我已经能够在我的应用程序上确认这一点，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，它建议我们可以通过将masterMOC设置为专用队列并将其绑定到持久存储来保存后台线程。如果您使用NSFetchedResultsController设置mainMOC（并且必须是mainMOC，因为那是绑定到用户界面的那个），并设置fetchBatchSize，批量大小被忽视，所有实体都立即出现故障。我启用了SQLite调试注释，当滚动数千行（批量大小为20）时，不会发生任何错误。
如果我进行一个简单的调整，即将持久性存储协调器绑定到mainMOC并使其成为根上下文（即，它不再是master的子节点），那么批量大小完美地工作，当我滚动数千行时，会发出几个故障。
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
答案 0 :(得分：6)
文档中对嵌套上下文的讨论很有限，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注释是：
获取和保存操作由父上下文而不是协调器调解。
鉴于缺乏与嵌套上下文批量提取功能相关的任何免责声明，我建议不要求批量提取和嵌套上下文不兼容。然而，这似乎是这种情况，因为最基本的例子不起作用。 （请参阅下面的测试代码）。
此处还有一个开放的雷达提交描述了同样的问题：http://openradar.appspot.com/11235622，以及FetchedResultsControllers和嵌套上下文中提到的其他问题：Duplication of entity when change made by a child ManagedObjectContext is pushed (saved) to its parent。
可能的部分解决方案可能是将NSManagedObjectContext NSMainQueueConcurrencyType NSPersistentStoreCoordinator直接添加到同一NSFetchedResultsController，仅用于提供NSFetchedResultsControllers。然后，当用户选择项目时，可以将ObjectID交还给嵌套的子上下文，然后可以在嵌套的上下文中执行任何后续编辑。
这显然降低了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#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
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：
NSFetchedResultsController 示例代码显示使用其他上下文为批处理工作#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
提供服务：
{{1}}
答案 1 :(得分：0)
来自NSFetchRequest Class Reference：
执行提取时，将评估整个请求并记录所有匹配对象的标识，但一次只能从持久性存储中提取batchSize对象的数据。从执行请求返回的数组将是一个代理对象，它根据需要透明地对批处理进行故障。
因此，如果托管对象上下文未附加到持久性存储，则无需提取任何内容 - 因此您看到的行为至少与文档一致。 fetchLimit可以在您描述的场景中工作。我还是会在bugreporter.apple.com上发射雷达。