NSFetchedResultsController自定义排序不被调用 [英] NSFetchedResultsController custom sort not getting called

问题描述

我目前正在尝试使用NSFetchedResultsController从Core Data填充我的项目中的UITableView。我使用一个自定义搜索比较器（虽然我也尝试过选择器，并有一个相同的问题）：

I am currently trying to populate a UITableView in my project from Core Data using NSFetchedResultsController. I am using a custom search with a comparator (although I have also tried a selector and had an identical problem):

    if (fetchedResultsController != nil) {
        return fetchedResultsController;
    }

    /*
     Set up the fetched results controller.
    */
    // Create the fetch request for the entity.
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    // Edit the entity name as appropriate.
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Object" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    // Set the batch size to a suitable number.
    [fetchRequest setFetchBatchSize:20];
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"objectName" ascending:YES comparator:^(id s1, id s2) {
            NSLog(@"Comparator");
      //custom compare here with print statement
    }];
    NSLog(@"Sort Descriptor Set");
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Object" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];
    [fetchRequest setSortDescriptors:sortDescriptors];

    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:@"firstLetterOfObject" cacheName:@"Objects"];
    [aFetchedResultsController release];
    [fetchRequest release];
    [sortDescriptor release];
    [sortDescriptors release];
    if (![fetchedResultsController performFetch:&error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return fetchedResultsController;

当我进入这个选项卡时，我已经记录了整个程序，发现NSFetchedResultsController甚至没有在获取时输入比较器块。

When I enter this tab, I have logged all over the program and found that the NSFetchedResultsController does not even enter the comparator block when fetching. It instead sorts it with some default sorting method.

如果我删除并添加一个带有objectName的Object，然后它会进入比较器块并正确地对表进行排序。

If I delete and add an Object with an objectName, however, it then does enter the comparator block and correctly sort the table.

为什么NSFetchedResultsController在管理对象模型更改之前不使用比较器进行排序？

Why does the NSFetchedResultsController not sort using the comparator until the managed object model is changed?

我也试过也关闭缓存，和/或执行在viewDidLoad：fetch，但似乎我获取了多少没有关系，但时间。因为某种原因，它只会在对象模型更改后使用我的排序。

Notes: I have tried also turning off caching, and/or performing a fetch in viewDidLoad:, but it seems that how many times I fetch does not matter, but when. For some reason it only uses my sorting after the object model has been changed.

推荐答案

的。首先，虽然这可能不是你的问题，你不能对瞬态属性进行排序。但更有可能的是，在由SQL存储支持的模型中排序时，比较器将编译为SQL查询，而不是所有的Objective-C函数都可用。在这种情况下，您需要在执行提取后在内存中进行排序。

There are a couple of things I can think of. First, though this may not be your problem, you cannot sort on transient properties. But more likely is that when sorting in a model backed by a SQL store, the comparator gets "compiled" to a SQL query, and not all Objective-C functions are available. In this case, you'd need to sort in memory after the fetch is performed.

编辑：请参阅此 doc ，特别是Fetch Predicates and Sort Descriptors部分。

See this doc, specifically the Fetch Predicates and Sort Descriptors section.

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