问题描述
我的 iOS Core Data 数据库中有一组实体对象，用于描述某个位置的某些内容.让我们称实体位置.我通过在 Location 上有两个引用位置的属性来实现这一点 - 纬度和经度，都是双精度的.还有其他元素，例如名称.
I have a set of entity objects in my iOS Core Data database that describe something at a location. Let's call the entity Location. I have implemented this by having two attributes on Location that refer to the location - latitude and longitude, both doubles. There are other elements, like name.
我正在使用 NSFetchedResultsController 将实体绑定到 UITableViewController.我想做的是让结果按到给定 CLLocationCoordinate2D 的距离排序.在一个非常理想的情况下，我可以刷新该列表以根据新位置重新计算排序.因此，这种排序将取决于两个键和第三个静态"变量(一个不会在集合中的项目之间变化的变量).
I am using a NSFetchedResultsController to bind the entities to a UITableViewController. What I would like to do is have the results sorted by distance to a given CLLocationCoordinate2D. In an really ideal scenario, I'm able to refresh that list to recalculate the sort based on a new location. Therefore, this sort would depend on two keys, and a third "static" variable (one that doesn't vary across the items in the collection).
如果我使用 NSSortDescriptors 对任意列表进行排序，我想我可以弄清楚如何做到这一点.但是，我不控制排序描述符在 NSFetchedResultsController 中的使用方式.
I think I could figure out how to do this if I was sorting an arbitrary list with NSSortDescriptors. However, I don't control how the sort descriptors are used in an NSFetchedResultsController.
有没有一种方法可以配置我的实体、我的 NSFetchedResultsController、我的 NSSortDescriptors 等来完成此任务?我怀疑答案不在于创建一个花哨的 NSSortDescriptor，而是在实体中创建一个表示与我的距离的瞬态属性，并定期重新计算该属性.但是，我对 Core Data 还很陌生，所以我不确定如何最好地做到这一点(遍历所有实体并重新计算一个字段).我也不确定 NSSortDescriptors 是否适用于瞬态属性.
Is there a way that I could configure my entities, my NSFetchedResultsController, my NSSortDescriptors, etc. to accomplish this? I suspect that the answer lies not in creating a fancy NSSortDescriptor but instead in creating a transient attribute in the entity that represents the distance-to-me, and recalculating that attribute periodically. However, I'm new enough to Core Data that I'm not sure how to best do this (iterate over all entities and recalculate a field). I'm also not sure if NSSortDescriptors will work on Transient attributes.
推荐答案
(来自评论:)
(基于 SQLite)Core Data 存储的获取请求不能使用基于瞬态属性或基于 Objective-C 谓词的排序描述符.
A fetch request for a (SQLite based) Core Data store cannot use sort descriptors based on transient attributes or Objective-C based predicates.
如果您不想失去获取结果控制器的优势(如动画表格视图更新、自动分组到部分等)，您必须预先计算到当前位置的距离并将其存储在 (对象的持久性)属性.
If you don't want to loose the advantages of a fetched results controller (like animated table view updates, automatic grouping into sections etc.) you have to pre-compute the distance to the current location and store it in a (persistent) attribute of your objects.
或者，您可以获取所有对象并在内存中对其进行排序.在这种情况下，您可以使用任意排序描述符.但这不能与获取的结果控制器结合使用，因此您必须注册托管对象上下文中的更改并在必要时重新加载表.
Alternatively, you could fetch all objects and sort them in memory. In that case you can use arbitrary sort descriptors. But that cannot be combined with a fetched results controller, so you would have to register for changes in the managed object context and reload the table if necessary.
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