为什么 PFQuery.cancel() 作为 parse.com-Framework 的一部分不起作用？

Question

我找到了很多与这个主题相关的帖子，但我可能会一直做错事......

在创建query（PFQuery 的一个实例）之后，请求被实现为query.findObjects()（在后台线程上运行）。在该请求期间，我无法像 query.cancel() 方法所期望的那样取消其进程。

场景：Internet 连接丢失，query.findObjects() 尝试连接，失败，重试。我实现了在请求第一次失败时执行的query.cancel()（在上述点执行的if objects == nil 中），但它仍在进行第二次尝试。在第二次尝试后（每一次都需要大约 15 秒）失败了，不会触发第三次。

为什么会出现这种情况，为什么调用query.cancel()时进程没有中断？

感谢您的帮助！

编辑 1：一些代码

func getPost() {

    let queue = dispatch_queue_create("SerialBgQueue", DISPATCH_QUEUE_SERIAL)

    dispatch_async(queue, {

        var query = PFQuery(className: "Post")

        var objects = query.findObjects()

        if objects != nil {

            // do something

        } else {

            println("This part is executed now")

            // doesn't stop the ongoing (second) connection attempt:
            query.cancel()

           // do something

            return
        }
    })
}

编辑 2：这就是日志 - 对我来说，这看起来像是第二次尝试，检查日志（尝试 2、3、4，...然后重新开始 2、3、4，...）。 else {-part 中间执行一次，最后执行第二次。之后，一切都停止了，不再“尝试”。

2014-10-08 13:37:10.104 Instagram[9210:391955] Error: Error Domain=NSURLErrorDomain Code=-1009 "The Internet connection appears to be offline." UserInfo=0x7fe708c545c0 {NSUnderlyingError=0x7fe708c53020 "The Internet connection appears to be offline.", NSErrorFailingURLStringKey=https://api.parse.com/2/find, NSErrorFailingURLKey=https://api.parse.com/2/find, _kCFStreamErrorDomainKey=12, _kCFStreamErrorCodeKey=8, NSLocalizedDescription=The Internet connection appears to be offline.} (Code: 100, Version: 1.3.0)

2014-10-08 13:37:10.105 Instagram[9210:391952] Network connection failed. Making attempt 2 after sleeping for 1.092226 seconds.

2014-10-08 13:37:11.311 ... (Code: 100, Version: 1.3.0) // same Error like in the very first line

2014-10-08 13:37:11.312 Instagram[9210:391945] Network connection failed. Making attempt 3 after sleeping for 2.184451 seconds.

2014-10-08 13:37:13.704 ... (Code: 100, Version: 1.3.0) 

2014-10-08 13:37:13.704 Instagram[9210:391952] Network connection failed. Making attempt 4 after sleeping for 4.368902 seconds.

2014-10-08 13:37:18.514 ... (Code: 100, Version: 1.3.0)

2014-10-08 13:37:18.514 Instagram[9210:391952] Network connection failed. Making attempt 5 after sleeping for 8.737804 seconds.

2014-10-08 13:37:27.257 ... (Code: 100, Version: 1.3.0)

This part is executed now

2014-10-08 13:37:27.265 ... (Code: 100, Version: 1.3.0)

2014-10-08 13:37:27.266 Instagram[9210:392183] Network connection failed. Making attempt 2 after sleeping for 1.387503 seconds.

2014-10-08 13:37:28.792 ... (Code: 100, Version: 1.3.0)

2014-10-08 13:37:28.793 Instagram[9210:392202] Network connection failed. Making attempt 3 after sleeping for 2.775006 seconds.

2014-10-08 13:37:31.843 ... (Code: 100, Version: 1.3.0)

2014-10-08 13:37:31.844 Instagram[9210:392205] Network connection failed. Making attempt 4 after sleeping for 5.550011 seconds.

2014-10-08 13:37:37.401 ... (Code: 100, Version: 1.3.0)

2014-10-08 13:37:37.401 Instagram[9210:392202] Network connection failed. Making attempt 5 after sleeping for 11.100023 seconds.

2014-10-08 13:37:49.050 ... (Code: 100, Version: 1.3.0)

This part is executed now

Answer 1

我认为您对findObjects() 的假设是不正确的。该品种会阻止启动它的线程（您在发布案例中的后台线程），并在成功或失败时返回。当您调用取消时，您的代码中没有什么可以取消。

典型的取消模式是代码在不同的线程上启动查询，然后决定它需要取消，就像这样（在 Objective-C 中......对不起）

PFQuery *query = [PFQuery queryWithClassName:@"Post"];    
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
    // this block might get called more than once, depending on the cache policy
    // but it will be called with an error only once and stop after that
    // (no second attempt that I know about) so as far as I can tell, there's
    // no reason to ever call cancel here.
}];

假设您不希望查询时间超过 10 秒。那么……

[query performSelector:@selector(cancel) withObject:nil afterDelay:10.0];