上周五在面试的时候被面试官问到一个问题：在Swift 3中如何取消一个正在执行的GCD线程？因为我使用Swift的时间还不长，这方面的知识点我还没覆盖到。回家后经过一番Google我终于找到了答案。
答案是我在Stack Overflow上找到的，原问题地址为：http://stackoverflow.com/questions/29492707/how-to-stop-cancel-suspend-resume-tasks-on-gcd-queue
回答原文摘抄如下：
To suspend a dispatch queue, it's simply dispatch_suspend(queue)
in Objective-C or Swift 2.3, or queue.suspend()
in Swift 3. That doesn't affect any tasks currently running, but merely prevents new tasks from starting on that queue. Also, you obviously only suspend queues that you created (not global queues, not main queue).
To resume a dispatch queue, it's dispatch_resume(queue)
in Objective-C or Swift 2.3, or queue.resume()
in Swift 3. There's no concept of "auto resume", so you'd just have to manually resume it when appropriate.
To pass a dispatch queue around, you simply pass the dispatch_queue_t
object that you created when you called dispatch_queue_create()
in Objective-C or Swift 2.3, or, in Swift 3, the DispatchQueue
object you create with [DispatchQueue(label:)
](https://developer.apple.com/reference/dispatch/dispatchqueue/2300059-init).
In terms of canceling tasks queued on dispatch queues, this is a new feature of iOS 8 and you'd call dispatch_block_cancel(block)
with your dispatch_block_t
object in Objective-C or Swift 2.3, or [item.cancel()
](https://developer.apple.com/reference/dispatch/dispatchworkitem/1780910-cancel) of a DispatchWorkItem
in Swift 3. This cancels queued blocks/items that have not started, but does not stop ones that are underway. If you want to be able to interrupt a dispatched block/item, you have to periodically examine [dispatch_block_testcancel()
](https://developer.apple.com/reference/dispatch/1431046-dispatch_block_testcancel?language=objc) in Objective-C and Swift 2.3, or [item.isCancelled
](https://developer.apple.com/reference/dispatch/dispatchworkitem/1780829-iscancelled) in Swift 3.
The canceling of GCD blocks is sufficiently complicated that I might refer you to the WWDC 2014 video [Power, Performance and Diagnostics: What's new in GCD and XPC](https://developer.apple.com/videos/wwdc/2014/?id=716), which introduces you to the concept of dispatch block objects, queuing them, canceling them, etc. While this is describing the older Objective-C and Swift 2.3 API, all of the concepts are equally applicable to the newer Swift 3 API.
If you want to cancel tasks, you might also consider using operation queues, NSOperationQueue
in Objective-C or Swift 2.3, a.k.a. [OperationQueue
](https://developer.apple.com/reference/foundation/operationqueue) in Swift 3, as its cancelable operations have been around for a while and you're likely to find lots of examples online. It also supports constraining the degree of concurrency with maxConcurrentOperationCount
(whereas with dispatch queues you can only choose between serial and concurrent, and controlling concurrency more than that requires a tiny bit of effort on your part).
If using operation queues, you suspend and resume by changing the suspended
property of the queue. And to pass it around, you just pass the NSOperationQueue
object you instantiated.
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