简而言之，我正在尝试使用后台队列将从 Web 服务中提取的 JSON 对象保存到 Core Data Sqlite3 数据库。保存发生在我通过 GCD 创建的序列化后台队列上，并保存到为该后台队列创建的 NSManagedObjectContext 的辅助实例中。保存完成后，我需要使用新创建/更新的对象更新主线程上的 NSManagedObjectContext 实例。我遇到的问题是主线程上的 NSManagedObjectContext 实例无法找到保存在后台上下文中的对象。下面是我对代码示例采取的操作列表。对我做错了什么有任何想法吗？
- 通过 GCD 创建一个后台队列，运行所有预处理逻辑，然后在该线程上保存后台上下文：
.
// process in the background queue
dispatch_async(backgroundQueue, ^(void){
if (savedObjectIDs.count > 0) {
[savedObjectIDs removeAllObjects];
}
if (savedObjectClass) {
savedObjectClass = nil;
}
// set the thead name
NSThread *currentThread = [NSThread currentThread];
[currentThread setName:VS_CORE_DATA_MANAGER_BACKGROUND_THREAD_NAME];
// if there is not already a background context, then create one
if (!_backgroundQueueManagedObjectContext) {
NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
if (coordinator != nil) {
_backgroundQueueManagedObjectContext = [[NSManagedObjectContext alloc] init];
[_backgroundQueueManagedObjectContext setPersistentStoreCoordinator:coordinator];
}
}
// save the JSON dictionary starting at the upper most level of the key path, and return all created/updated objects in an array
NSArray *objectIds = [self saveJSON:jsonDict objectMapping:objectMapping class:managedObjectClass managedObjectContext:_backgroundQueueManagedObjectContext level:0];
// save the object IDs and the completion block to global variables so we can access them after the save
if (objectIds) {
[savedObjectIDs addObjectsFromArray:objectIds];
}
if (completion) {
saveCompletionBlock = completion;
}
if (managedObjectClass) {
savedObjectClass = managedObjectClass;
}
// save all changes object context
[self saveManagedObjectContext];
});
“saveManagedObjectContext”方法基本上是查看哪个线程正在运行并保存适当的上下文。我已验证此方法工作正常，因此我不会将代码放在这里。
所有这些代码都驻留在单例中，在单例的 init 方法中，我为“NSManagedObjectContextDidSaveNotification”添加了一个侦听器，它调用了 mergeChangesFromContextDidSaveNotification: 方法
.
// merge changes from the context did save notification to the main context
- (void)mergeChangesFromContextDidSaveNotification:(NSNotification *)notification
{
NSThread *currentThread = [NSThread currentThread];
if ([currentThread.name isEqual:VS_CORE_DATA_MANAGER_BACKGROUND_THREAD_NAME]) {
// merge changes to the primary context, and wait for the action to complete on the main thread
[_managedObjectContext performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:) withObject:notification waitUntilDone:YES];
// on the main thread fetch all new data and call the completion block
dispatch_async(dispatch_get_main_queue(), ^{
// get objects from the database
NSMutableArray *objects = [[NSMutableArray alloc] init];
for (id objectID in savedObjectIDs) {
NSError *error;
id object = [_managedObjectContext existingObjectWithID:objectID error:&error];
if (error) {
[self logError:error];
} else if (object) {
[objects addObject:object];
}
}
// remove all saved object IDs from the array
[savedObjectIDs removeAllObjects];
savedObjectClass = nil;
// call the completion block
//completion(objects);
saveCompletionBlock(objects);
// clear the saved completion block
saveCompletionBlock = nil;
});
}
}
正如您在上面的方法中看到的那样，我在主线程上调用“mergeChangesFromContextDidSaveNotification：”，并且我已将操作设置为等到完成。根据苹果文档，后台线程应该等到该操作完成，然后才能继续执行该调用下面的其余代码。正如我上面提到的，一旦我运行此代码，一切似乎都可以正常工作，但是当我尝试将获取的对象打印到控制台时，我什么也得不到。似乎合并实际上并没有发生，或者可能在我的其余代码运行之前没有完成。是否还有其他通知我应该收听以确保合并已完成？或者我是否需要在合并之后但在 fecth 之前保存主对象上下文？
另外，我为错误的代码格式道歉，但似乎 SO 的代码标签不喜欢方法定义。
多谢你们！
更新：
我已经进行了下面推荐的更改，但仍然遇到同样的问题。以下是我拥有的更新代码。
这是调用后台线程保存进程的代码
// process in the background queue
dispatch_async(backgroundQueue, ^(void){
if (savedObjectIDs.count > 0) {
[savedObjectIDs removeAllObjects];
}
if (savedObjectClass) {
savedObjectClass = nil;
}
// set the thead name
NSThread *currentThread = [NSThread currentThread];
[currentThread setName:VS_CORE_DATA_MANAGER_BACKGROUND_THREAD_NAME];
// if there is not already a background context, then create one
if (!_backgroundQueueManagedObjectContext) {
NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
if (coordinator != nil) {
_backgroundQueueManagedObjectContext = [[NSManagedObjectContext alloc] init];
[_backgroundQueueManagedObjectContext setPersistentStoreCoordinator:coordinator];
}
}
// save the JSON dictionary starting at the upper most level of the key path
NSArray *objectIds = [self saveJSON:jsonDict objectMapping:objectMapping class:managedObjectClass managedObjectContext:_backgroundQueueManagedObjectContext level:0];
// save the object IDs and the completion block to global variables so we can access them after the save
if (objectIds) {
[savedObjectIDs addObjectsFromArray:objectIds];
}
if (completion) {
saveCompletionBlock = completion;
}
if (managedObjectClass) {
savedObjectClass = managedObjectClass;
}
// listen for the merge changes from context did save notification
