ios - Swift:同步 Web 服务调用

Question

在 Swift 中，我正在调用 Web 服务(Google Places)并成功获取 Google Place ID。

当我遍历 JSON 响应并获取 Google Place ID 时，我想调用另一个 Web 服务(Google Place Details)

使用下面的代码，我得到的响应是:

estPlace_ID_1
Return Number Is: Nothing
estPlace_ID
Return Number Is: Nothing
.....
Function Phone Number is: 867-5309
Function Phone Number is: 867-5309

似乎在 for result in results 循环完成之前函数 get Details 没有被执行。

如何更改代码，使其等到执行 getDetails 后再继续迭代？

class func getDetails(id: String) -> String {
    <Setup the request>
    let session = NSURLSession.sharedSession()

    //Second Request
    let task = session.dataTaskWithRequest(request) { data, response, error in 
        do {
            //Parse Result
            print("Function Phone Number is" + phoneNumber)
        }
        catch {
        }
    }
    task.resume()

    return phoneNumber
}

//First request
<Setup the request>
let task = session.dataTaskWithRequest(request) { data, response, error in
    //a few checks with guard statements

    do {        
        //Extract results from JSON response
        results = <FROM_JSON>

        for result in results {
            estPlace_ID = result["value"]

            print(estPlace_ID)
            print("return number is" + getDetails(estPlace_ID))              
        }
        catch {       
        }
    }
    task.resume()
}

Answer 1

可以通过调度信号量实现函数调用阻塞，直到异步调用的结果到达。模式是:

create_semaphore()
someAyncCall() {
    signal_semaphore()
}
wait_for_semaphore()
rest_of_the_code()

在您的情况下，您可以按如下方式修改 getDetails 方法:

class func getDetails(id: String) -> String {
    <Setup the request>
    let session = NSURLSession.sharedSession()
    let sem = dispatch_semaphore_create(0)

    //Second Request
    let task = session.dataTaskWithRequest(request) { data, response, error in 
        do {
            //Parse Result
            print("Function Phone Number is" + phoneNumber)

        } catch {
        }
        // the task has completed, signal this
        dispatch_semaphore_signal(sem)
    }
    task.resume()

    // wait until the semaphore is signaled
    dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER)

    // we won't get here until dispatch_semaphore_signal() is called
    return phoneNumber
}

要记住的一件重要事情(感谢 Rob 指出这一点)是您需要在不同的队列上调用 getDetails，否则您会遇到死锁:

dispatch_async(dispatch_get_global_queue(0, 0) ){
    for result in results {
        let estPlace_ID = result["value"]

        print(estPlace_ID)
        print("return number is" + getDetails(estPlace_ID))
    }
}

请注意，在上面的示例中，dispatch_semaphore_wait 的第二个参数是 DISPATCH_TIME_FOREVER，这意味着调用代码将无限期地等待异步调用完成。如果你想设置一些超时，你可以创建一个 dispatch_time_t 值并传递它:

// want to wait at most 30 seconds
let timeout = 30
let dispatchTimeout = dispatch_time(DISPATCH_TIME_NOW, timeout * Int64(NSEC_PER_SEC))
dispatch_semaphore_wait(sem, dispatchTimeout)