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This is a SyncEngine from an RW tutorial. I need help understanding how only UPDATED records from the web are fetched and processed into Core Data.

- (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];
}

From what I understand, on the first or initial sync, it fetches JSONDictionaryForClassWithName which reads the downloaded data from disk and creates a newManagedObjectWithClassName.

My confusion is in the update else block. downloadedRecords is populated from JSONDataRecordsForClass which simply calls JSONDictionaryForClassWithName. Then it checks to see if there is at least 1 object in that array. If there is it does this:

NSArray *storedRecords = [self managedObjectsForClass:className sortedByKey:@"objectId" usingArrayOfIds:[downloadedRecords valueForKey:@"objectId"] inArrayOfIds:YES];

This fetches all managedObjectsForClass:sortedByKey which is below:

- (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;
}

The next bit which compares the [storedRecords count] > currentIndex is confusing. Can someone please explain this? I think my confusion lies in what the managedObjectsForClass method does with the usingArraysOfIds & inArrayOfIds.

I would expect that at some point it gets the the updatedAt field from the downloaded records and compares it to the updatedAt field of the CoreData fetched records.

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1 回答 1

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此函数正在处理存储的 JSON。实际的远程获取和 updateAt 检查发生在downloadDataForRegisteredObjects和中mostRecentUpdatedAtDateForEntityWithName。

[storedRecords count] > currentIndex有点疯狂。尽管为原始程序员辩护，但编写任何像样的同步引擎都会很快让你走上神坛。基本上，他需要确定哪些记录是现有的，哪些是新的，并相应地更新本地数据存储，仅此而已。

我又看了看，这段代码实际上被严重破坏了。只有当您在本地和远程都拥有相同的记录时，它才会起作用。或者，如果新对象有一个 objectId，排序方式是在本地存储的最后一个对象之后。Parse objectId 的情况并非如此。

如果您仅使用一台设备进行测试，则此方法可行，因为新对象将在本地插入，然后再推送到服务器。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，则此代码会做一些奇怪的事情。

于 2013-04-26T23:56:17.477 回答