获得NSFetchedResultsController，NSSortDescription和sectionNameForKeyPath一起工作 [英] Getting NSFetchedResultsController, NSSortDescription and sectionNameForKeyPath to work together

问题描述

我目前正在处理一个应用程式，该应用程式有几个实体和关系，如下所示：

I'm currently working on an App that has a couple of Entities and relationships as illustrated below:

项目< - > 类别。

Item <<--> Category.

我目前正在抓取项目实例，并使用项目的 category.name 。在这种情况下，我可以使用排序描述符按名称对类别进行排序，这是非常简单和工作正常（相关代码如下）：

I am currently fetching Item instances and displaying them in sections using the item's category.name. In this case I can use a sort descriptor to sort the categories by name, which is pretty straightforward and working fine (relevant code below):

-(NSFetchedResultsController*)fetchedResultsController {
    if (fetchedResultsController_ != nil)
        return fetchedResultsController_;
    NSManagedObjectContext *moc = [order_ managedObjectContext];
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Item" inManagedObjectContext:moc];
    [fetchRequest setEntity:entity];
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"category.name" ascending:YES];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
    [fetchRequest setSortDescriptors:sortDescriptors];
    [sortDescriptors release];
    [sortDescriptor release];
    NSFetchedResultsController *controller = [[NSFetchedResultsController alloc]
                                          initWithFetchRequest:fetchRequest
                                          managedObjectContext:moc
                                          sectionNameKeyPath:@"category.name"
                                          cacheName:nil];
    controller.delegate = self;
    self.fetchedResultsController = controller;
    [controller release];
    [fetchRequest release];

    NSError *error = nil;
    if (![fetchedResultsController_ performFetch:&error]) {
        // Error handling
    }
    return fetchedResultsController_;
}

我现在的问题是，我需要排序这些类别不是名称，由属于类别实体一部分的（NSNumber *）displayOrder 属性。 但我需要表格视图中的章节标题，继续使用类别的名称。

My problem now is that I need to sort these categories not by name, but by a (NSNumber*) displayOrder attribute that is part of the Category entity. BUT I need the section titles on the tableview to continue to use the categorie's name.

如果我设置sortDescriptor使用 category.displayOrder 并保持sectionNameKeyPath为 category.name 工作正常，但sortDescriptor被fetchedResultsController忽略，表部分按类别的名称排序（不知道为什么？）。

If I set the sortDescriptor to use category.displayOrder and keep the sectionNameKeyPath as category.name, the section titles work fine but the sortDescriptor is simply ignored by the fetchedResultsController and the table sections are ordered by the category's name (not sure why??).

我的下一个想法是覆盖 displayOrder getter方法，但是没有让我太远，因为返回类型不同，加上我需要实际的displayOrder值进行段排序。

My next idea was to overwrite the displayOrder getter method but that didn't get me too far as the return types are different, plus I needed the actual displayOrder value for the section sorting.

现在我有一个解决方案感觉有点笨拙（下面的代码），我想知道是否有一个更好的方法实现相同的东西，使用fetchedResultsController单独。

So right now I have a solution which feels a bit clunky (code below), and I am wondering if there is a better way of achieving the same thing using the fetchedResultsController alone.

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section
// The code below grabs a reference to first object for a given section
// and uses it to return the associated category name
{
    id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
    NSArray *menuItems = [sectionInfo objects];
    if ([menuItems count] > 0)
    {
        MenuItem *menuItem = [menuItems objectAtIndex:0];
        NSString *categoryName = menuItem.category.name;
        return categoryName;
    }
    return [sectionInfo name];
}

我在这里缺少基本的东西吗？

Am I missing something basic here?

先感谢您的想法。

Rog

推荐答案

这是一个很好的解决方案，Rog。

That's a perfectly good solution to the problem, Rog.

你肯定不想/需要子类NSFetchedResultsController。

You certainly don't want/need to subclass NSFetchedResultsController.

@aroth，我们没有足够的信息来了解他的对象模型的细节，但是这些名称肯定意味着不拥有项。项目有一个类别。他的目的是显示一个项目列表，这就是为什么他正在抓取项目。

@aroth, we don't have enough information to know the details of his object model, but the names certainly imply that category does not own item. Item has a category. His intent is to display a list of items, that is why he is fetching items.

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