Question

在下面的代码中（在ViewDidLoad中），我将一个JSON文件导入iOS项目，然后使用Core Data持久化数据，最后成功执行获取请求。但是，在我删除导入文件并保留数据的代码后，获取请求开始返回(null)。根据下面的代码，你能解释为什么会这样吗？

   id delegate = [[UIApplication sharedApplication] delegate];
   self.managedObjectContext = [delegate managedObjectContext];


    NSError* err = nil;
    NSString* dataPath = [[NSBundle mainBundle] pathForResource:@"inventorydata" ofType:@"json"];
    NSArray* inventoryData = [NSJSONSerialization JSONObjectWithData:[NSData dataWithContentsOfFile:dataPath]
                                                         options:kNilOptions
                                                           error:&err];
    if(err) NSLog(@"Error %@",[err description]);
    NSLog(@"Imported Data: %@", inventoryData);

    [Questions enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {

        Inventory *partsInfo= [NSEntityDescription
                                      insertNewObjectForEntityForName:@"Inventory"
                                      inManagedObjectContext:self.managedObjectContext];
        partsInfo.name = [obj objectForKey:@"name"];
        partsInfo.sku = [obj objectForKey:@"sku"];



        Supplier *supplierInfo = [NSEntityDescription
                          insertNewObjectForEntityForName:@"Supplier"
                          inManagedObjectContext:self.managedObjectContext];
        supplierInfo.supplierId = [obj objectForKey:@"supplierId"];
        [supplierInfo setValue:[NSSet setWithObject:partsInfo ] forKey:@"partData"];


    }];
    NSError *error;
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Supplier"
                                              inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];
    self.fetchedObjects = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
    for (Supplier *info in self.fetchedObjects) {
        NSLog(@"supplierId: %@", info.supplierId);

    }

Answer 1

试试这个。

   [Questions enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
    Inventory *partsInfo= [NSEntityDescription
                                  insertNewObjectForEntityForName:@"Inventory"
                                  inManagedObjectContext:self.managedObjectContext];
    partsInfo.name = [obj objectForKey:@"name"];
    partsInfo.sku = [obj objectForKey:@"sku"];



    Supplier *supplierInfo = [NSEntityDescription
                      insertNewObjectForEntityForName:@"Supplier"
                      inManagedObjectContext:self.managedObjectContext];
    supplierInfo.supplierId = [obj objectForKey:@"supplierId"];
    [supplierInfo setValue:[NSSet setWithObject:partsInfo ] forKey:@"partData"];
    }];

   //Add this code : need to save database. 
   [delegate saveContext];

    // You should add this method in AppDelegate.m
    - (void)saveContext{
    NSError *error = nil;
    NSManagedObjectContext *managedObjectContext = self.managedObjectContext;
    if (managedObjectContext != nil) {
    if ([managedObjectContext hasChanges] && ![managedObjectContext save:&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();
    } 
    }  
    }

在删除插入数据的代码后，获取请求返回null

1 个答案: