类 NSFetchedResultsController详解

用好NSFetchedResultsController，程序的performance能大 大提高。原因在于你用NSFetchedResultsController去读取数据的话，苹果公司那些顶尖的工程师已经帮你想好了如何最大效率地读取 数据库。 所以，干脆利落地去掉NSArray和NSMutableArray，只用NSFetchedResultsController! 1,首先在h文件加入下面这段代码。

@interfaceFailedBanksListViewController:UITableViewController{NSFetchedResultsController*_fetchedResultsController;NSManagedObjectContext*_context;}@property(nonatomic, retain)NSFetchedResultsController*fetchedResultsController;@property(nonatomic, retain)NSManagedObjectContext*context;@end

2,在m文件中，加入下面的代码

@synthesize fetchedResultsController = _fetchedResultsController;

3，在dealloc函数中，加入下面的代码:

self.fetchedResultsController.delegate=nil;self.fetchedResultsController =nil;

4，在viewDidUnload函数中，加入下面的代码:

-(void)viewDidUnload {self.fetchedResultsController =nil;}

5，在m文件中，加入如下函数（以FailedBackInfo为数据对象）

-(NSFetchedResultsController*)fetchedResultsController {if(_fetchedResultsController !=nil){return _fetchedResultsController;NSFetchRequest*fetchRequest =[[NSFetchRequest alloc] init];NSEntityDescription*entity =[NSEntityDescription        entityForName:@"FailedBankInfo" inManagedObjectContext:_context];[fetchRequest setEntity:entity];NSSortDescriptor*sort =[[NSSortDescriptor alloc]        initWithKey:@"details.closeDate" ascending:NO];[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];[fetchRequest setFetchBatchSize:20];NSFetchedResultsController*theFetchedResultsController =[[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:_context sectionNameKeyPath:nil cacheName:@"Root"];self.fetchedResultsController = theFetchedResultsController;    _fetchedResultsController.delegate=self;[sort release];[fetchRequest release];[theFetchedResultsController release];return _fetchedResultsController;}

上面的代码中，entity为实体对象，_context是coredata的NSManagedObject对象，它是连接数据库的一个上下文对象，相当于一个连接器。sort用来排序。cacheName用来组织数据成sections（这样组织方便后面展示） 6，在viewDidLoad中加入下面代码：

-(void)viewDidLoad {[super viewDidLoad];NSError*error;if(![[self fetchedResultsController] performFetch:&error]){// Update to handle the error appropriately.NSLog(@"Unresolved error %@, %@", error,[error userInfo]);exit(-1);// Fail}self.title =@"Failed Banks";}

7，接下来，在UITableView中显示。

-(NSInteger)tableView:(UITableView*)tableView numberOfRowsInSection:(NSInteger)section {id  sectionInfo = [[_fetchedResultsController sections] objectAtIndex:section];return[sectionInfo numberOfObjects];}-(void)configureCell:(UITableViewCell*)cell atIndexPath:(NSIndexPath*)indexPath {FailedBankInfo*info =[_fetchedResultsController objectAtIndexPath:indexPath];cell.textLabel.text = info.name;cell.detailTextLabel.text =[NSString stringWithFormat:@"%@, %@", info.city, info.state];}-(UITableViewCell*)tableView:(UITableView*)tableView cellForRowAtIndexPath:(NSIndexPath*)indexPath {staticNSString*CellIdentifier=@"Cell";UITableViewCell*cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];if(cell ==nil){        cell =[[[UITableViewCell alloc]            initWithStyle:UITableViewCellStyleSubtitle            reuseIdentifier:CellIdentifier] autorelease];}// Set up the cell...[self configureCell:cell atIndexPath:indexPath];return cell;}

8,当数据发生变化时，coredata会自动通知显示层即UITableView作刷新。

-(void)controllerWillChangeContent:(NSFetchedResultsController*)controller {// The fetch controller is about to start sending change notifications, so prepare the table view for updates.[self.tableView beginUpdates];}

-(void)controller:(NSFetchedResultsController*)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath*)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath*)newIndexPath {UITableView*tableView =self.tableView;switch(type){case NSFetchedResultsChangeInsert:[tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];break;case NSFetchedResultsChangeDelete:[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];break;case NSFetchedResultsChangeUpdate:[self configureCell:[tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];break;case NSFetchedResultsChangeMove:[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];// Reloading the section inserts a new row and ensures that titles are updated appropriately.[tableView reloadSections:[NSIndexSet indexSetWithIndex:newIndexPath.section] withRowAnimation:UITableViewRowAnimationFade];break;}}

-(void)controller:(NSFetchedResultsController*)controller didChangeSection:(id )sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {switch(type){case NSFetchedResultsChangeInsert:[self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];break;case NSFetchedResultsChangeDelete:[self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];break;}}

-(void)controllerDidChangeContent:(NSFetchedResultsController*)controller {// The fetch controller has sent all current change notifications, so tell the table view to process all updates.[self.tableView endUpdates];}