类 NSFetchedResultsController详解

最新推荐文章于 2020-06-09 08:54:10 发布

转载最新推荐文章于 2020-06-09 08:54:10 发布 · 3.5k 阅读

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#notifications #performance #数据库 #table #up

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本文详细介绍了如何使用NSFetchedResultsController优化iOS应用的数据读取性能，通过减少内存消耗和提高效率，实现更流畅的应用体验。重点包括NSFetchedResultsController的初始化、属性设置、数据刷新机制及在UITableView中的应用。

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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];
}