我在实现两个 NSManagedObjectContext 之间的父/子关系时遇到问题。我的应用程序从 Web 服务导入大量数据,这些数据在保存上下文时导致 UI 滞后。因此,在我的 AppDelegate 中,我使用 NSPrivateQueueConcurrencyType 创建了一个父上下文(master):
- (NSManagedObjectContext *)masterMOC
{
if (_masterMOC != nil) {
return _masterMOC;
}
NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
if (coordinator != nil) {
_masterMOC = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[_masterMOC setPersistentStoreCoordinator:coordinator];
}
return _masterMOC;
}
...和一个带有 NSMainQueueConcurrencyType 的子上下文(主)。我将主 MOC 的 parentContext 设置为 masterMOC:
- (NSManagedObjectContext *)mainMOC
{
if (_mainMOC != nil) {
return _mainMOC;
}
_mainMOC = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
[_mainMOC setUndoManager:nil];
[_mainMOC setParentContext:[self masterMOC]];
return _mainMOC;
}
在我的 applicationDidFinishLaunching 中,我启动了一个导入操作,该操作查询 Web 服务并将结果保存在主 (PrivateQueue) 上下文中。我还注册了 NSManagedObjectContextDidSaveNotification 并尝试将这些更改合并到子 mainMOC 中。
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];
[notificationCenter addObserver:self selector:@selector(contextChanged:) name:NSManagedObjectContextDidSaveNotification object:self.masterMOC];
RequestHandler *handler = [[RequestHandler alloc] initWithManagedObjectContext:self.masterMOC];
[handler importAllViews];
...
return YES;
}
- (void) contextChanged: (NSNotification *) notification
{
// Only interested in merging from master into main.
if ([notification object] != self.masterMOC) return;
[self.mainMOC performBlock:^{
[self.mainMOC mergeChangesFromContextDidSaveNotification:notification];
}];
}
RequestHandler 类有一个 saveContext 方法,可以在正确的线程上保存主上下文:
@implementation KDBRequestHandler
...
- (void) saveContext
{
[self.managedObjectContext performBlock:^{
NSError *error;
if (![self.managedObjectContext save:&error]) {
[NSException raise:@"Unable to save build details." format:@"Error saving context: %@", error];
}
}];
}
...
@end
我已经验证了导入正确地将其对象保存在 Instruments 的后台线程上。 我的问题在于具有 NSMainQueueConcurrencyType 的子托管对象上下文。导入开始后,应用程序 didFinishLaunching 按标准初始化 UI。 View Controller 分配有 mainMOC。
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
// Override point for customization after application launch.
if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPad) {
UISplitViewController *splitViewController = (UISplitViewController *)self.window.rootViewController;
UINavigationController *navigationController = [splitViewController.viewControllers lastObject];
splitViewController.delegate = (id)navigationController.topViewController;
UINavigationController *masterNavigationController = splitViewController.viewControllers[0];
KDBMasterViewController *controller = (KDBMasterViewController *)masterNavigationController.topViewController;
controller.managedObjectContext = self.mainMOC;
} else {
UINavigationController *navigationController = (UINavigationController *)self.window.rootViewController;
KDBMasterViewController *controller = (KDBMasterViewController *)navigationController.topViewController;
controller.managedObjectContext = self.mainMOC;
}
return YES;
}
MasterViewController 本质上是在创建核心数据项目时为您创建的样板 Controller 。它的 fetchedResultsController 最终在 mainMOC 上执行其工作,因此是主线程。
- (NSFetchedResultsController *)fetchedResultsController
{
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
// Edit the entity name as appropriate.
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Job" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
// Set the batch size to a suitable number.
[fetchRequest setFetchBatchSize:20];
// Edit the sort key as appropriate.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:NO];
NSArray *sortDescriptors = @[sortDescriptor];
[fetchRequest setSortDescriptors:sortDescriptors];
// Edit the section name key path and cache name if appropriate.
// nil for section name key path means "no sections".
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Master"];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;
NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error]) {
// Replace this implementation with code to handle the error appropriately.
// abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
abort();
}
return _fetchedResultsController;
}
在初始加载时,当数据库是新鲜且空的时,一切都会按预期进行。数据在后台下载并按预期填充到 MasterViewController 的 UITableView 中。但是在后续运行中,应用程序经常在 MasterViewController 的 fetchedResultsController 中崩溃。提取失败并出现此错误:
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '+entityForName: could not locate an NSManagedObjectModel for entity name 'Job''
*** First throw call stack:
(
0 CoreFoundation 0x01c6a1e4 __exceptionPreprocess + 180
1 libobjc.A.dylib 0x019678e5 objc_exception_throw + 44
2 CoreData 0x002c6a1b +[NSEntityDescription entityForName:inManagedObjectContext:] + 251
3 JMobile 0x00032ad4 -[KDBMasterViewController fetchedResultsController] + 340
4 JMobile 0x00031d6e -[KDBMasterViewController numberOfSectionsInTableView:] + 78
5 UIKit 0x0088e712 -[UITableViewRowData(UITableViewRowDataPrivate) _updateNumSections] + 102
6 UIKit 0x0088f513 -[UITableViewRowData invalidateAllSections] + 69
7 UIKit 0x006fa6ea -[UITableView _updateRowData] + 197
....
由于崩溃不会 100% 发生,所以我怀疑是并发问题。我通过注意到在 fetchedResultsController 查询时 mainMOC 的父上下文为零来确认。在尝试查询子上下文 (mainMOC) 之前,如何确保子上下文 (mainMOC) 与父上下文正确设置?
最佳答案
访问您的 mainMOC
时存在竞争条件:
在将观察者添加到 masterMOC 的保存之后、开始导入之前添加行:[self mainMOC];
,将解决 MOC 初始化竞争问题。
您可以阅读THIS用于讨论类似情况下的竞争条件(关于哪个线程并行访问您的上下文)。
Apple 提供的模板代码非常适合只有一个线程初始化上下文(非临时上下文)的情况,否则,您应该使用某种线程锁定机制来同步初始化。
关于ios - 多个 NSManagedObjectContexts 问题,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/23225507/