在后台线程上保存 coredata 会导致获取死锁和崩溃

Question

我将 coredata 保存在后台方法（父子）中，但获取是在主线程上完成的。所以我在 fetch 方法中遇到了死锁，有时应用程序崩溃了。我做错了什么吗？如何在不影响主线程的情况下同时改进保存和获取？我已经阅读了很多文档，但没有一篇解释我如何在项目中使用这两者。如果这是一个错误的问题，请指导我找到正确的解决方案并让我知道我的错误。

-(ThreadInfo *)retrieveSolicitationInfoForThreadID:(NSString*)inThreadID;
{
NSString *loginUser=[[NSUserDefaults standardUserDefaults]    valueForKey:@"currentUser"];

AppDelegate *sharedDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
NSManagedObjectContext *context = [sharedDelegate managedObjectContext];
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setReturnsObjectsAsFaults:NO];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"ThreadInfo"
                                      inManagedObjectContext:context];
[fetchRequest setEntity:entity];
 NSPredicate *userPredicate = [NSPredicate predicateWithFormat:@"userEmail ==   %@",loginUser];
 NSPredicate *threadPredicate = [NSPredicate predicateWithFormat:@"threadID == %@",inThreadID];

NSPredicate *compoundPredicate = [NSCompoundPredicate andPredicateWithSubpredicates: @[userPredicate, threadPredicate]];

[fetchRequest setPredicate:compoundPredicate];

 NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:nil];
if(fetchedObjects.count!=0)
{
    ThreadInfo *threadInfo=[fetchedObjects objectAtIndex:0];
    return threadInfo;
}
 return nil;
}  

保存

-(void)updateThreadEntityWithSyncDetails:(NSMutableDictionary *)inDictionary
  {

 NSString *loginUser=[[NSUserDefaults standardUserDefaults] valueForKey:@"currentUser"];

 AppDelegate *sharedDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
NSManagedObjectContext *context = [sharedDelegate managedObjectContext];

  //    NSManagedObjectContext *writerContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
  //   [writerContext setPersistentStoreCoordinator:[sharedDelegate persistentStoreCoordinator]];

 // create main thread MOC
   context = [[NSManagedObjectContext alloc]        initWithConcurrencyType:NSMainQueueConcurrencyType];
     context.parentContext = writerContext;

    NSManagedObjectContext *contextforThread = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];

   contextforThread.parentContext = context;

  [contextforThread performBlock:^{




NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setReturnsObjectsAsFaults:NO];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"ThreadInfo"
                                          inManagedObjectContext:contextforThread];
[fetchRequest setEntity:entity];
NSPredicate *userPredicate = [NSPredicate predicateWithFormat:@"userEmail == %@",loginUser];
NSPredicate *threadPredicate = [NSPredicate predicateWithFormat:@"threadID == %@",[inDictionary valueForKey:@"thread"]];
NSPredicate *compoundPredicate = [NSCompoundPredicate andPredicateWithSubpredicates: @[userPredicate, threadPredicate]];
[fetchRequest setPredicate:compoundPredicate];

NSArray *fetchedObjects = [contextforThread executeFetchRequest:fetchRequest error:nil];
for (ThreadInfo *threadInfo in fetchedObjects)
{
    if([[inDictionary allKeys] containsObject:@"userEmail"])
    {
        if([inDictionary valueForKey:@"userEmail"]!=[NSNull null])
        {
            threadInfo.userEmail=[inDictionary valueForKey:@"userEmail"];
        }
    }
    if([[inDictionary allKeys] containsObject:@"badgeValue"])
    {
        if([inDictionary valueForKey:@"badgeValue"]!=[NSNull null])
        {
            threadInfo.badgeValue=[inDictionary valueForKey:@"badgeValue"];
        }
    }


    if([[inDictionary allKeys] containsObject:@"choice4Percentage"])
    {
        if([inDictionary valueForKey:@"choice4Percentage"]!=[NSNull null])
        {
            threadInfo.choice4Percentage=[inDictionary valueForKey:@"choice4Percentage"];
        }
    }
    if([[inDictionary allKeys] containsObject:@"choice5Percentage"])
    {
        if([inDictionary valueForKey:@"choice5Percentage"]!=[NSNull null])
        {
            threadInfo.choice5Percentage=[inDictionary valueForKey:@"choice5Percentage"];
        }
    }

 }

    NSError *error;
    if(![context save:&error]) {
        NSLog(@"Child error : %@",error);

    }

    [context performBlock:^{
        NSError *error;
        if(![context save:&error]) {
            NSLog(@"%@",error);
        }
               }];
    }];


 }    

Answer 1

问题在于您的获取。您正在获取两个字符串，我猜您的商店中有大量实体。您的抓取性能是您问题的 100%。

A) 如果可以避免，您不应该进行字符串比较 B）你不应该做两次字符串比较

您的数据模型需要重构。无论如何，对具有大量实体的表进行两个字符串比较都会表现得很糟糕。

更新

你的数据结构不好。我不知道我可以说多少种方式。理想情况下，您需要一个整数或其他数字结构作为您的唯一标识符。

由于您没有告诉我/我们您正在处理多少实体，我只能猜测您的数量很大。预期结果如何？一？还是很多？这是您正在努力应对的独特约束吗？如果没有，为什么没有来自您的服务器的唯一约束？如果这是一个独特的约束，你为什么不像我建议的那样限制获取大小？

你没有给我任何信息，但希望我能神奇地解决你的问题。

您可以尝试将其减少为一个字符串，但您是在问我您的数据将如何表现。测试它。你有仪器，你有数据。改变事物并比较结果。

字符串很昂贵。他们一直都是，而且永远都是。

更新

由于它是一个独特的约束，您可以帮自己一个大忙并稍微更改您的代码：

- (ThreadInfo *)retrieveSolicitationInfoForThreadID:(NSString*)inThreadID;
{
    NSString *loginUser = [[NSUserDefaults standardUserDefaults] valueForKey:@"currentUser"];

    AppDelegate *sharedDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *context = [sharedDelegate managedObjectContext];
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"ThreadInfo"];
    [fetchRequest setReturnsObjectsAsFaults:NO];

    //Since there is every only one tell SQL to stop looking after one
    [fetchRequest setFetchBatchSize:1]; 

    /* Change these to your single constraint
    NSPredicate *userPredicate = [NSPredicate predicateWithFormat:@"userEmail ==   %@",loginUser];
    NSPredicate *threadPredicate = [NSPredicate predicateWithFormat:@"threadID == %@",inThreadID];
    NSPredicate *compoundPredicate = [NSCompoundPredicate andPredicateWithSubpredicates: @[userPredicate, threadPredicate]];
    [fetchRequest setPredicate:compoundPredicate];
    */

    NSError *error = nil;
    NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
    if (fetchedObjects == nil) {
        NSLog(@"Error fetching: %@\n%@", [error localizedDescription], [error userInfo]);
        abort();
    }
    return [fetchedObjects lastObject];
}

设置获取限制将导致存储停止查看第一个匹配项，这将减少获取时间。将其移动到单个字符串也可以减少获取时间。

以后在这种情况下避免使用字符串。使用整数主键非常快，可以完全避免这个性能问题。

Answer 2

面对coreData和multiplyThreads等问题，应在对应方法中添加如下代码：

[[NSNotificationCenter defaultCenter] addObserver: cdm.managedObjectContext selector: @selector(mergeChangesFromContextDidSaveNotification:) name:NSManagedObjectContextDidSaveNotification object: nil];

然后coreData在一个线程发生变化，它会立即在另一个正在使用coreData的线程中更新它。

希望答案对你有所帮助！