使用循环运行后台获取时出现 NSInternalInconsistencyException

Question

我有这段代码正在尝试对 HealthKit 数据进行后台提取。当我第一次运行该应用程序时，代码运行良好，但如果我手动执行后台提取（使用调试命令），我会抛出一个异常和一个错误，上面写着reason: 'this request has been neutered - you can't call -sendResponse: twice nor after encoding it'，我不太清楚为什么。

这是获取数据的代码：

- (void)fetchNewDataWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler {

    if (!self.healthStore) {
        self.healthStore = [HKHealthStore new];
    }

    dataTypes = [NSDictionary dictionaryWithObjectsAndKeys:
                 [NSNumber numberWithInt:1], HKQuantityTypeIdentifierStepCount,
                 [NSNumber numberWithInt:2], HKQuantityTypeIdentifierFlightsClimbed,
                 [NSNumber numberWithInt:3], HKQuantityTypeIdentifierDistanceWalkingRunning,
                 [NSNumber numberWithInt:4], HKQuantityTypeIdentifierDistanceCycling, nil];
    achievementData = [NSMutableDictionary new];

    NSCalendar *calendar = [NSCalendar currentCalendar];
    NSDate *startDate = [calendar startOfDayForDate:[NSDate date]];
    NSDate *endDate = [calendar dateByAddingUnit:NSCalendarUnitDay value:1 toDate:startDate options:0];
    NSPredicate *predicate = [HKQuery predicateForSamplesWithStartDate:startDate endDate:endDate options:HKQueryOptionNone];

    for (NSString *key in dataTypes) {

        HKSampleType *sampleType = [HKSampleType quantityTypeForIdentifier:key];

        HKSampleQuery *query = [[HKSampleQuery alloc] initWithSampleType:sampleType predicate:predicate limit:HKObjectQueryNoLimit sortDescriptors:nil resultsHandler:^(HKSampleQuery *query, NSArray *results, NSError *error) {

            if (!error) {

                if (!results) {

                    NSLog(@"No results were returned form query");

                    completionHandler(UIBackgroundFetchResultNoData);

                } else {

                    dispatch_async(dispatch_get_main_queue(), ^{

                        [self processNewDataWithResults:results andType:key];

                        completionHandler(UIBackgroundFetchResultNewData);

                    });

                }

            } else {

                NSLog(@"Error: %@ %@", error, [error userInfo]);

                completionHandler(UIBackgroundFetchResultFailed);

            }

        }];

        [self.healthStore executeQuery:query];

    }

}

然后我有一个单独的函数来处理你可以看到的数据，当找到结果时会被调用。如果你愿意，我可以把它贴在这里，但它很长，不确定它是否与它有关。

我尝试在其中设置断点以查看何时调用完成处理程序，但据我所知，它只被调用一次，除非我在这里遗漏了一些愚蠢的东西。

如果有人有任何建议，请告诉我:) 谢谢！

编辑 以下是错误消息的样子：

2015-05-13 10:11:54.467 appName[379:169163] *** Assertion failure in -[UIFetchContentInBackgroundAction sendResponse:], /SourceCache/BaseBoard/BaseBoard-98.3/BaseBoard/BSAction.m:221
2015-05-13 10:11:54.470 appName[379:169163] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'this request has been neutered - you can't call -sendResponse: twice nor after encoding it'
*** First throw call stack:
(0x28ed4137 0x36e30c77 0x28ed400d 0x29bd2bc9 0x2dbde865 0x397ed5 0x2dbde7cf 0x2ca82a3d 0x39019b 0x390187 0x393e9d 0x28e99889 0x28e97fa9 0x28de39a1 0x28de37b3 0x305951a9 0x2c56e695 0xdff29 0x373d8aaf)
libc++abi.dylib: terminating with uncaught exception of type NSException

Answer 1

通过查看您发布的代码，除了for 循环，我没有看到后台获取完成处理程序将被准确调用 4 次。

HKSampleQuery 的每个实例的 resultHandlers 中的代码路径在任何情况下都会以 completionHandler(UIBackgroundFetchResult...) 调用结束，并且您总是实例化其中的四个，所以这就是断言 'you can't call -sendResponse: twice' 所抱怨的关于 IMO。

通过注释掉dataTypes字典中的3个查询应该很容易检查这是否是问题。

编辑：根据 cmets 的要求，这是一个可能的解决方案（.. 只是在我的头上，所以带着一粒盐..）：

它 (a) 使用信号量锁将异步查询结果回调转换为“同步”调用 & (b) 在执行完成处理程序之前使用调度组等待所有 4 个查询完成。

// NOTE: This example assumes that the fetch has "new data" if any of the queries returned something
//       Also it skips the 'NSError' part (which could work exactly like the 'result' var)

// Define one or more block-global result variables for queries can put their end state into
UIBackgroundFetchResult __block result = UIBackgroundFetchResultNoData;

// Create a dispatch group that will manage all the concurrent queries
dispatch_queue_t queue = dispatch_queue_create([@"my.query.queue" UTF8String], DISPATCH_QUEUE_CONCURRENT);
dispatch_group_t queries = dispatch_group_create();

// For each dataType dispatch a group block containing the query to run on the concurrent queue
for (NSString *key in dataTypes) {
    dispatch_group_async(queries, queue, ^{
        // To work around the asynchronous callback, I'll use a lock to wait for the query to return their result, so..

        // ..like above, a block-global var will hold the result of the query
        BOOL __block success = NO;

        // ..create a one-time lock..
        dispatch_semaphore_t lock = dispatch_semaphore_create(0);
        HKSampleQuery *query = [[HKSampleQuery alloc] initWithSampleType:sampleType predicate:predicate limit:HKObjectQueryNoLimit sortDescriptors:nil resultsHandler:^(HKSampleQuery *query, NSArray *results, NSError *error) {
            success = YES; // ..or however you define success.. ;)

            dispatch_semaphore_signal(lock);    // ..open lock to signal result available..
        }];
        dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);   // ..wait for callback result lock to open..

        // ..determine & set result.
        if (success) {
            result = UIBackgroundFetchResultNewData;
        }
    });
}

// Schedule a final block to execute (on the main queue) when all the other group blocks have finished running
dispatch_group_notify(queries, dispatch_get_main_queue(), ^{
    // Determine final result and call completion handler
    completionHandler(result);
});