Question

我在使用UITableView时让我的Core Data实体玩得很好并且订购时遇到了一些麻烦。

我在StackOverflow上已经完成了一些教程和其他问题，但似乎没有明确或优雅的方法来做到这一点 - 我真的希望我错过了一些东西。

我有一个Core Data实体，它上面有一个名为“displayOrder”的int16属性。我使用已经在“displayOrder”上排序的NSFetchRequest来返回我的UITableView的数据。除了重新排序之外的一切都得到了尊重。这是我的（低效）moveRowAtIndePath方法：

- (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)fromIndexPath toIndexPath:(NSIndexPath *)toIndexPath {         

    NSUInteger fromIndex = fromIndexPath.row;  
    NSUInteger toIndex = toIndexPath.row;

    FFObject *affectedObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:fromIndex];  
    affectedObject.displayOrderValue = toIndex;

    [self FF_fetchResults];


    for (NSUInteger i = 0; i < [self.fetchedResultsController.fetchedObjects count]; i++) {  
        FFObject *otherObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:i];  
        NSLog(@"Updated %@ / %@ from %i to %i", otherObject.name, otherObject.state, otherObject.displayOrderValue, i);  
        otherObject.displayOrderValue = i;  
    }

    [self FF_fetchResults];  
}

有人能指出我的示例代码的方向，或者看看我做错了什么？ tableview显示更新OK，我可以通过我的日志消息看到displayOrder属性正在更新。它不是一直保存和重新加载，并且对于这个实现感觉非常“偏离”（除了我所有FFObjects的浪费迭代）。

提前感谢您提供的任何建议。

Answer 1

我看了一下你的代码，这可能会更好：

- (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)fromIndexPath toIndexPath:(NSIndexPath *)toIndexPath {         

    NSUInteger fromIndex = fromIndexPath.row;  
    NSUInteger toIndex = toIndexPath.row;

    if (fromIndex == toIndex) {
        return;
    }

    FFObject *affectedObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:fromIndex];  
    affectedObject.displayOrderValue = toIndex;

    NSUInteger start, end;
    int delta;

    if (fromIndex < toIndex) {
        // move was down, need to shift up
        delta = -1;
        start = fromIndex + 1;
        end = toIndex;
    } else { // fromIndex > toIndex
        // move was up, need to shift down
        delta = 1;
        start = toIndex;
        end = fromIndex - 1;
    }

    for (NSUInteger i = start; i <= end; i++) {
        FFObject *otherObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:i];  
        NSLog(@"Updated %@ / %@ from %i to %i", otherObject.name, otherObject.state, otherObject.displayOrderValue, otherObject.displayOrderValue + delta);  
        otherObject.displayOrderValue += delta;
    }

    [self FF_fetchResults];  
}

Answer 2

（这是对gerry3上面回答的评论，但我还不能评论其他用户的问题和答案。）

gerry3的一个小改进 - 非常优雅 - 解决方案。如果我没有弄错，那就行了

otherObject.displayOrderValue += delta;

如果displayOrderValue不是基本类型，

将实际执行指针算术。这可能不是你想要的。相反，为了设置实体的价值，我建议：

otherObject.displayOrderValue = [NSNumber numberWithInt:[otherObject.displayOrderValue intValue] + delta];

这应该正确更新您的实体属性并避免任何EXC_BAD_ACCESS崩溃。

Answer 3

这是一个完整的解决方案，如何使用核心数据管理索引表。您的属性名为displayOrder，我称之为index。首先，您最好将视图控制器和模型分开。为此，我使用模型控制器，它是视图和模型之间的接口。

您需要管理3种情况，用户可以通过视图控制器进行影响。

添加新对象
删除现有对象
重新排序对象。

前两个案例添加和删除非常简单。删除调用名为renewObjectIndicesUpwardsFromIndex的例程，以便在删除的对象之后更新索引。

- (void)createObjectWithTitle:(NSString*)title {
    FFObject* object = [FFObject insertIntoContext:self.managedObjectContext];

    object.title = title;
    object.index = [NSNumber numberWithInteger:[self numberTotalObjects]];
    [self saveContext];
}

- (void)deleteObject:(FFObject*)anObject {
  NSInteger objectIndex = [anObject.index integerValue];
  [anObject deleteObject];
  [self renewObjectIndicesUpwardsFromIndex:objectIndex];
  [self saveContext];
}

- (void)renewObjectIndicesUpwardsFromIndex:(NSInteger)fromIndex {
  NSFetchRequest* fetchRequest = [[NSFetchRequest alloc] init];
  [fetchRequest setEntity:[NSEntityDescription entityForName:@"Object" inManagedObjectContext:self.managedObjectContext]];

  NSPredicate* predicate = [NSPredicate predicateWithFormat:@"(index > %d)", fromIndex];
  [fetchRequest setPredicate:predicate];

  NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"index" ascending:YES];
  NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptor, nil];

  [fetchRequest setSortDescriptors:sortDescriptors];

