如何使用CoreData随机排序 [英] How to sort randomly with CoreData

问题描述

我使用NSFetchRequest来获取一些项目，可以按热门排序，或随机。

I am using NSFetchRequest to fetch some items, which could be sorted by Popular, or Random.

按照指南，我可以很容易地使用NSSortDescriptor按照流行度对项目进行排序。

Following the guides, I could sort the items by popularity easily using NSSortDescriptor.

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"popularity" ascending:NO];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
    [request setSortDescriptors:sortDescriptors];

但是如何随机排序？

推荐答案

有没有一个随机排序的东西？

Is there such a thing as a "random sort"?

如果您要随机排列显示通过 NSFetchedResultsController 获得的结果，请记住您正在填充表视图基于对控制器提取结果（ -objectAtIndexPath：）中的元素的 indexPath 引用。

If you want to randomly permute the display of results obtained via NSFetchedResultsController, remember that you are populating your table view based on an indexPath reference to an element in your controller's fetched results (-objectAtIndexPath:).

因此，你可以做的事情是洗刷一个 NSMutableArray （或数组）的索引路径，编写一个方法将索引路径映射到置换索引路径。此方法将索引路径用作输入，查找数组（或多个数组）并输出一个新的索引路径作为输出。

So one thing you can do is shuffle an NSMutableArray (or arrays) of index paths, writing a method that maps an index path to a permuted index path. This method takes an index path as input, looks up the array (or arrays) and spits out a new index path as output. You use this new index path to populate your table view.

或者，您也可以为您的表格视图添加 randomizerTag 属性 Item 实体。您可以使用任何排列函数生成整数 {1 ... n} 的排列，并将这些数字保存到商店中的每个记录。保存后，您可以重新应用排序在记录的 randomizerTag 值的排序描述符。

Alternatively, you can add a randomizerTag attribute to your Item entity. You use any permutation function to generate a permutation of integers { 1 ... n } and save those numbers to each record in your store. Once saved, you can refetch, applying a sort descriptor that sorts on the randomizerTag values of your records.

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