ios - UITableView 的 NSFetchedResultsController 多个实体

我有两个实体，一个叫做 Post，一个叫做 User。 Post
初始化时，我不从服务器下载个人资料图片，我只在它滚动经过该单元格时下载(延迟加载)。下载后，我将下载的图像保存到核心数据堆栈中相应的 User.profilePicture。

当我更新 User 实体时，有没有办法调用 controllerDidChangeContent ？我目前的理解是我的 NSFetchedResultsController 只能跟随 Post 实体，因为这是我最初设置它做的事情并且不能遍历和监视关系中的更新，是真的吗？

最佳答案

可悲的是，我只知道针对此问题的丑陋解决方案。

在您的 User .m 文件中，像这样实现 setProfilePicture::

//NOT TESTED IN A MULTITHREADED ENV
- (void) setProfilePicture:(NSData *)data
{
    [self willChangeValueForKey:@"profilePicture"];
    [self setPrimitiveValue:data forKey:@"profilePicture"];
    [self.posts enumerateObjectsUsingBlock:^(Post* p, BOOL *stop) {
        [p willChangeValueForKey:@"user"];
        [p didChangeValueForKey:@"user"];
    }];
    [self didChangeValueForKey:@"profilePicture"];
}

这将通知 FRC Post 元素有变化。

您可能会找到其他信息 here

编辑:

要获取访问数据，您可以将其添加到您的 User .m 中:

//UNTESTED
+ (void) mergeToMain:(NSNotification*)notification
{
    AppDelegate* appDel = (AppDelegate*)[[UIApplication sharedApplication] delegate];
    [appDel.managedObjectContext performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:)
                                                  withObject:notification
                                               waitUntilDone:YES];
}

- (NSData*)_profilePicture
{
    return [self primitiveValueForKey:@"profilePicture"];
}

- (NSData*) profilePicture
{
    [self willAccessValueForKey:@"profilePicture"];
    NSData* picData = [self primitiveValueForKey:@"profilePicture"];
    if (!name) {
        __block NSManagedObjectID* objectID = self.objectID;
        //This solves the multiple downloads per item by using a single queue
        //for all profile pictures download.
        //There are more concurrent ways to accomplish that
        dispatch_async(downloadSerialQueue, ^{ //define some serial queue for assuring you down download multiple times the same object
            NSError* error = nil;
            AppDelegate* appDel = (AppDelegate*)[[UIApplication sharedApplication] delegate];
            NSManagedObjectContext* context = [[NSManagedObjectContext alloc] init];
            [context setPersistentStoreCoordinator:appDel.persistentStoreCoordinator];
            [context setUndoManager:nil];
            User* user = (User*)[context existingObjectWithID:objectID error:&error];
            if (user && [user _profilePicture] == nil) {
                NSData *data = //[method to retrieve data from server];
                if (data) {
                    if (user) {
                        user.profilePicture = data;
                    } else {
                        NSLog(@"ERROR:: error fetching user: %@",error);
                        return;
                    }
                    [[NSNotificationCenter defaultCenter] addObserver:[self class] selector:@selector(mergeToMain:) name:NSManagedObjectContextDidSaveNotification object:context];
                    [context save:&error];
                    [[NSNotificationCenter defaultCenter] removeObserver:[self class] name:NSManagedObjectContextDidSaveNotification object:context];
                }
            }
        });
    }
    [self didAccessValueForKey:@"profilePicture"];
    return picData;
}