如何在移动到下一行代码之前完成 NSURLSession 请求

Question

任何帮助都将不胜感激，我有一堆用于调用 NSURLConnection Synchronous 方法的类，但不再存在。如何以在继续前等待响应的方式调用此代码块？

class func login(email:String,password:String)->Int{
    var userId = 0
    let postEndpoint:String = "https://www.testsite.com/user/login.php"
    let url = NSURL(string: postEndpoint)!
    let urlRequest = NSMutableURLRequest(URL: url, cachePolicy: .ReloadIgnoringLocalAndRemoteCacheData, timeoutInterval: 15.0)
    urlRequest.HTTPMethod = "POST"
    let body = "email=\(email)&password=\(password)".dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: false)
    urlRequest.HTTPBody = body
    let task = NSURLSession.sharedSession().dataTaskWithRequest(urlRequest) {(data, response, error) in
        if data != "" && error == nil{
            do {
                let json = try NSJSONSerialization.JSONObjectWithData(data!, options: []) as! [String: AnyObject]
                if let id = json["id"] as? Int {
                    print("found id:\(id)")
                    userId = id
                }                
            } catch let error as NSError {
                print("Failed to load: \(error.localizedDescription)")
                userId = 2
            }

        }
        else{
            userId = 2
        }
    }
    task.resume()
    return userId
}

调用这个的代码...

success = User.login(emailTextField.text!, password: passwordTextField.text!)
print("success last: \(success)")

问题是在返回用户 ID 之前调用了“最后一次成功：”。啊啊啊啊啊啊！

Answer 1

几年前我问过同样的问题。我没有收到那个答案。那时人们制造的 cmets 和现在一样。我确实认为完成处理程序是一个更好的主意，但是现在您使用的是 Swift，您可以使用 Grand Central Dispatch 来做到这一点：

class func login(email:String,password:String)->Int{
    var userId = 0
    let postEndpoint:String = "https://www.testsite.com/user/login.php"
    let url = NSURL(string: postEndpoint)!
    let urlRequest = NSMutableURLRequest(URL: url, cachePolicy: .ReloadIgnoringLocalAndRemoteCacheData, timeoutInterval: 15.0)
    urlRequest.HTTPMethod = "POST"
    let body = "email=\(email)&password=\(password)".dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: false)
    urlRequest.HTTPBody = body

    let group = dispatch_group_create()

    // Signal the beginning of the async task. The group won't
    // finish until you call dispatch_group_leave()
    dispatch_group_enter(group)

    dispatch_group_async(group, dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0)) {
        let task = NSURLSession.sharedSession().dataTaskWithRequest(urlRequest) {(data, response, error) in
            if data != "" && error == nil{
                do {
                    let json = try NSJSONSerialization.JSONObjectWithData(data!, options: []) as! [String: AnyObject]
                    if let id = json["id"] as? Int {
                        print("found id:\(id)")
                        userId = id
                    }
                } catch let error as NSError {
                    print("Failed to load: \(error.localizedDescription)")
                    userId = 2
                }
            }
            else {
                userId = 2
            }

            // Signal the end of the async task
            dispatch_group_leave(group)
        }
        task.resume()
    }

    // Wait 30 seconds for the group to be done
    let timeout = dispatch_time(DISPATCH_TIME_NOW, 30_000_000_000)
    dispatch_group_wait(group, timeout)

    return userId
}

如果您不希望它超时，请将倒数第二行替换为：

dispatch_group_wait(group, DISPATCH_TIME_FOREVER)

---

这有什么用处：

假设您必须调用 3 个 Web 服务。在所有 3 个都完成之前，您的申请无法继续。您可以调用第一个服务，然后在其完成块中调用第二个服务，依此类推。但是，您需要为等待付出时间代价。

使用此模式，您可以同时调用所有 3 个：

let group = dispatch_group_create()
dispatch_group_enter(group)
dispatch_group_enter(group)
dispatch_group_enter(group)

dispatch_group_async(group, dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0)) {
    let task1 = NSURLSession.sharedSession().dataTaskWithRequest(request1) {...
        // handle response & error
        dispatch_group_leave()
    }

    let task2 = NSURLSession.sharedSession().dataTaskWithRequest(request2) {...
        // handle response & error
        dispatch_group_leave()
    }

    let task3 = NSURLSession.sharedSession().dataTaskWithRequest(request3) {...
        // handle response & error
        dispatch_group_leave()
    }
}

let timeout = dispatch_time(DISPATCH_TIME_NOW, 30_000_000_000)
dispatch_group_wait(group, timeout)

// proceed with your code...