Question

这是我用于表视图控制器的代码。问题是行没有被删除而且没有显示在我的表视图中？我应该怎么做才能从我的行中的coredata数据库中删除我的数据？

我如何使用下面的代码执行此操作：

- (void)viewDidLoad {
    [super viewDidLoad];
    [self fetchdata];
}
-(void)viewDidAppear:(BOOL)animated
{
    [self fetchdata];
    [self.tableView reloadData];
}
-(void)fetchdata
{
    self.fetchrequest=[[NSFetchRequest alloc]init];
    NSEntityDescription *entity=[NSEntityDescription entityForName:@"Student" inManagedObjectContext:_managedObjectContext];
    [_fetchrequest setEntity:entity];
    NSError *error;
    self.fetchedobjects=[_managedObjectContext executeFetchRequest:_fetchrequest error:&error];
    if (_fetchedobjects != nil)
    {
        _studentattributes=[[NSMutableArray alloc]initWithArray:_fetchedobjects];
    }
}

#pragma mark - Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {

    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {

    return _studentattributes.count;
}


- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"cellreuse" forIndexPath:indexPath];

    _STUDENT=[_studentattributes objectAtIndex: indexPath.row];
    cell.textLabel.text=_STUDENT.name;

    _namearray=[[NSMutableArray alloc]init];
    [self.namearray addObject:_STUDENT.name];
    _classarray=[[NSMutableArray alloc]init];
    [self.classarray addObject:_STUDENT.standard];
    _sectionarray=[[NSMutableArray alloc]init];
    [self.sectionarray addObject:_STUDENT.section];
    _rollarray=[[NSMutableArray alloc]init];
    [self.rollarray addObject:_STUDENT.roll];

    return cell;
}
-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([segue.identifier isEqualToString:@"seguereuse"])
    {
        EnterDetailViewController *enterdetail=(EnterDetailViewController *)[segue destinationViewController];
        enterdetail.managedObjectContext=self.managedObjectContext;
    }

    else if ([segue.identifier isEqualToString:@"seguedisplay"])
    {
         NSIndexPath *indexpath=[self.tableView indexPathForSelectedRow];
        DisplayViewController *display=(DisplayViewController *)[segue destinationViewController];
        display.studentdetails=[[NSMutableArray alloc]initWithObjects:[_namearray objectAtIndex:indexpath.row],[_classarray objectAtIndex:indexpath.row],[_sectionarray objectAtIndex:indexpath.row],[_rollarray objectAtIndex:indexpath.row], nil];
    }
}



- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (editingStyle == UITableViewCellEditingStyleDelete)
    {

       [_managedObjectContext deleteObject:[_studentattributes objectAtIndex:indexPath.row]];


       [self.tableview reloaddata];

            }
    else if (editingStyle == UITableViewCellEditingStyleInsert)
    {

    }

}

现在如何理清问题？

Answer 1

基于视图的表视图中的常用方法是从数据库，内容数组和更新块中的表视图中删除对象。

- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (editingStyle == UITableViewCellEditingStyleDelete)
    {
       [tableView beginUpdates];
       NSManagedObject *objectToDelete = [_studentattributes objectAtIndex:indexPath.row];
       [_managedObjectContext deleteObject: objectToDelete];
       [_studentattributes removeObject: objectToDelete];
       [tableView deleteRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];
       [tableView endUpdates];
       NSError *error;
       [_managedObjectContext save:&error];
       if (error) {
         NSLog(@"An error occurred while deleting a row: %@", error);
       }
    }
    else if (editingStyle == UITableViewCellEditingStyleInsert)
    {

    }
}

Answer 2

问题是因为您持有fetchedObjects的强引用，删除后只会重新加载表格视图。

另外，您没有要求managedObjectContext保留/保存更改

所以你的代码应该更像：

- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (editingStyle == UITableViewCellEditingStyleDelete)
    {

       [_managedObjectContext deleteObject:[_studentattributes objectAtIndex:indexPath.row]];
       [_managedObjectContext save:nil]; // Tell the context to save the changes.

       [self fetchdata]; // Needs to fetch data again so your _studentattributes you have the right count.

       [self.tableview reloaddata];

    }
    else if (editingStyle == UITableViewCellEditingStyleInsert)
    {

    }

}

Answer 3

首先，我发现你多次打电话给fetchdata。我建议您在loadView中拨打一次，然后在[self.tableview reloadData]中拨打viewWillAppear。另请注意，您在表格视图中呼叫reloaddata而不是reloadData。

接下来，管理对象上下文是一个便笺簿，您可以在其中进行更改。但最终你需要通过调用来保存它：

if (![context save:&error]) {
     NSLog(@"Couldn't save: %@", error);
}

PS：根据您对Adeel的评论，我假设您的数据源_studentattributes在您的桌面视图上再次调用reloadData之前已正确更新，因为这会驱动表视图外观和感觉。

如何删除核心数据库中的数据？

3 个答案: