ios – NSFetchedResultsController用于UITableView的多个实体

2023年5月25日 203次阅读

我有两个实体,一个叫做Post,另一个叫做User.发布<< —->用户是核心数据中的关系.我使用NSFetchedResultsController获取核心数据堆栈中的所有Post记录,然后在UITableView中显示它们.每个单元格都有一个图像,该图像对应于User.profilePicture.

在初始化时,我不从服务器下载配置文件图片,我只在它滚过该单元格时下载(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中.

当我更新User实体时,是否有一种方法可以调用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并且无法遍历和监视关系中的更新,这是真的吗？

最佳答案 可悲的是,我只知道这个问题的UGLY解决方案.

在您的User .m文件中实现setProfilePicture：如下所示：

//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元素有更改.

您可能会发现其他信息here

编辑：

要获取访问数据,您可以将其添加到User.m：

//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;
}
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