ios - 限制 iOS 应用中的 dispatch_async 任务时间

Question

我使用 dispatch_async 进行一些服务器调用。有时，由于互联网连接速度较慢，此调用会花费很长时间。

我如何设置以秒为单位的限制 - 例如:任务有 3 秒要完成，否则它会停止。

Answer 1

作为MultiColour Pixel says ，你真的应该为此使用 NSURLSession。

但是，如果您仍在寻找 GCD 答案，这里是...

---

您可以轻松地使用信号量为您的请求实现超时。

您只需要创建一个初始值为 0 的信号量，然后使用 dispatch_semaphore_wait 等待网络任务完成时发出信号，或者请求超时。

因此，如果您的网络任务是同步的，您会想要做这样的事情......

let queue = dispatch_queue_create("serialQueue", DISPATCH_QUEUE_SERIAL) // your GCD queue

...

dispatch_async(queue, {

    let semaphore = dispatch_semaphore_create(0) // create semaphore

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
        self.doNetworkTask()
        dispatch_semaphore_signal(semaphore); // signal semaphore on completion
    })

    let timeout = dispatch_time(DISPATCH_TIME_NOW, Int64(3.0 * Double(NSEC_PER_SEC))) // 3 second time-out
    let returnValue = dispatch_semaphore_wait(semaphore, timeout)

    guard returnValue == 0 else { // return value is non-zero if the request timed out

        // request timed out, cancel the request here.
        self.stopNetworkTask()
        return // prevent further execution if network task was unsucessful
    }

    dispatch_async(dispatch_get_main_queue(), {
        // do processing after request success
    })

})

或者如果您的网络任务是异步的...

let queue = dispatch_queue_create("serialQueue", DISPATCH_QUEUE_SERIAL) // your GCD queue

...

dispatch_async(queue, {

    let semaphore = dispatch_semaphore_create(0) // create semaphore

    self.doNetworkTaskWithCompletion({
        dispatch_semaphore_signal(semaphore); // signal semaphore on completion
    })

    let timeout = dispatch_time(DISPATCH_TIME_NOW, Int64(3.0 * Double(NSEC_PER_SEC))) // 3 second time-out
    let returnValue = dispatch_semaphore_wait(semaphore, timeout)

    guard returnValue == 0 else { // return value is non-zero if the request timed out

        // request timed out, cancel the request here.
        self.stopNetworkTask()
        return // prevent further execution if network task was unsucessful
    }

    dispatch_async(dispatch_get_main_queue(), {
        // do processing after request success
    })

})