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(mergeChangesFromBackground:) name:NSManagedObjectContextDidSaveNotification object:_backgroundQueueManagedObjectContext];
// save all changes object context
[self saveManagedObjectContext];
});
这是 NSManagedObjectContextDidSaveNotification 通知调用的代码
// merge changes from the context did save notification to the main context
- (void)mergeChangesFromBackground:(NSNotification *)notification
{
// kill the listener
[[NSNotificationCenter defaultCenter] removeObserver:self name:NSManagedObjectContextDidSaveNotification object:_backgroundQueueManagedObjectContext];
NSThread *currentThread = [NSThread currentThread];
// merge changes to the primary context, and wait for the action to complete on the main thread
[[self managedObjectContext] performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:) withObject:notification waitUntilDone:YES];
// dispatch the completion block
dispatch_async(dispatch_get_main_queue(), ^{
// get objects from the database
NSMutableArray *objects = [[NSMutableArray alloc] init];
for (id objectID in savedObjectIDs) {
NSError *error;
id object = [[self managedObjectContext] existingObjectWithID:objectID error:&error];
if (error) {
[self logError:error];
} else if (object) {
[objects addObject:object];
}
}
// remove all saved object IDs from the array
[savedObjectIDs removeAllObjects];
savedObjectClass = nil;
// call the completion block
//completion(objects);
saveCompletionBlock(objects);
// clear the saved completion block
saveCompletionBlock = nil;
});
}
更新：
所以我找到了解决方案。事实证明，我在后台线程上保存对象 ID，然后尝试在主线程上使用它们重新获取它们的方式并没有奏效。所以我最终从与 NSManagedObjectContextDidSaveNotification 通知一起发送的 userInfo 字典中提取插入/更新的对象。以下是我现在正在运行的更新代码。
和之前一样，这段代码开始预处理和保存逻辑
// process in the background queue
dispatch_async(backgroundQueue, ^(void){
// set the thead name
NSThread *currentThread = [NSThread currentThread];
[currentThread setName:VS_CORE_DATA_MANAGER_BACKGROUND_THREAD_NAME];
[self logMessage:[NSString stringWithFormat:@"(%@) saveJSONObjects:objectMapping:class:completion:", [managedObjectClass description]]];
// if there is not already a background context, then create one
if (!_backgroundQueueManagedObjectContext) {
NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
if (coordinator != nil) {
_backgroundQueueManagedObjectContext = [[NSManagedObjectContext alloc] init];
[_backgroundQueueManagedObjectContext setPersistentStoreCoordinator:coordinator];
}
}
// save the JSON dictionary starting at the upper most level of the key path
[self saveJSON:jsonDict objectMapping:objectMapping class:managedObjectClass managedObjectContext:_backgroundQueueManagedObjectContext level:0];
// save the object IDs and the completion block to global variables so we can access them after the save
if (completion) {
saveCompletionBlock = completion;
}
// listen for the merge changes from context did save notification
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(mergeChangesFromBackground:) name:NSManagedObjectContextDidSaveNotification object:_backgroundQueueManagedObjectContext];
// save all changes object context
[self saveManagedObjectContext];
});
这是处理 NSManagedObjectContextDidSaveNotification 的修改方法
- (void)mergeChangesFromBackground:(NSNotification *)notification
{
// kill the listener
[[NSNotificationCenter defaultCenter] removeObserver:self name:NSManagedObjectContextDidSaveNotification object:_backgroundQueueManagedObjectContext];
// merge changes to the primary context, and wait for the action to complete on the main thread
[[self managedObjectContext] performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:) withObject:notification waitUntilDone:YES];
// dispatch the completion block
dispatch_async(dispatch_get_main_queue(), ^{
// pull the objects that were saved from the notification so we can get them on the main thread MOC
NSDictionary *userInfo = [notification userInfo];
NSMutableArray *modifiedObjects = [[NSMutableArray alloc] init];
NSSet *insertedObject = (NSSet *)[userInfo objectForKey:@"inserted"];
NSSet *updatedObject = (NSSet *)[userInfo objectForKey:@"updated"];
if (insertedObject && insertedObject.count > 0) {
[modifiedObjects addObjectsFromArray:[insertedObject allObjects]];
}
if (updatedObject && updatedObject.count > 0) {
[modifiedObjects addObjectsFromArray:[updatedObject allObjects]];
}
NSMutableArray *objects = [[NSMutableArray alloc] init];
// iterate through the updated objects and find them in the main thread MOC
for (NSManagedObject *object in modifiedObjects) {
NSError *error;
NSManagedObject *obj = [[self managedObjectContext] existingObjectWithID:object.objectID error:&error];
if (error) {
[self logError:error];
}
if (obj) {
[objects addObject:obj];
}
}
modifiedObjects = nil;
// call the completion block
saveCompletionBlock(objects);
// clear the saved completion block
saveCompletionBlock = nil;
});
}