {
                self.networkCompletion = nil
                self.finish()
            }

            guard let data = data, error == nil else {
                self.networkCompletion?(.failure(error!))
                return
            }

            guard
                let httpResponse = response as? HTTPURLResponse,
                200..<300 ~= httpResponse.statusCode
                else {
                    self.networkCompletion?(.failure(NetworkError.invalidResponse(data, response)))
                    return
            }

            self.networkCompletion?(.success(data))
        }
        networkCompletion = completion
    }

    convenience init(url: URL, completion: @escaping NetworkCompletion) {
        self.init(request: URLRequest(url: url), completion: completion)
    }

    override func main() {
        task.resume()
    }

    override func cancel() {
        task.cancel()
    }
}

这回传了Data，但是您可以编写排列/子类，将其进一步解析为使用JSONDecoder或返回的Web服务返回的任何内容.但是希望这可以说明基本思想.

This is passing back Data, but you can write permutations/subclasses that further parse that into whatever your web service is returning using JSONDecoder or whatever. But hopefully this illustrates the basic idea.

上面使用了这个AsynchronousOperation类:

/// Asynchronous operation base class
///
/// This is abstract to class performs all of the necessary KVN of `isFinished` and
/// `isExecuting` for a concurrent `Operation` subclass. You can subclass this and
/// implement asynchronous operations. All you must do is:
///
/// - override `main()` with the tasks that initiate the asynchronous task;
///
/// - call `completeOperation()` function when the asynchronous task is done;
///
/// - optionally, periodically check `self.cancelled` status, performing any clean-up
///   necessary and then ensuring that `finish()` is called; or
///   override `cancel` method, calling `super.cancel()` and then cleaning-up
///   and ensuring `finish()` is called.

public class AsynchronousOperation: Operation {

    /// State for this operation.

    @objc private enum OperationState: Int {
        case ready
        case executing
        case finished
    }

    /// Concurrent queue for synchronizing access to `state`.

    private let stateQueue = DispatchQueue(label: Bundle.main.bundleIdentifier! + ".rw.state", attributes: .concurrent)

    /// Private backing stored property for `state`.

    private var _state: OperationState = .ready

    /// The state of the operation

    @objc private dynamic var state: OperationState {
        get { stateQueue.sync { _state } }
        set { stateQueue.sync(flags: .barrier) { _state = newValue } }
    }

    // MARK: - Various `Operation` properties

    open         override var isReady:        Bool { return state == .ready && super.isReady }
    public final override var isAsynchronous: Bool { return true }
    public final override var isExecuting:    Bool { return state == .executing }
    public final override var isFinished:     Bool { return state == .finished }

    // KVN for dependent properties

    open override class func keyPathsForValuesAffectingValue(forKey key: String) -> Set<String> {
        if ["isReady", "isFinished", "isExecuting"].contains(key) {
            return [#keyPath(state)]
        }

        return super.keyPathsForValuesAffectingValue(forKey: key)
    }

    // Start

    public final override func start() {
        if isCancelled {
            state = .finished
            return
        }

        state = .executing

        main()
    }

    /// Subclasses must implement this to perform their work and they must not call `super`. The default implementation of this function throws an exception.

    open override func main() {
        fatalError("Subclasses must implement `main`.")
    }

    /// Call this function to finish an operation that is currently executing

    public final func finish() {
        if isExecuting { state = .finished }
    }
}

有很多写基础AsynchronousOperation的方法，我不想在细节上迷失方向，但是想法是我们现在有了一个Operation，我们可以将其用于任何异步过程.

There are lots of ways to write a base AsynchronousOperation, and I don’t want to get lost in the details, but the idea is that we now have an Operation that we can use for any asynchronous process.