  NSError* fetchError = nil;
  NSArray* objects = [self.managedObjectContext executeFetchRequest:fetchRequest error:&fetchError];

  NSInteger index = fromIndex;
  for (FFObject* object in objects) {
    object.index = [NSNumber numberWithInteger:index];
    index += 1;
  }
  [self saveContext];
}

在我来到重新订购的控制器例程之前，这里是视图控制器中的部分。我使用类似于this answer的bool isModifyingOrder。请注意，视图控制器调用控制器moveObjectOrderUp和moveObjectOrderDown中的两个函数。根据您在表格视图中显示对象的方式 - 最新的或最新的 - 您可以切换它们。

- (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)sourceIndexPath toIndexPath:(NSIndexPath *)destinationIndexPath {

  isModifyingOrder = YES;

  NSUInteger fromIndex = sourceIndexPath.row;
  NSUInteger toIndex = destinationIndexPath.row;

  if (fromIndex == toIndex) {
    return;
  }

  FFObject *affectedObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:fromIndex];

  NSInteger delta;
  if (fromIndex < toIndex) {
    delta = toIndex - fromIndex;
    NSLog(@"Moved down by %lu cells", delta);
    [self.objectController moveObjectOrderUp:affectedObject by:delta];
  } else {
    delta = fromIndex - toIndex;
    NSLog(@"Moved up by %lu cells", delta);
    [self.objectController moveObjectOrderDown:affectedObject by:delta];
  }

  isModifyingOrder = NO;
}

这里是控制器中的部分。这可以写得更好，但为了理解这可能是最好的。

- (void)moveObjectOrderUp:(FFObject*)affectedObject by:(NSInteger)delta {
  NSInteger fromIndex = [affectedObject.index integerValue] - delta;
  NSInteger toIndex = [affectedObject.index integerValue];

  if (fromIndex < 1) {
    return;
  }

  NSFetchRequest* fetchRequest = [[NSFetchRequest alloc] init];
  [fetchRequest setEntity:[NSEntityDescription entityForName:@"Object" inManagedObjectContext:self.managedObjectContext]];

  NSPredicate* predicate = [NSPredicate predicateWithFormat:@"(index >= %d) AND (index < %d)", fromIndex, toIndex];
  [fetchRequest setPredicate:predicate];

  NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"index" ascending:YES];
  NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptor, nil];

  [fetchRequest setSortDescriptors:sortDescriptors];

  NSError* fetchError = nil;
  NSArray* objects = [self.managedObjectContext executeFetchRequest:fetchRequest error:&fetchError];

  for (FFObject* object in objects) {
    NSInteger newIndex = [object.index integerValue] + 1;
    object.index = [NSNumber numberWithInteger:newIndex];
  }

  affectedObject.index = [NSNumber numberWithInteger:fromIndex];

  [self saveContext];
}

- (void)moveObjectOrderDown:(FFObject*)affectedObject by:(NSInteger)delta {
  NSInteger fromIndex = [affectedObject.index integerValue];
  NSInteger toIndex = [affectedObject.index integerValue] + delta;

  NSFetchRequest* fetchRequest = [[NSFetchRequest alloc] init];
  [fetchRequest setEntity:[NSEntityDescription entityForName:@"Object" inManagedObjectContext:self.managedObjectContext]];

  NSPredicate* predicate = [NSPredicate predicateWithFormat:@"(index > %d) AND (index <= %d)", fromIndex, toIndex];
  [fetchRequest setPredicate:predicate];

  NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"index" ascending:YES];
  NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptor, nil];

  [fetchRequest setSortDescriptors:sortDescriptors];

  NSError* fetchError = nil;
  NSArray* objects = [self.managedObjectContext executeFetchRequest:fetchRequest error:&fetchError];

  for (FFObject* object in objects)
  {
    NSInteger newIndex = [object.index integerValue] - 1;
    object.index = [NSNumber numberWithInteger:newIndex];
  }

  affectedObject.index = [NSNumber numberWithInteger:toIndex];

  [self saveContext];
}

不要忘记在视图控制器中使用第二个BOOL进行删除操作，以防止移动通知执行任何操作。我将其称为isDeleting并将其放在此处。

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject
   atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type
  newIndexPath:(NSIndexPath *)newIndexPath {
  if (isModifyingOrder) return;

  ...

  switch(type) {

    ...

    case NSFetchedResultsChangeMove:
        if (isDeleting == false) {
            [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:localIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            [self.tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:localNewIndexPath]withRowAnimation:UITableViewRowAnimationFade];
        }
        break;

    ...

  }
}

Answer 4

我认为：

    affectedObject.displayOrderValue = toIndex;

必须放在：

之后

   for (NSUInteger i = start; i <= end; i++) {
    FFObject *otherObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:i];  
    NSLog(@"Updated %@ / %@ from %i to %i", otherObject.name, otherObject.state, otherObject.displayOrderValue, otherObject.displayOrderValue + delta);  
    otherObject.displayOrderValue += delta;
}

之前：

    [self FF_fetchResults];

如何使用Core Data在UITableView中维护显示顺序？

4 个答案: