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ios - CoreData 保存在 iPad Mini 上的 GCD 后台队列挂起

转载 作者:塔克拉玛干 更新时间:2023-11-02 20:14:44 25 4
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我有一个单例类,它使用 GCD(Grand Central Dispatch)队列在后台将 JSON 对象保存到 CoreData 数据库。这在大多数情况下都能完美运行,但在 iPad 2 和 iPad Mini 设备上,我遇到了一些进程卡住问题。

我的设置非常简单。我有一个设置为串行运行的后台调度队列 (backgroundQueue),并且我有一个单独的 NSManagedObjectContext 实例用于后台队列。当我想将某些内容保存到数据库时,我调用开始保存过程的方法,在该方法中,我使用 dispatch_async 在后台线程上调用我的保存逻辑。

所有处理逻辑运行后,我保存后台 MOC,并使用 NSManagedObjectContextDidSaveNotification 将后台 MOC 的更改合并到主线程 MOC。后台线程等待它完成,一旦完成,它将运行队列中的下一个 block (如果有的话)。下面是我的代码。

dispatch_queue_t backgroundQueue;

- (id)init
{
if (self = [super init])
{
// init the background queue
backgroundQueue = dispatch_queue_create(VS_CORE_DATA_MANAGER_BACKGROUND_QUEUE_NAME, DISPATCH_QUEUE_SERIAL);
}
return self;
}

- (void)saveJsonObjects:(NSDictionary *)jsonDict
objectMapping:(VS_ObjectMapping *)objectMapping
class:(__unsafe_unretained Class)managedObjectClass
completion:(void (^)(NSArray *objects, NSError *error))completion
{
[VS_Log logDebug:@"**** Queue Save JSON Objects for Class: %@", managedObjectClass];

// process in the background queue
dispatch_async(backgroundQueue, ^(void)
{
[VS_Log logDebug:@"**** Dispatch Save JSON Objects for Class: %@", managedObjectClass];

// create a new process object and add it to the dictionary
currentRequest = [[VS_CoreDataRequest alloc] init];

// 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)
{
// create the background queue
_backgroundQueueManagedObjectContext = [[NSManagedObjectContext alloc] init];
[_backgroundQueueManagedObjectContext setPersistentStoreCoordinator:coordinator];
[_backgroundQueueManagedObjectContext setMergePolicy:NSMergeByPropertyStoreTrumpMergePolicy];
_backgroundQueueManagedObjectContext.undoManager = nil;

// listen for the merge changes from context did save notification on the background queue
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(mergeChangesFromBackground:) name:NSManagedObjectContextDidSaveNotification object:_backgroundQueueManagedObjectContext];
}
}

// save the JSON dictionary
NSArray *objects = [self saveJSON:jsonDict objectMapping:objectMapping class:managedObjectClass managedObjectContext:_backgroundQueueManagedObjectContext level:0];

// save the objects so we can access them later to be re-fetched and returned on the main thread
if (objects.count > 0)
[currentRequest.objects addObjectsFromArray:objects];

// save the object IDs and the completion block to global variables so we can access them after the save
if (completion)
currentRequest.completionBlock = completion;

[VS_Log logDebug:@"**** Save MOC for Class: %@", managedObjectClass];

// save all changes object context
[self saveManagedObjectContext];
});
}

- (void)mergeChangesFromBackground:(NSNotification *)notification
{
[VS_Log logDebug:@"**** Merge Changes From Background"];

// save the current request to a local var since we're about to be processing it on the main thread
__block VS_CoreDataRequest *request = (VS_CoreDataRequest *)[currentRequest copy];
__block NSNotification *bNotification = (NSNotification *)[notification copy];

// clear out the request
currentRequest = nil;

dispatch_sync(dispatch_get_main_queue(), ^(void)
{
[VS_Log logDebug:@"**** Start Merge Changes On Main Thread"];

// merge changes to the primary context, and wait for the action to complete on the main thread
[_managedObjectContext mergeChangesFromContextDidSaveNotification:bNotification];

NSMutableArray *objects = [[NSMutableArray alloc] init];

// iterate through the updated objects and find them in the main thread MOC
for (NSManagedObject *object in request.objects)
{
NSError *error;
NSManagedObject *obj = [[self managedObjectContext] existingObjectWithID:object.objectID error:&error];
if (error)
[self logError:error];

if (obj)
[objects addObject:obj];
}

// call the completion block
if (request.completionBlock)
{
void (^saveCompletionBlock)(NSArray *, NSError *) = request.completionBlock;
saveCompletionBlock(objects, nil);
}

[VS_Log logDebug:@"**** Complete Merge Changes On Main Thread"];

// clear the request
request = nil;
});

[VS_Log logDebug:@"**** Complete Merge Changes From Background"];
}

当问题发生时,一切似乎都运行良好,它进入了 mergeChangesFromBackground: 方法,但没有调用 dispatch_async()。

正如我上面提到的,代码在大多数情况下都能完美运行。只有当我在 iPad 2 或 iPad Mini 上测试并保存大型对象时,它才会出现运行问题。

有没有人知道为什么会这样?

