我正在阅读“iOS 7编程手册”，我似乎无法根据我的问题标题运行脚本。基本上，我试图在后台队列中创建一些记录，然后在主队列中获取结果，但在运行代码时没有任何内容传递给控制台。以下是示例中使用的方法。我在这里缺少什么？
- (void) populateDatabase{
for (NSUInteger counter = 0; counter < 1000; counter++){
Person *person =
[NSEntityDescription
insertNewObjectForEntityForName:NSStringFromClass([Person class])
inManagedObjectContext:self.managedObjectContext];
person.firstName = [NSString stringWithFormat:@"First name %lu",
(unsigned long)counter];
person.lastName = [NSString stringWithFormat:@"Last name %lu",
(unsigned long)counter];
person.age = @(counter);
}
NSError *error = nil;
if ([self.managedObjectContext save:&error]){
NSLog(@"Managed to populate the database.");
} else {
NSLog(@"Failed to populate the database. Error = %@", error);
}
}
- (void) processPersons:(NSArray *)paramPersons{
for (Person *person in paramPersons){
NSLog(@"First name = %@, last name = %@, age = %ld",
person.firstName,
person.lastName,
(long)person.age.integerValue);
}
}
- (NSFetchRequest *) newFetchRequest{
NSFetchRequest *request = [[NSFetchRequest alloc]
initWithEntityName:
NSStringFromClass([Person class])];
request.fetchBatchSize = 20;
request.predicate =
[NSPredicate predicateWithFormat:@"(age >= 100) AND (age <= 200)"];
request.resultType = NSManagedObjectIDResultType;
return request;
}
- (BOOL) application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{
__weak NSManagedObjectContext *mainContext = self.managedObjectContext;
__weak AppDelegate *weakSelf = self;
__block NSMutableArray *mutablePersons = nil;
/* Set up the background context */
NSManagedObjectContext *backgroundContext =
[[NSManagedObjectContext alloc]
initWithConcurrencyType:NSPrivateQueueConcurrencyType];
backgroundContext.persistentStoreCoordinator =
self.persistentStoreCoordinator;
/* Issue a block on the background context */
[backgroundContext performBlock:^{
NSError *error = nil;
NSArray *personIds = [backgroundContext
executeFetchRequest:[weakSelf newFetchRequest]
error:&error];
if (personIds != nil && error == nil){
mutablePersons = [[NSMutableArray alloc]
initWithCapacity:personIds.count];
/* Now go on the main context and get the objects on that
context using their IDs */
dispatch_async(dispatch_get_main_queue(), ^{
for (NSManagedObjectID *personId in personIds){
Person *person = (Person *)[mainContext
objectWithID:personId];
[mutablePersons addObject:person];
}
[weakSelf processPersons:mutablePersons];
});
} else {
NSLog(@"Failed to execute the fetch request.");
}
}];
self.window = [[UIWindow alloc]
initWithFrame:[[UIScreen mainScreen] bounds]];
self.window.backgroundColor = [UIColor whiteColor];
ViewController *viewController = [[ViewController alloc] init];
self.window.rootViewController = viewController;
[self.window makeKeyAndVisible];
return YES;
}