ios – [NSFunctionExpression _propertyType]：发送到实例的无法识别的选择器

我的项目实现了与iCloud的核心数据同步.当我尝试删除重复项时,我得到以下异常：

– [NSFunctionExpression _propertyType]：发送到实例的无法识别的选择器

我已经读过这种问题的主要原因是在不同的线程上使用相同的托管对象上下文.但是,在我的情况下,所有进程都在主线程上完成.我不知道如何解决这个问题.

在初始化方法中,我添加以下观察者：

[[NSNotificationCenter defaultCenter] addObserverForName:NSPersistentStoreDidImportUbiquitousContentChangesNotification
                                                      object:self.managedObjectContext.persistentStoreCoordinator
                                                       queue:[NSOperationQueue mainQueue]
                                                  usingBlock:^(NSNotification *note) {
                                                      NSLog(@"NSPersistentStoreDidImportUbiquitousContentChangesNotification received");
                                                      [self.managedObjectContext performBlock:^{
                                                          [self.managedObjectContext mergeChangesFromContextDidSaveNotification:note];
                                                          [self filterDuplicates]; <-- Here I call method to call filtering duplicates
                                                      }];
                                                  }];

重复方法：

- (void) filterDuplicates {

    NSString *uniquePropertyKey = @"name";
    NSString *entityName        = @"SFGallery";

    NSExpression *countExpression                       = [NSExpression expressionWithFormat:@"count:(%@)",uniquePropertyKey];
    NSExpressionDescription *countExpressionDescription = [[NSExpressionDescription alloc] init];
    [countExpressionDescription setName:@"count"];
    [countExpressionDescription setExpression:countExpression];
    [countExpressionDescription setExpressionResultType:NSInteger64AttributeType];

    NSEntityDescription *entity             = [NSEntityDescription entityForName:entityName inManagedObjectContext:self.managedObjectContext];
    NSAttributeDescription *uniqueAttribute = [[entity attributesByName] objectForKey:uniquePropertyKey];

    // Fetch the number of times each unique value appears in the store.
    NSFetchRequest *fetchRequest            = [NSFetchRequest fetchRequestWithEntityName:entityName];
    fetchRequest.propertiesToFetch          = @[uniqueAttribute,countExpression];
    fetchRequest.propertiesToGroupBy        = @[uniqueAttribute];
    fetchRequest.resultType                 = NSDictionaryResultType;
    fetchRequest.sortDescriptors            = @[[NSSortDescriptor sortDescriptorWithKey:@"order" ascending:YES]];

    NSArray *fetchedDictionaries            = [self fetchAlbumsWithFetchRequest:fetchRequest];

    // Filter out unique values that have no duplicates.
    NSMutableArray *valuesWithDupes = [NSMutableArray array];
    for (NSDictionary *dict in fetchedDictionaries) {
        NSNumber *count = dict[@"count"];
        if ([count integerValue] > 1) {
            [valuesWithDupes addObject:dict[uniquePropertyKey]];
        }
    }

    NSFetchRequest *dupeFetchRequest        = [NSFetchRequest fetchRequestWithEntityName:entityName];
    dupeFetchRequest.includesPendingChanges = NO;
    dupeFetchRequest.fetchBatchSize         = 20;
    NSPredicate *predicate                  = [NSPredicate predicateWithFormat:@"name IN (%@)",valuesWithDupes];
    dupeFetchRequest.predicate              = predicate;
    NSArray *dupes                          = [self fetchAlbumsWithFetchRequest:dupeFetchRequest];

    SFGallery *prevObject;
    for (SFGallery *duplicate in dupes) {
        if (prevObject) {
            if ([duplicate.name isEqualToString:prevObject.name]) {
                if ([duplicate.timestamp compare:prevObject.timestamp] == NSOrderedAscending) {
                    [self.managedObjectContext deleteObject:duplicate];
                } else {
                    [self.managedObjectContext deleteObject:prevObject];
                    prevObject = duplicate;
                }
            } else {
                prevObject = duplicate;
            }
        } else {
            prevObject = duplicate;
        }
    }
}

以及发生异常时的方法：

- (NSArray *)fetchAlbumsWithFetchRequest:(NSFetchRequest *)fetchRequest {
  Exception Here ---> NSFetchedResultsController *fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
                                                                                               managedObjectContext:self.managedObjectContext
                                                                                                 sectionNameKeyPath:nil
                                                                                                          cacheName:nil];

    NSError *error = nil;
    if (![fetchedResultsController performFetch:&error]) {
        NSLog(@"Error when fetching Albums: %@",[error userInfo]);
        return nil;
    }

    return fetchedResultsController.fetchedObjects;
}

有帮助吗？

编辑1：

我很确定它与NSFetchRequest有关.
NSFetchRequest的实例具有属性属性集：

fetchRequest.propertiesToFetch = @ [uniqueAttribute,countExpression];

这就是带来问题的countExpression.

