ios - DispatchQueue 不等待异步函数完成

Question

我正在尝试创建一个使用 USPS API 获取数据的简单包裹跟踪应用程序。 callRestService 方法成功获取数据，其完成处理程序 serviceCallback(设置 unparsedXml 属性)工作。但是，我从中调用 callRestService 的方法不会等待它及其完成处理程序完成后再继续，导致我的 print(unparsedXml) 语句返回 nil。

如下所示，我尝试使用 DispatchGroup 对象和 DispatchQueue 使函数等待 callRestService 完成，但无论如何它都会继续。如何让函数等待调用完成？

var unparsedXml:String?

public func getTrackingInfo(_ trackingNumber: String) -> TrackingInfo {
    let group = DispatchGroup()
    group.enter()
    DispatchQueue.global(qos: DispatchQoS.default.qosClass).async {
        self.callRestService(requestUrl: self.getRequest(trackingNumber))
        group.leave()
    }
    group.wait()
    print(unparsedXml)
    return TrackingInfo()
}

private func getRequest(_ trackingNumber: String) -> String {
    let APIUsername = "Intentionally Omitted"
    let trackingXmlLink = "http://production.shippingapis.com/ShippingAPI.dll?API=TrackV2&XML=%3CTrackFieldRequest%20USERID=%22" + APIUsername + "%22%3E%20%3CRevision%3E1%3C/Revision%3E%20%3CClientIp%3E127.0.0.1%3C/ClientIp%3E%20%3C/SourceId%3E%20%3CTrackID%20ID=%22" + trackingNumber + "%22%3E%20%3CDestinationZipCode%3E66666%3C/DestinationZipCode%3E%20%3CMailingDate%3E2010-01-01%3C/MailingDate%3E%20%3C/TrackID%3E%20%3C/TrackFieldRequest%3E"
    return trackingXmlLink
}

public func callRestService(requestUrl:String) ->Void
{
    var request = URLRequest(url: URL(string: requestUrl)!)
    request.httpMethod = "GET"
    
    let session = URLSession.shared
    let task = session.dataTask(with: request, completionHandler: serviceCallback)
    
    task.resume()
}

private func serviceCallback(data:Data? , response:URLResponse? , error:Error? ) -> Void
{
    unparsedXml = String(data: data!, encoding: .utf8)
    //print(unparsedXml) Works correctly when uncommented
}

Answer 1

您的问题是 callRestService 将调度一个异步网络操作，因此您的 group.leave 将立即被调用，触发您的 group.notify.

您可以将 group.leave 放在完成处理程序中，但您应该避免阻塞代码。我建议您将 getTrackingInfo 构造为采用完成处理程序的异步函数:

public func getTrackingInfo(_ trackingNumber: String, completion:(TrackingInfo?,Error?) -> Void) {
    self.callRestService(requestUrl: self.getRequest(trackingNumber)) { (data, response, error) in
        guard error == nil, let returnData = data else {
            completion(nil,error)
            return
        }

        completion(TrackingInfo(returnData),nil)
    }
}

private func getRequest(_ trackingNumber: String) -> String {
    let APIUsername = "Intentionally Omitted"
    let trackingXmlLink = "http://production.shippingapis.com/ShippingAPI.dll?API=TrackV2&XML=%3CTrackFieldRequest%20USERID=%22" + APIUsername + "%22%3E%20%3CRevision%3E1%3C/Revision%3E%20%3CClientIp%3E127.0.0.1%3C/ClientIp%3E%20%3CSourceId%3EFaiz%20Surani%3C/SourceId%3E%20%3CTrackID%20ID=%22" + trackingNumber + "%22%3E%20%3CDestinationZipCode%3E66666%3C/DestinationZipCode%3E%20%3CMailingDate%3E2010-01-01%3C/MailingDate%3E%20%3C/TrackID%3E%20%3C/TrackFieldRequest%3E"
    return trackingXmlLink
}

public func callRestService(requestUrl:String, completion:(Data? , URLResponse? , Error? ) -> Void) ->Void
{
    var request = URLRequest(url: URL(string: requestUrl)!)
    request.httpMethod = "GET"

    let session = URLSession.shared
    let task = session.dataTask(with: request, completionHandler: completion)

    task.resume()
}