谢谢

** 编辑 **

自从写这篇文章后,我了解了嵌套的 NSManagedObjectContexts。我修改了我的代码以使用它,它似乎运行良好。但是,我仍然有同样的问题。想法?另外,对嵌套 MOC 有什么意见吗?

- (void)saveJsonObjects:(NSDictionary *)jsonDict
objectMapping:(VS_ObjectMapping *)objectMapping
class:(__unsafe_unretained Class)managedObjectClass
completion:(void (^)(NSArray *objects, NSError *error))completion
{
// process in the background queue
dispatch_async(backgroundQueue, ^(void)
{
// create a new process object and add it to the dictionary
__block VS_CoreDataRequest *currentRequest = [[VS_CoreDataRequest alloc] init];

// 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)
{
// create the background queue
_backgroundQueueManagedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[_backgroundQueueManagedObjectContext setParentContext:[self managedObjectContext]];
[_backgroundQueueManagedObjectContext setMergePolicy:NSMergeByPropertyStoreTrumpMergePolicy];
_backgroundQueueManagedObjectContext.undoManager = nil;
}
}

// save the JSON dictionary starting at the upper most level of the key path
NSArray *objects = [self saveJSON:jsonDict objectMapping:objectMapping class:managedObjectClass managedObjectContext:_backgroundQueueManagedObjectContext level:0];

// if no objects were processed, then return with an error
if (!objects || objects.count == 0)
{
currentRequest = nil;

// dispatch the completion block
dispatch_async(dispatch_get_main_queue(), ^(void)
{
NSError *error = [self createErrorWithErrorCode:100 description:@"No objects were found for the object mapping"];
[self logMessage:error.debugDescription];
completion(nil, error);
});
}

// save the objects so we can access them later to be re-fetched and returned on the main thread
if (objects.count > 0)
[currentRequest.objects addObjectsFromArray:objects];

// save the object IDs and the completion block to global variables so we can access them after the save
if (completion)
currentRequest.completionBlock = completion;

// save all changes object context
NSError *error = nil;
[_backgroundQueueManagedObjectContext save:&error];
if (error)
[VS_Log logError:error.localizedDescription];

dispatch_sync(dispatch_get_main_queue(), ^(void)
{
NSMutableArray *objects = [[NSMutableArray alloc] init];

// iterate through the updated objects and find them in the main thread MOC
for (NSManagedObject *object in currentRequest.objects)
{
NSError *error;
NSManagedObject *obj = [[self managedObjectContext] existingObjectWithID:object.objectID error:&error];
if (error)
[self logError:error];

if (obj)
[objects addObject:obj];
}

// call the completion block
if (currentRequest.completionBlock)
{
void (^saveCompletionBlock)(NSArray *, NSError *) = currentRequest.completionBlock;
saveCompletionBlock(objects, nil);
}
});

// clear out the request
currentRequest = nil;
});
}

** 编辑 **

大家好,

首先,我要感谢大家对此的意见,因为正是这些意见促使我找到了解决问题的方法。长话短说,我完全放弃了使用 GCD 和自定义后台队列,而是现在使用嵌套上下文和“performBlock”方法在后台线程上执行所有保存。这很好用,我解决了挂断问题。

但是,我现在有一个新的错误。每当我第一次运行我的应用程序时,每当我尝试保存具有子对象关系的对象时,我都会收到以下异常。

-_referenceData64 仅为抽象类定义。定义 -[NSTemporaryObjectID_default _referenceData64]!

下面是我的新代码。

- (void)saveJsonObjects:(NSDictionary *)jsonDict
objectMapping:(VS_ObjectMapping *)objectMapping
class:(__unsafe_unretained Class)managedObjectClass
completion:(void (^)(NSArray *objects, NSError *error))completion
{
[_backgroundManagedObjectContext performBlock:^(void)
{
// create a new process object and add it to the dictionary
VS_CoreDataRequest *currentRequest = [[VS_CoreDataRequest alloc] init];
currentRequest.managedObjectClass = managedObjectClass;

