使用循环运行后台提取时出现NSInternalIncontincyException

Question

我有这样一段代码，它试图对HealthKit数据进行后台提取。当我第一次运行应用程序时，代码运行得很好，但如果我手动执行后台提取(使用debug命令)，我会抛出一个异常和一个显示reason: 'this request has been neutered - you can't call -sendResponse: twice nor after encoding it'的错误，我不太确定原因。

以下是获取数据的代码：

- (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数据，您可以看到这些数据在找到结果时被调用。如果你愿意，我可以把它粘贴在这里，但它很长，不确定它是否与它有关。

我试着插入断点来查看何时调用完成处理程序，但据我所知，它只被调用过一次，除非我在这里遗漏了一些愚蠢的东西。

如果有人有什么建议，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

从您发布的代码来看，由于for循环，除了后台获取完成处理程序将被准确调用4次之外，我看不到任何其他方法。

在任何情况下，HKSampleQuery的每个实例的resultHandlers中的代码路径都将以completionHandler(UIBackgroundFetchResult...)调用结束，并且您总是实例化其中的四个，因此这就是断言'you can't call -sendResponse: twice'所抱怨的IMO.

通过注释dataTypes字典中的3个查询，应该可以很容易地检查这是否是问题所在。

编辑：根据评论中的要求，这里有一个可能的解决方案(..就在我的脑海中，所以对此持怀疑态度..)：

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) 使用调度组等待所有4个查询完成，然后再执行完成处理程序。

// 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);
});