Ios 此代码如何仅使用核心数据的更新？

这是RW教程中的SyncEngine。我需要帮助了解如何仅从web获取更新的记录并将其处理为核心数据

- (void)processJSONDataRecordsIntoCoreData {
    NSManagedObjectContext *managedObjectContext = [[SDCoreDataController sharedInstance] backgroundManagedObjectContext];
    // Iterate over all registered classes --- CHECK!
    for (NSString *className in self.registeredClassesToSync) {
        if (![self initialSyncComplete]) { 
            NSDictionary *JSONDictionary = [self JSONDictionaryForClassWithName:className];
            NSArray *records = [JSONDictionary objectForKey:@"results"];
            for (NSDictionary *record in records) {
                [self newManagedObjectWithClassName:className forRecord:record];
            }
        } else {
            NSArray *downloadedRecords = [self JSONDataRecordsForClass:className sortedByKey:@"objectId"];
            if ([downloadedRecords lastObject]) {
                 NSArray *storedRecords = [self managedObjectsForClass:className sortedByKey:@"objectId" usingArrayOfIds:[downloadedRecords valueForKey:@"objectId"] inArrayOfIds:YES];
                int currentIndex = 0;
                //if dl count is < current index, there is an updated object dl from the web
                for (NSDictionary *record in downloadedRecords) {
                    NSManagedObject *storedManagedObject = nil;
                    //Quick check to see if they indeed match, if they do, update the stored object with remote service objects
                    if ([storedRecords count] > currentIndex) {
                        storedManagedObject = [storedRecords objectAtIndex:currentIndex];
                    }
                    //Othwerwise its a new object and you need to create a new NSManagedObject to represent it in CDdb
                    if ([[storedManagedObject valueForKey:@"objectId"] isEqualToString:[record valueForKey:@"objectId"]]) {
                        [self updateManagedObject:[storedRecords objectAtIndex:currentIndex] withRecord:record];
                    } else {
                        [self newManagedObjectWithClassName:className forRecord:record];
                    }
                    currentIndex++;
                }
            }
        }
        // After all NSMO are created in your context, save it!
        [managedObjectContext performBlockAndWait:^{
            NSError *error = nil;
            if (![managedObjectContext save:&error]) {
                NSLog(@"Unable to save context for class %@", className);
            }
        }];
        // Cleanup time
        [self deleteJSONDataRecordsForClassWithName:className];
        [self executeSyncCompletedOperations];
    }

   [self downloadDataForRegisteredObjects:NO];
}

这将获取所有managedObjectsForClass:sortedByKey，如下所示：

- (NSArray *)managedObjectsForClass:(NSString *)className sortedByKey:(NSString *)key usingArrayOfIds:(NSArray *)idArray inArrayOfIds:(BOOL)inIds {
    __block NSArray *results = nil;
    NSManagedObjectContext *managedObjectContext = [[SDCoreDataController sharedInstance] backgroundManagedObjectContext];
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:className];
    NSPredicate *predicate;
    if (inIds) {
        predicate = [NSPredicate predicateWithFormat:@"objectId IN %@", idArray];
    } else {
        predicate = [NSPredicate predicateWithFormat:@"NOT (objectId IN %@)", idArray];
    }

    [fetchRequest setPredicate:predicate];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:
                                      [NSSortDescriptor sortDescriptorWithKey:@"objectId" ascending:YES]]];
    [managedObjectContext performBlockAndWait:^{
        NSError *error = nil;
        results = [managedObjectContext executeFetchRequest:fetchRequest error:&error];
    }];

    return results;
}

比较[storedRecords count]>currentIndex的下一位比较混乱。有人能解释一下吗？我认为我的困惑在于managedObjectsForClass方法如何使用ArraySofids和inArrayOfIds

---

我希望在某个时刻，它从下载的记录中获取updatedAt字段，并将其与CoreData获取的记录的updatedAt字段进行比较。

此函数正在处理存储的JSON。实际的远程获取和更新检查发生在

downloadDataForRegisteredObjects

和

mostrecentupdatedatedateforentitywithname

中

[storedRecords count]>currentIndex

有点疯狂。尽管是为了保护原程序员，但编写任何像样的同步引擎都会让你很快变得棒极了。基本上，他需要找出哪些记录是存在的，哪些是新的，并相应地更新本地数据存储，仅此而已

---

我又看了一眼，这个代码实际上坏得很厉害。只有在本地和远程都有相同记录的情况下，它才会起作用。或者，如果新对象的objectId排序方式在本地存储的最后一个对象之后。这与Parse objectId的情况不同

---

如果您只使用一台设备进行测试，这是有效的，因为新对象将在推送到服务器之前在本地插入。因此，您将始终拥有相同数量的记录。如果以任何其他方式插入其他记录，此代码将执行奇怪的操作。

这正是我想了解其工作原理的原因，即“确定哪些记录存在，哪些记录是新的”。相信我，我一直在努力理解这段代码，我很欣赏它的复杂性！：）。你能解释一下现有的和新的是如何工作的吗？

- (NSArray *)managedObjectsForClass:(NSString *)className sortedByKey:(NSString *)key usingArrayOfIds:(NSArray *)idArray inArrayOfIds:(BOOL)inIds {
    __block NSArray *results = nil;
    NSManagedObjectContext *managedObjectContext = [[SDCoreDataController sharedInstance] backgroundManagedObjectContext];
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:className];
    NSPredicate *predicate;
    if (inIds) {
        predicate = [NSPredicate predicateWithFormat:@"objectId IN %@", idArray];
    } else {
        predicate = [NSPredicate predicateWithFormat:@"NOT (objectId IN %@)", idArray];
    }

    [fetchRequest setPredicate:predicate];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:
                                      [NSSortDescriptor sortDescriptorWithKey:@"objectId" ascending:YES]]];
    [managedObjectContext performBlockAndWait:^{
        NSError *error = nil;
        results = [managedObjectContext executeFetchRequest:fetchRequest error:&error];
    }];

    return results;
}