// save the JSON dictionary starting at the upper most level of the key path
NSArray *objects = [self saveJSON:jsonDict objectMapping:objectMapping class:managedObjectClass managedObjectContext:_backgroundManagedObjectContext level:0];

if (objects.count == 0)
{
currentRequest.error = [self createErrorWithErrorCode:100 description:@"No objects were found for the object mapping"];
[self performSelectorOnMainThread:@selector(backgroundSaveProcessDidFail:) withObject:currentRequest waitUntilDone:NO];
}
else
{
// save the objects so we can access them later to be re-fetched and returned on the main thread
[currentRequest.objects addObjectsFromArray:objects];

// save the object IDs and the completion block to global variables so we can access them after the save
currentRequest.completionBlock = completion;

[_backgroundManagedObjectContext lock];

@try
{
[_backgroundManagedObjectContext processPendingChanges];
[_backgroundManagedObjectContext save:nil];
}
@catch (NSException *exception)
{
currentRequest.error = [self createErrorWithErrorCode:100 description:exception.reason];
}

[_backgroundManagedObjectContext unlock];

// complete the process on the main thread
if (currentRequest.error)
[self performSelectorOnMainThread:@selector(backgroundSaveProcessDidFail:) withObject:currentRequest waitUntilDone:NO];
else
[self performSelectorOnMainThread:@selector(backgroundSaveProcessDidSucceed:) withObject:currentRequest waitUntilDone:NO];

// clear out the request
currentRequest = nil;
}
}];
}

- (void)backgroundSaveProcessDidFail:(VS_CoreDataRequest *)request
{
if (request.error)
[self logError:request.error];

if (request.completionBlock)
{
void (^saveCompletionBlock)(NSArray *, NSError *) = request.completionBlock;
saveCompletionBlock(nil, request.error);
}
}

- (void)backgroundSaveProcessDidSucceed:(VS_CoreDataRequest *)request
{
// get objects from main thread
NSArray *objects = nil;
if (request.objects.count > 0)
{
NSFetchRequest *fetchReq = [NSFetchRequest fetchRequestWithEntityName:[NSString stringWithFormat:@"%@", request.managedObjectClass]];
fetchReq.predicate = [NSPredicate predicateWithFormat:@"self IN %@", request.objects];
objects = [self executeFetchRequest:fetchReq];
}

// call the completion block
if (request.completionBlock)
{
void (^saveCompletionBlock)(NSArray *, NSError *) = request.completionBlock;
saveCompletionBlock(objects, nil);
}
}

- (NSManagedObjectContext *)managedObjectContext
{
if (_managedObjectContext != nil) {
return _managedObjectContext;
}

NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
if (coordinator != nil)
{
// create the MOC for the backgroumd thread
_backgroundManagedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[_backgroundManagedObjectContext setPersistentStoreCoordinator:coordinator];
[_backgroundManagedObjectContext setMergePolicy:NSMergeByPropertyStoreTrumpMergePolicy];
_backgroundManagedObjectContext.undoManager = nil;

// create the MOC for the main thread
_managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
[_managedObjectContext setParentContext:_backgroundManagedObjectContext];
[_managedObjectContext setMergePolicy:NSMergeByPropertyStoreTrumpMergePolicy];
}

return _managedObjectContext;
}

你们中有人知道为什么会发生这种崩溃吗?

最佳答案

您可能需要考虑这种上下文架构(未经测试的代码):
mainManagedObjectContext 将以相同的方式初始化,但使用 NSMainQueueConcurrencyType 并且没有观察者(顺便说一句,记得删除你的观察者)

- (void) mergeChangesFromBackground:(NSNotification*)notification
{
__block __weak NSManagedObjectContext* context = self.managedObjectContext;
[context performBlockAndWait:^{
[context mergeChangesFromContextDidSaveNotification:notification];
}];
}

- (NSManagedObjectContext*) bgContext
{
if (_bgContext) {
return _bgContext;
}

NSPersistentStoreCoordinator* coordinator = [self persistentStoreCoordinator];
if (!coordinator) {
return nil;
}
_bgContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[_bgContext setPersistentStoreCoordinator:coordinator];
[_bgContext setMergePolicy:NSMergeByPropertyStoreTrumpMergePolicy];
[_bgContext setUndoManager:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(mergeChangesFromBackground:)
name:NSManagedObjectContextDidSaveNotification
object:_bgContext];
return _bgContext;
}

- (void) doSomethingOnBGContextWithoutBlocking:(void(^)(NSManagedObjectContext*))contextBlock
{
__block __weak NSManagedObjectContext* context = [self bgContext];
[context performBlock:^{
NSThread *currentThread = [NSThread currentThread];
NSString* prevName = [currentThread name];
[currentThread setName:@"BGContextThread"];

contextBlock(context);

[context reset];
[currentThread setName:prevName];
}];
}

关于ios - CoreData 保存在 iPad Mini 上的 GCD 后台队列挂起,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/15820629/

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