如何正确配置NSFetchedResultsController [英] How to properly configure NSFetchedResultsController

问题描述

我把一个NSFetchedResultsController放入我的代码，所以我得到了很好的自动分区我的表视图数据。

所以我运行一个测试，以确保一切正常正确。我在我的持久存储中有一个单一的Book实体。我将首先执行fetch旧的方式，然后我将尝试使用NSFetchedResultsController。

代码 NSFetchedResultsController：

  NSFetchRequest * request = [[NSFetchRequest alloc] init]; 
 NSEntityDescription * entity = [NSEntityDescription entityForName：kBookEntityName inManagedObjectContext：self.managedObjectContext]; 
 [request setEntity：entity]; 
 
 NSSortDescriptor * sortDescriptor = [[NSSortDescriptor alloc] initWithKey：@titleascending：NO]; 
 NSArray * sortDescriptors = [[NSArray alloc] initWithObjects：sortDescriptor，nil]; 
 [request setSortDescriptors：sortDescriptors]; 
 [sortDescriptor release]; 
 [sortDescriptors release]; 
 
 //以下2行将被NSFetchedResultsController替换
 NSMutableArray * mutableFetchResults = [[[self.managedObjectContext executeFetchRequest：request error：nil] mutableCopy] autorelease]; 
 Book * result =（Book *）[mutableFetchResults objectAtIndex：0]; 
 
 NSString * title = [result valueForKey：@title]; 
 NSString * priority = [result valueForKeyPath：@priority.name]; 
 
 
 [request release]; 
  

现在我在行中替换NSFetchedResultsController：

  NSFetchedResultsController * fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest：request managedObjectContext：self.managedObjectContext sectionNameKeyPath：@titlecacheName：@BookList 
 
 Book * result =（Book *）[fetchedResultsController objectAtIndexPath：[NSIndexPath indexPathForRow：0 inSection：0]]; 
  

看起来很干燥。第一代码块适当地提取单个Book实体。但是，使用NSFetchedResultsController的代码不会。相反，它返回nil。

我的问题是：我在这个例子中正确配置NSFetchedResultsController吗？

NSFetchedResultsController也是nil）

解决方案

我想你还需要告诉 NSFetchedResultsController 实际执行fetch：

  NSError * error; 
 BOOL success = [controller performFetch：& error]; 
  

（取自 NSFetchedResultsController 参考）

另一件似乎奇怪的事情：你真的想使用title作为 sectionNameKeyPath ？将不会为每本书创建一个单独的部分？

I am placing an NSFetchedResultsController into my code so I get that nice automatic sectioning of my table view data.

So I am running a test to make sure everything works properly. I have a single Book entity in my persistent store. I will first perform the fetch the old way, then I will try to use NSFetchedResultsController. The difference between the 2 blocks of code is just 2 lines.

Code without NSFetchedResultsController:

 NSFetchRequest *request = [[NSFetchRequest alloc] init];
 NSEntityDescription *entity = [NSEntityDescription entityForName:kBookEntityName inManagedObjectContext:self.managedObjectContext];
 [request setEntity:entity];

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

  //The following 2 lines will be replaced by the NSFetchedResultsController
 NSMutableArray *mutableFetchResults = [[[self.managedObjectContext executeFetchRequest:request error:nil] mutableCopy] autorelease];
 Book *result = (Book*)[mutableFetchResults objectAtIndex:0];

 NSString* title = [result valueForKey:@"title"];
 NSString* priority = [result valueForKeyPath:@"priority.name"];


 [request release];

Now I substitute in the lines for the NSFetchedResultsController:

NSFetchedResultsController* fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:self.managedObjectContext sectionNameKeyPath:@"title" cacheName:@"BookList"];                                                                                                                                                                                                                                                                                                           

Book *result = (Book*)[fetchedResultsController objectAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:0]];

Seems pretty cut and dry. The first code block properly fetches the single Book entity. The code with the NSFetchedResultsController, however, does not. Instead it returns nil.

My question is: Am I properly configuring the NSFetchedResultsController in this example?

(note, the fetchedObjects property of the NSFetchedResultsController is also nil)

解决方案

I think you still need to tell the NSFetchedResultsController to actually perform the fetch:

NSError *error;
BOOL success = [controller performFetch:&error];

(taken from the example in the NSFetchedResultsController reference)

one other thing that seems odd: do you really want to use "title" as the sectionNameKeyPath? won't that basically create a separate section for each book?

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