使用父/子上下文时，批量大小不起作用

Question

我已经能够在我的应用程序中确认这一点，并且我创建了一个快速示例应用程序来确认这一点。这是设置：

您有两个托管对象上下文：

masterMOC: NSPrivateQueueConcurrencyType, tied to persistent store coordinator
mainMOC: NSMainQueueConcurrencyType, child of masterMOC, NOT tied to any store coordinator

此设置的灵感来自 WWDC 视频，这表明我们可以通过将 masterMOC 设置为私有队列并将其绑定到持久存储来保存在后台线程上。如果您使用mainMOC 设置NSFetchedResultsController（它必须是mainMOC，因为它与UI 相关联），并设置fetchBatchSize，批量大小将被忽略，所有实体都出现故障立刻。我启用了 SQLite 调试注释，当滚动数千行（批量大小为 20）时，不会触发任何错误。

如果我做一个简单的调整，即将持久存储协调器绑定到 mainMOC 并使其成为根上下文（即，它不再是 master 的子级），那么批量大小就可以完美地工作，并且作为我滚动了数千行，触发了几个错误。

这是预期的行为吗？我错过了什么吗？

您可以下载示例项目here

Answer 1

文档中对嵌套上下文的讨论有限，它只出现在“iOS v5.0 的核心数据发行说明”和UIManagedDocument 中。关于获取和嵌套上下文的唯一评论是：

获取和保存操作由父上下文而不是协调器进行调解。

鉴于没有任何与嵌套上下文的批量获取功能相关的免责声明，我建议不要期望批量获取和嵌套上下文不兼容。但是，这似乎是这种情况，因为最基本的示例不起作用。 （见下面的测试代码）.

这里还有一个公开的雷达提交描述了相同的问题：http://openradar.appspot.com/11235622，以及与 FetchedResultsControllers 和嵌套上下文有关的其他问题：Duplication of entity when change made by a child ManagedObjectContext is pushed (saved) to its parent。

一个可能的部分解决方案是直接向同一个NSPersistentStoreCoordinator 添加一个额外的NSManagedObjectContext 或NSMainQueueConcurrencyType，其唯一目的是为NSFetchedResultsController 提供服务。然后，当用户选择项目时，ObjectID 可以返回给嵌套的子上下文，然后可以在嵌套的上下文中执行任何后续编辑。

这显然会降低使用嵌套上下文的好处，并且需要更频繁地保存以在嵌套上下文和NSFetchedResultsControllers 上下文之间进行同步。但是，根据应用程序的设计以及嵌套上下文与批量加载的相对优势，这可能很有用。 （参见下面的示例代码）

测试代码显示嵌套上下文中最简单的案例批量提取失败：

#import "AppDelegate.h"

// Xcode 4.3.3:
// Create a new iOS Master-Detail project called "BatchTest" tick the "Use Core Data" check box.
// Delete all files except the AppDelegate and the BatchTest data model (leave supporting files).
// Delete all properties and methods from AppDelegate.h
// Paste this code into AppDelegate.m
// Switch on core data debugging by editing the "BatchTest" scheme and adding 
//   -com.apple.CoreData.SQLDebug 1
// To the "arguments passed on launch" list in the "Run" step
// Run.

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

    /////////////////////////////////////////////////////////////////////////////////////
    // Setup the core data stack.
    /////////////////////////////////////////////////////////////////////////////////////
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"BatchTest" withExtension:@"momd"];
    NSManagedObjectModel *model = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];

    NSURL *appDocsDirectory = [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
    NSURL *storeURL = [appDocsDirectory URLByAppendingPathComponent:@"BatchTest.sqlite"];

    NSPersistentStoreCoordinator *coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:model];
    [coordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:nil];

    NSManagedObjectContext *parentContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
    parentContext.persistentStoreCoordinator = coordinator;

    NSManagedObjectContext *childContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
    childContext.parentContext = parentContext;


    /////////////////////////////////////////////////////////////////////////////////////
    // Load some test data and reset the context.
    /////////////////////////////////////////////////////////////////////////////////////
    [parentContext performBlockAndWait:^{
        for (int i=0; i<1000; i++) {
            [NSEntityDescription insertNewObjectForEntityForName:@"Event" inManagedObjectContext:parentContext];
        }
        [parentContext save:nil];
        [parentContext reset];
    }];


    /////////////////////////////////////////////////////////////////////////////////////
    // Test Batched Fetching
    /////////////////////////////////////////////////////////////////////////////////////

    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Event"];
    request.fetchBatchSize = 10;

    // Fetch from the child.
    NSArray *results = [childContext executeFetchRequest:request error:nil];
    NSLog(@"Object 500: %@", [results objectAtIndex:500]);
    // Result is all 1000 rows fetched in full, no subsequent batch fetching for event 500.

