如何以编程方式切换UITableView的NSFetchedResultsController（或其谓词）？ [英] How to switch UITableView's NSFetchedResultsController (or its predicate) programmatically?

问题描述

我有一个UITableView显示大量名为文档的实体的子集。子集由另一个实体选择定义。选择是命名的文档的有序列表。

I have a UITableView that displays a subset of a large number of entities named "Documents". The subset is defined by another entity "Selection". Selections are named, ordered list of documents.

它工作正常，除非我想在运行时更改显示的选择。

It Works fine, except when I want to change the displayed selection at run time. I get only a blank list.

基本上，我需要更改NSFetchedResultsController保存的谓词，以便新的谓词使用另一个Selection。我不能让它工作。我最后一次尝试是摆脱NSFetchedResultsController完全和重新分配它：

Basically, I need to change the predicate that my NSFetchedResultsController holds so that the new predicate uses the another Selection. I couldn't make it work. My last attempt is to get rid of the NSFetchedResultsController altogether and reallocate it:

- (void) displaySelection:(Selection *)aSet
{
 self.currentSelection = aSet;
 self.fetchedResultsController = nil;
 // methods here don't all use the property but directly the ivar, so we must trigger the getter
 [self fetchedResultsController];
 [self.tableView reloadData];
}

当然，NSFetchedResultsController getter做正确的事情：

And of course, the NSFetchedResultsController getter does the right thing:

- (NSFetchedResultsController *)fetchedResultsController
{
    if (fetchedResultsController != nil) { return fetchedResultsController; }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"DocInSelection" inManagedObjectContext:managedObjectContext];
    [fetchRequest setEntity:entity];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"selection.identifier like %@", currentSelection.identifier];
    [fetchRequest setPredicate:predicate];
<snip>
    [fetchRequest setSortDescriptors:sortDescriptors];
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;
<snip>
    return fetchedResultsController;
}

此代码第一次工作，因为设置了初始选择。当 displaySelection：被调用时，表视图将变为空白。

This code works the first time, because the Initial Selection is set. When displaySelection: is called, though, the tableview becomes blank.

href =http://stackoverflow.com/questions/2528121/nsfetchedresultscontroller-fetch-request-updating-predicate-and-uitableview> NSFetchedResultsController撷取要求 - 更新谓词和UITableView

A very similar question was asked at NSFetchedResultsController fetch request - updating predicate and UITableView

而答案是摆脱NSFetchedResultsController。我不想这样做，因为NSFetchedResultsController在这里带来了很多有用的东西（例如缓存，部分加载...）。问题仍然是：如何切换由NSFetchedResultsController支持的UITableView中的数据，其中switch意味着有不同的谓词，或甚至（在我的情况下）不同的实体。

And the answer was to get rid of the NSFetchedResultsController. I don't want to do that, because NSFetchedResultsController brings a lot of useful goodies here (eg caching, partial loading...). The question still stands: how to "switch" data in a UITableView backed by a NSFetchedResultsController, where "switch" means having a different predicate, or even (not in my case) a different entity.

注意，为了完整起见，由于从Selection到Document的多对多关系是有序的，它通过一个名为DocInSelection的中间轻量级实体来处理，该实体具有ordering属性，两个多对一的关系到文档和选择。

Note for the sake of completeness, that since the many-to-many relationship from Selection to Document is ordered, it is handled through an in-between lightweight entity called DocInSelection, which has an "ordering" property and two many-to-one relationships to Document and Selection.

感谢任何建议。

推荐答案

在我发布我的问题后，我尝试了另一个问题的OP的代码的变体。它适用于我。这里是：

After I posted my question, I tried a variant of the code the OP of the other question showed. It works for me. Here it is:

- (void) displaySelection:(Selection *)aSet
{
    if (aSet != self.currentSelection) {
        self.currentSelection = aSet;

        NSFetchRequest *fetchRequest = [[self fetchedResultsController] fetchRequest];
        NSPredicate *predicate = nil;
        NSEntityDescription *entity = nil;
        entity = [NSEntityDescription entityForName:@"DocInSelection" inManagedObjectContext:managedObjectContext];
        predicate = [NSPredicate predicateWithFormat:@"selection.identifier like %@", currentSelection.identifier];
        [fetchRequest setEntity:entity];
        [fetchRequest setPredicate:predicate];

        [NSFetchedResultsController deleteCacheWithName:@"Root"];

        NSError *error = nil;
        if (![[self fetchedResultsController] performFetch:&error]) {
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
            abort();
        }       
    }
    [self.tableView reloadData];
}

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