在异常断点上,我记录了获取请求对象,我得到了以下内容：

<NSFetchRequest: 0x10c58b4a0> (entity: SFGallery; predicate: ((null)); sortDescriptors: ((
    "(timestamp,descending,compare:)"
)); type: NSDictionaryResultType; includesPendingChanges: NO; propertiesToFetch: ((
    "(<NSAttributeDescription: 0x10c56e5c0>),name title,isOptional 1,isTransient 0,entity SFGallery,renamingIdentifier title,validation predicates (\n),warnings (\n),versionHashModifier (null)\n userInfo {\n},attributeType 700,attributeValueClassName NSString,defaultValue (null)","count:(\"title\")"
)); propertiesToGroupBy: ((
    "(<NSAttributeDescription: 0x10c56e5c0>),defaultValue (null)"
)); )

这是抛出调用堆栈：

0   CoreFoundation                      0x0000000103692495 __exceptionPreprocess + 165
    1   libobjc.A.dylib                     0x00000001033ca99e objc_exception_throw + 43
    2   CoreFoundation                      0x000000010372365d -[NSObject(NSObject) doesNotRecognizeSelector:] + 205
    3   CoreFoundation                      0x0000000103683d8d ___forwarding___ + 973
    4   CoreFoundation                      0x0000000103683938 _CF_forwarding_prep_0 + 120
    5   CoreData                            0x0000000100d7812d -[NSFetchRequest _newValidatedProperties:groupBy:error:] + 541
    6   CoreData                            0x0000000100d0da63 -[NSFetchRequest(_NSInternalMethods) _resolveEntityWithContext:] + 179
    7   CoreData                            0x0000000100e03c2b -[NSFetchedResultsController initWithFetchRequest:managedObjectContext:sectionNameKeyPath:cacheName:] + 555
    8   myAppli                             0x000000010003d2d3 -[SFCoreDataManager fetchAlbumsWithFetchRequest:] + 163
    9   myAppli                             0x000000010003c901 -[SFCoreDataManager filterDuplicates] + 1073
    10  myAppli                             0x00000001000271cb -[SFDebugViewController removeDuplicates:] + 91
    11  UIKit                               0x0000000101d0af06 -[UIApplication sendAction:to:from:forEvent:] + 80
    12  UIKit                               0x0000000101d0aeb4 -[UIApplication sendAction:toTarget:fromSender:forEvent:] + 17
    13  UIKit                               0x0000000101de7880 -[UIControl _sendActionsForEvents:withEvent:] + 203
    14  UIKit                               0x0000000101de6dc0 -[UIControl touchesEnded:withEvent:] + 530
    15  UIKit                               0x0000000101d41d05 -[UIWindow _sendTouchesForEvent:] + 701
    16  UIKit                               0x0000000101d426e4 -[UIWindow sendEvent:] + 925
    17  UIKit                               0x0000000101d1a29a -[UIApplication sendEvent:] + 211
    18  UIKit                               0x0000000101d07aed _UIApplicationHandleEventQueue + 9579
    19  CoreFoundation                      0x0000000103621d21 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
    20  CoreFoundation                      0x00000001036215f2 __CFRunLoopDoSources0 + 242
    21  CoreFoundation                      0x000000010363d46f __CFRunLoopRun + 767
    22  CoreFoundation                      0x000000010363cd83 CFRunLoopRunSpecific + 467
    23  GraphicsServices                    0x000000010482cf04 GSEventRunModal + 161
    24  UIKit                               0x0000000101d09e33 UIApplicationMain + 1010
    25  myAppli                             0x0000000100010473 main + 115
    26  libdyld.dylib                       0x0000000103ea55fd start + 1
    27  ???                                 0x0000000000000001 0x0 + 1

编辑2：

当我得到这个例外时：
[NSFunctionExpression _propertyType]：无法识别的选择器发送到实例0x10c433d90

用po打印实例后[0x10c433d90类]

我得到了NSFunctionExpression

解决方法

上面包含的代码部分来自苹果示例代码.
这是来源： https://developer.apple.com/library/ios/documentation/DataManagement/Conceptual/UsingCoreDataWithiCloudPG/UsingSQLiteStoragewithiCloud/UsingSQLiteStoragewithiCloud.html

在提供样本之前,我无法相信苹果工程师不会测试代码.

首先,表达式应该以这种方式定义：

NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:uniquePropertyKey];
NSExpression *countExpression = [NSExpression expressionForFunction: @"count:" arguments:@[keyPathExpression]];

在这种情况下,实际上我很惊讶为什么这个实现不起作用：

NSExpression *countExpression = [NSExpression expressionWithFormat:@"count:(%@)",uniquePropertyKey];

可能是一个知道为什么可以分享一些知识的人

而苹果的闪亮花朵：

fetchRequest.propertiesToFetch = @[uniqueAttribute,countExpression];

应该

fetchRequest.propertiesToFetch = @[uniqueAttribute,countExpressionDescription];

我希望这个答案可以节省很多时间