    [childContext reset];    
    [parentContext performBlockAndWait:^{
        [parentContext reset];

        // Fetch from the parent.
        NSArray *results = [parentContext executeFetchRequest:request error:nil];
        NSLog(@"Object 500: %@", [results objectAtIndex:500]);
        // Result is 1000 primary keys fetched, followed by a batch of 10 rows to find event 500.

    }];

    return YES;
}

@end

示例代码显示了使用附加上下文来为 NSFetchedResultsController 提供批处理工作：

#import "AppDelegate.h"

// Xcode 4.3.3:
// Create a new iOS Master-Detail project called "BatchTest" tick the "Use Core Data" check box.
// Delete all files except the AppDelegate and the BatchTest data model (leave supporting files).
// Delete all properties and methods from AppDelegate.h
// Paste this code into AppDelegate.m
// Switch on core data debugging by editing the "BatchTest" scheme and adding 
//   -com.apple.CoreData.SQLDebug 1
// To the "arguments passed on launch" list in the "Run" step
// Run.

@interface AppDelegate () {
    NSManagedObjectContext *backgroundContext;
    NSManagedObjectContext *editingContext;
    NSManagedObjectContext *fetchedResultsControllerContext;
    NSManagedObject *selectedObject;
}

@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

    /////////////////////////////////////////////////////////////////////////////////////
    // Setup the core data stack.
    /////////////////////////////////////////////////////////////////////////////////////
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"BatchTest" withExtension:@"momd"];
    NSManagedObjectModel *model = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];

    NSURL *appDocsDirectory = [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
    NSURL *storeURL = [appDocsDirectory URLByAppendingPathComponent:@"BatchTest.sqlite"];

    NSPersistentStoreCoordinator *coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:model];
    [coordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:nil];

    backgroundContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
    backgroundContext.persistentStoreCoordinator = coordinator;

    editingContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
    editingContext.parentContext = backgroundContext;

    fetchedResultsControllerContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
    fetchedResultsControllerContext.persistentStoreCoordinator = coordinator;

    /////////////////////////////////////////////////////////////////////////////////////
    // Load some test data and reset the context.
    /////////////////////////////////////////////////////////////////////////////////////
    [backgroundContext performBlockAndWait:^{
        for (int i=0; i<1000; i++) {
            [NSEntityDescription insertNewObjectForEntityForName:@"Event" inManagedObjectContext:backgroundContext];
        }
        [backgroundContext save:nil];
        [backgroundContext reset];
    }];

    /////////////////////////////////////////////////////////////////////////////////////
    // Example of three contexts performing different roles.
    /////////////////////////////////////////////////////////////////////////////////////

    // The fetchedResultsControllerContext will batch correctly as it is tied directly 
    // to the persistent store.  It can be used to drive the UI as it is a Main Queue context. 
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Event"];
    request.fetchBatchSize = 10;
    NSArray *fetchResults = [fetchedResultsControllerContext executeFetchRequest:request error:nil];

    // User selects an object in the fetchedResultsControllerContext (i.e. in a UITableView).
    selectedObject = [fetchResults lastObject];
    NSLog(@"**** selectedObject.timeStamp before editing:%@", [selectedObject valueForKey:@"timeStamp"]);

    // Pass the object to the editing context for editing using its objectID.
    NSManagedObjectID *selectedObjectID = selectedObject.objectID;
    NSManagedObject *objectForEditing = [editingContext objectWithID:selectedObjectID];

    // Edit the object
    [objectForEditing setValue:[NSDate date] forKey:@"timeStamp"];

    // Subscribe to save notifications of the background context so the 
    // fetchedResultsControllerContext will be updated after the background save occurs.
    [[NSNotificationCenter defaultCenter] addObserver:self 
                                             selector:@selector(backgroundContextDidSave:) 
                                                 name:NSManagedObjectContextDidSaveNotification 
                                               object:backgroundContext];

    // Save the editing context to push changes up to the parent, then background save.
    [editingContext save:nil];
    [backgroundContext performBlock:^{
        [backgroundContext save:nil];
    }];

    return YES;
}

- (void)backgroundContextDidSave:(NSNotification *)notification {

    [fetchedResultsControllerContext mergeChangesFromContextDidSaveNotification:notification];
    NSLog(@"**** selectedObject.timeStamp after editing:%@", [selectedObject valueForKey:@"timeStamp"]);
    // Merging changes into the fetchedResultsControllerContext would trigger updates
    // to an NSFetchedResultsController and it's UITableView where these set up.

}

@end

Answer 2

来自NSFetchRequest Class Reference：

当执行 fetch 时，会评估整个请求并记录所有匹配对象的身份，但一次从持久存储中获取不超过 batchSize 对象的数据。执行请求返回的数组将是一个代理对象，可以根据需要透明地对批次进行故障处理。

因此，如果托管对象上下文未附加到持久存储，则没有可取的东西 - 因此您看到的行为至少与文档有些一致。 fetchLimit 可能适用于您描述的场景。我仍然会在 bugreporter.apple.com 上发射雷达。