关于NSFetchedResultsController的一些用法

如何在iPhone等设备持久保存数据，需要用到Core Data，它能帮你快速而有效率的完成数据储存，Core Data 编程经常用到 NSFetchedResultsController这个类，刚开始不太了解这个类的用法，不过后来查看官方的说明文档，有一点小体会，所以现在写下来和大家一起分享讨论。

NSFetchedResultsController是用于高效获取Coredata内的数据，然后将获取的数据现实在是UITableView；

Coredata生成的xcdatamodeld文件是用于持久数据保持，也就是关机之后保存在xcdatamodeld里面的数据不会丢失，如果网上请求数据然后保存到 xcdatamodeld文件里面，那么 NSFetchedResultsController就自动从 NSManagedObjectContext ＊Context更新数据，然后刷新列表；

也是就是NSFetchedResultsController会自动更新Context的内容并且现实，同时它也作为一个提供给Coredata数据给用户的接口，下面贴下代码

- (void)setFetchedResultsController:(NSFetchedResultsController *)newfrc
{
    NSFetchedResultsController *oldfrc = _fetchedResultsController;
    if (newfrc != oldfrc) {
        _fetchedResultsController = newfrc;
        newfrc.delegate = self;
        if ((!self.title || [self.title isEqualToString:oldfrc.fetchRequest.entity.name]) && (!self.navigationController || !self.navigationItem.title)) {
            self.title = newfrc.fetchRequest.entity.name;
        }
        if (newfrc) {
            if (self.debug) NSLog(@"[%@ %@] %@", NSStringFromClass([self class]), NSStringFromSelector(_cmd), oldfrc ? @"updated" : @"set");
            [self performFetch];
        } else {
            if (self.debug) NSLog(@"[%@ %@] reset to nil", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
            [self.tableView reloadData];
        }
    }
}

#pragma mark - UITableViewDataSource

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    NSInteger sections = [[self.fetchedResultsController sections] count];
    return sections;
    
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    
    NSInteger rows = 0;
    if ([[self.fetchedResultsController sections] count] > 0) {
        id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
        rows = [sectionInfo numberOfObjects];
    }
    return rows;
    
}

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section
{
    return [[[self.fetchedResultsController sections] objectAtIndex:section] name];
}

- (NSInteger)tableView:(UITableView *)tableView sectionForSectionIndexTitle:(NSString *)title atIndex:(NSInteger)index
{
    return [self.fetchedResultsController sectionForSectionIndexTitle:title atIndex:index];
   
}

- (NSArray *)sectionIndexTitlesForTableView:(UITableView *)tableView
{
    return [self.fetchedResultsController sectionIndexTitles];
}

#pragma mark - NSFetchedResultsControllerDelegate
//当Context将会改变内容执行该委托
- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller
{
    [self.tableView beginUpdates];
}
//当该部分要被修改执行该委托
- (void)controller:(NSFetchedResultsController *)controller
  didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo
           atIndex:(NSUInteger)sectionIndex
     forChangeType:(NSFetchedResultsChangeType)type
{
    switch(type)//判对是添加还是删除
    {
            NSLog(@"sectionIndex is %d",sectionIndex);
        case NSFetchedResultsChangeInsert:
            [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;
            
        case NSFetchedResultsChangeDelete:
            [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}

//当内容的对象被改变时候执行
- (void)controller:(NSFetchedResultsController *)controller
   didChangeObject:(id)anObject
       atIndexPath:(NSIndexPath *)indexPath
     forChangeType:(NSFetchedResultsChangeType)type
      newIndexPath:(NSIndexPath *)newIndexPath
{
    switch(type)
    {
        case NSFetchedResultsChangeInsert:
            [self.tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
            
        case NSFetchedResultsChangeDelete:
            [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
            
        case NSFetchedResultsChangeUpdate:
            [self.tableView reloadRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
            
        case NSFetchedResultsChangeMove:
            [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            [self.tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}

//当内容修改完成执行
- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
    [self.tableView endUpdates];
}