iOS中的NSOperation-如何处理循环和嵌套的NSOperation调用以获取图像

我需要帮助了解如何适当处理以下用例：

假设我正在编写聊天应用程序：

发布应用程序

要求服务器（

AFHTTPRequestOperation

）提供所有新消息的列表

循环浏览这些消息，看看我是否需要下载任何图像

如果是，则再次调用服务器（

AFImageRequestOperation

）以获取图像

我的托管对象“消息”不再在同一上下文中，但我只使用了一个

managedObjectContext

，因此不断发生崩溃

这是因为我嵌套调用的方式，因为它们是异步的吗？我几乎可以肯定的是，这条消息没有在其他地方被删除，因为我看到了它

需要注意的一点是，当我更改视图控制器时，这种情况似乎会发生。我启动应用程序，在根视图控制器（RVC）上，它执行上面的第2步。如果我在下载所有图像之前点击进入“

MessageListViewController

”（MLVC），则未完成下载的图像的关联消息突然变为nil

managedObjectContext

以下是相关代码：

    AFHTTPRequestOperation * operation = [AFJSONRequestOperation JSONRequestOperationWithRequest:requestImageInfoURL
     success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) {

         NSDictionary * JSONresponse = (NSDictionary *)JSON;

         if( [[[JSONresponse objectForKey:@"status"] uppercaseString] isEqualToString:@"ERROR"] )
         {
             for( id<CommsObserver> observer in _observers )
                 [observer errorOccurred:[JSONresponse objectForKey:@"message"]];
         }
         else
         {
             if( [JSONresponse containsKey:@"convoMessages"] )
             {
                 NSArray * messageList = [JSON objectForKey:@"messages"];

                 for( int i = 0 ; i < messageList.count ; i++ )
                 {
                     __block ConversationMessage * message = [JSONUtility convoMessageFromJSON:[messageList objectAtIndex:i]];

                     if( !message )
                         NSLog( @"Couldn't create the new message..." );
                     else
                     {
                         message.unread = [NSNumber numberWithBool:YES];
                         [[DataController sharedController] saveContext];

                         if( (!message.text || [message.text isEqualToString:@""]) && (message.image || message.imageInfoKey) )
                         {
                             NSString * imageURL = (message.image ? [Comms urlStringForImageInfo:message.image] : [Comms urlStringForImageKey:message.imageInfoKey extension:message.imageInfoExt]);

                             NSURLRequest *requestImageURL = [NSURLRequest requestWithURL:[NSURL URLWithString:imageURL]];
                             AFImageRequestOperation * imageOperation;
                             imageOperation = [AFImageRequestOperation imageRequestOperationWithRequest:requestImageURL
                                   imageProcessingBlock:^UIImage *(UIImage *image) {
                                       return image;
                                   } success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image) {

                                       if( message.image )
                                       {
                                           NSLog( @"updating imageInfo for message" );

                                           [Utilities updateImageInfo:message.image
                                                            withImage:image
                                                            asPreview:YES
                                                          asThumbnail:YES
                                                         preserveSize:YES];
                                       }
                                       else
                                       {
                                           NSLog( @"creating a new imageInfo for message" );

                                           ImageInfo * imageInfo = [Utilities createImageInfoFromImage:image asPreview:NO asThumbnail:NO preserveSize:YES];

                                           if( imageInfo.managedObjectContext == nil )
                                               NSLog( @"imageInfo MOC is NIL" );
                                           else if( message.managedObjectContext == nil )
                                           {
                                               NSLog( @"message MOC is NIL" );

                                               message = [[DataController sharedController] convoMessageForKey:message.key];

                                               if( !message )
                                                   NSLog( @"message is NIL, meaning it wasn't found in the MOC" );
                                               else if( !message.managedObjectContext )
                                                   NSLog( @"message MOC was STILL NIL" );
                                               else
                                                   NSLog( @"problem solved..." );
                                           }

                                           if( imageInfo )
                                               [[DataController sharedController] associateImageInfo:imageInfo withMessage:message];
                                       }

                                       [[DataController sharedController] saveContext];

                                   } failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error) {
                                       NSLog( @"Image DOWNLOAD error... \n%@" , [NSString stringWithFormat:@"%@" , error] );
                                   }];

                             [imageOperation start];
                         }

                         for( id<CommsObserver> observer in _observers )
                             [observer newConvoMessages:@[message.key]];
                     }
                 } // End for loop of messageList

             } // End if JSONresponse

         } // End outer if ERROR statement

     } // End Success 

     failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error, id JSON) {
         NSLog( @"Error: \n%@" , [NSString stringWithFormat:@"%@" , error] );
     }
];

[operation start];

AFHTTPRequestOperation*operation=[AFJSONRequestOperation JSONRequestOperationWithRequest:requestImageInfoURL
成功：^（NSURLRequest*请求，NSHTTPURLResponse*响应，id JSON）{
NSDictionary*JSONresponse=（NSDictionary*）JSON；
如果（[[JSONresponse objectForKey:@“状态”]uppercaseString]IseQualtString:@“错误”]）
{
for（id观察员在_观察员中）
[observer ErrorOccessed:[JSONresponse objectForKey:@“message”]；
}
其他的
{
if（[JSONresponse containsKey:@“ConvalMessages”]）
{
NSArray*messageList=[JSON objectForKey:@“messages”]；
for（int i=0；i [[DataController sharedController] saveContext];

[[DataController sharedController].managedObjectContext performBlock:^{
    assert(message.managedObjectContext == [DataController sharedController].managedObjectContext);
    message.unread = [NSNumber numberWithBool:YES];
    [[DataController sharedController] saveContext];
    ...

}];

NSManagedObjectContext* context =  [[NSManagedObjectContext alloc] 
    initWithConcurrencyType:NSPrivateQueueConcurrencyType];

__block NSManagedObject* obj;
[context performBlockAndWait:^{
    obj = [context objectRegisteredForID:theObjectID];
}];

NSString* name = obj.name;

__block NSString* name;
[obj.managedObjectContext performBlockAndWait:^{
    name = obj.name;
}];

NSManagedObjectID* moid = obj.objectID;  // safe from any thread

NSManagedObjectID* moid = obj.objectID

NSManagedObjectContext* otherContext = [[NSManagedObjectContext alloc] 
    initWithConcurrencyType:NSPrivateQueueConcurrencyType];

[otherContext performBlock:^{
    NSManagedObject* obj = [otherContext objectWithID: moid];
    ...
}];

__block NSManagedObject* obj;
__block NSError* error;
[context performBlockAndWait:^{
    obj = [context existingObjectWithID:theObjectID error:&error];
}];
if (obj==nil) {
    NSLog(@"Error:%@", error);
}

__block NSManagedObject* obj;
[context performBlock:^{
    NSError* error;
    obj = [context existingObjectWithID:theObjectID error:&error];
    if (obj==nil) {
        NSLog(@"Error:%@", error);
    }
}];