问题描述
我在测试中看到一个错误，偶尔会出现以下iOS错误：
I am seeing an error in my tests where occasionally I get the following iOS error:
标识为{GUID}的后台URLSession已经存在！
A background URLSession with identifier {GUID} already exists!
即使我在每次测试后在清理调用中调用NSURLSession上的invalidateAndCancel。我正在寻找一种方法，等到我确定NSURLSession对象已经失效，然后再进行下一次测试。
Even though I call invalidateAndCancel on the NSURLSession in the cleanup call after every test. I am looking for a way to wait until I am sure the NSURLSession object has been invalidated, before proceeding to the next test.
推荐答案
当您创建 NSURLSession 对象时，使用 sessionWithConfiguration：（NSURLSessionConfiguration *）配置委托：（id< NSURLSessionDelegate>）委托delegateQueue：（NSOperationQueue * ）queue; 方法并指定你自己作为委托。
When you create your NSURLSession object use the sessionWithConfiguration:(NSURLSessionConfiguration *)configuration delegate:(id <NSURLSessionDelegate>)delegate delegateQueue:(NSOperationQueue *)queue; method and specify yourself as the delegate.
然后你将得到一个回调： - （void ）URLSession：（NSURLSession *）session didBecomeInvalidWithError：（NSError *）error; 会话失效时。
You will then get a callback in: - (void)URLSession:(NSURLSession *)session didBecomeInvalidWithError:(NSError *)error; when the session is invalidated.
你不能相信 invalidateAndCancel 将立即停止所有任务，因为它取决于处理取消呼叫的任务。
You cannot trust that invalidateAndCancel will instantly stop all tasks since it is up to the tasks to handle the cancel-call.
从标题中 NSURLSession ：
/* -invalidateAndCancel acts as -finishTasksAndInvalidate, but issues
* -cancel to all outstanding tasks for this session. Note task
* cancellation is subject to the state of the task, and some tasks may
* have already have completed at the time they are sent -cancel.
*/
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