带有CoreData的中央调度(GCD)

正如您可能知道或注意到的,您必须在主线程上执行UI操作。正如您所提到的,当您保存UI更新时。你可以通过嵌套一个调用来解决这个问题 dispatch_sync 在主线上。

dispatch_queue_t main_queue = dispatch_get_main_queue();
dispatch_queue_t request_queue = dispatch_queue_create("com.app.request", NULL);

__block __typeof__(self) blockSelf = self;

dispatch_async(request_queue, ^{
    MyNSManagedObject *mObject = [blockSelf.fetchedResultsController objectAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:0]];

    // update and heavy lifting...

    dispatch_sync(main_queue, ^{
      [blockSelf saveManagedObjectContext];
    });
});     

使用 blockSelf 是为了避免意外创建引用循环。 ( Practical blocks )

对于核心数据,有一条黄金法则——每个线程一个托管对象上下文。托管对象上下文不是线程安全的,因此如果在后台任务中工作,可以使用主线程避免与UI操作的线程冲突,也可以创建一个新上下文来完成工作。如果这项工作需要几秒钟的时间,那么您应该执行后者来阻止您的UI锁定。

要执行此操作,请创建一个新上下文,并为其提供与主上下文相同的持久存储:

NSManagedObjectContext *backgroundContext = [[[NSManagedObjectContext alloc] init] autorelease];
[backgroundContext setPersistentStoreCoordinator:[mainContext persistentStoreCoordinator]];

执行所需的任何操作,然后在保存新上下文时,需要处理保存通知并将更改合并到主上下文中 mergeChangesFromContextDidSaveNotification: 信息。代码应该如下所示:

/* Save notification handler for the background context */
- (void)backgroundContextDidSave:(NSNotification *)notification {
    /* Make sure we're on the main thread when updating the main context */
    if (![NSThread isMainThread]) {
        [self performSelectorOnMainThread:@selector(backgroundContextDidSave:)
                               withObject:notification
                            waitUntilDone:NO];
        return;
    }

    /* merge in the changes to the main context */
    [self.managedObjectContext mergeChangesFromContextDidSaveNotification:notification];
}

/* ... */

/* Save the background context and handle the save notification */
[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(backgroundContextDidSave:)
                                             name:NSManagedObjectContextDidSaveNotification
                                           object:backgroundContext];

[backgroundContext save:NULL];

[[NSNotificationCenter defaultCenter] removeObserver:self
                                                name:NSManagedObjectContextDidSaveNotification
                                              object:syncContext];

处理保存通知和合并很重要,否则主UI/上下文将看不到您所做的更改。通过合并,您的主fetchresultcontroller等将获得更改事件,并按预期更新您的UI。

另一个需要注意的重要事项是,NSManagedObject实例只能在从中获取它们的上下文中使用。如果您的操作需要对某个对象的引用,则必须传递该对象的 objectID 使用 existingObjectWithID: . 比如说:

/* This can only be used in operations on the main context */
MyNSManagedObject *objectInMainContext =
    [self.fetchedResultsController objectAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:0]];

/* This can now be used in your background context */
MyNSManagedObject *objectInBackgroundContext =
    (MyNSManagedObject *) [backgroundContext existingObjectWithID:[objectInMainContext objectID]];

由于核心数据要求每个线程有一个托管对象上下文,可能的解决方案是在全局管理器中跟踪每个线程的上下文,然后跟踪保存通知并传播到所有线程:

假设:

@property (nonatomic, strong) NSDictionary* threadsDictionary;

下面是如何获取托管对象(每个线程):

- (NSManagedObjectContext *) managedObjectContextForThread {

// Per thread, give one back
NSString* threadName = [NSString stringWithFormat:@"%d",[NSThread currentThread].hash];

NSManagedObjectContext * existingContext = [self.threadsDictionary objectForKey:threadName];
if (existingContext==nil){
    existingContext = [[NSManagedObjectContext alloc] init];
    [existingContext setPersistentStoreCoordinator: [self persistentStoreCoordinator]];
    [self.threadsDictionary setValue:existingContext forKey:threadName];
}

return existingContext;

}

在全局管理器的init方法中的某个时刻(我使用了一个singleton):

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(backgroundContextDidSave:)                                                    name:NSManagedObjectContextDidSaveNotification                                                   object:nil];

然后接收保存通知并传播到所有其他托管上下文对象:

- (void)backgroundContextDidSave:(NSNotification *)notification {
    /* Make sure we're on the main thread when updating the main context */
    if (![NSThread isMainThread]) {
        [self performSelectorOnMainThread:@selector(backgroundContextDidSave:)
                               withObject:notification
                            waitUntilDone:NO];
        return;
    }

    /* merge in the changes to the main context */
    for (NSManagedObjectContext* context in [self.threadsDictionary allValues]){
            [context mergeChangesFromContextDidSaveNotification:notification];
    }
}

(为清楚起见,删除了一些其他方法)