使用 FetchResultController 和 ManagedObjectContext 获取对象之间的区别
使用 FetchResultController 或 ManagedObjectContext 从 Core Data 获取元素有什么区别？
1) FetchResultController
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Item" inManagedObjectContext: managedObjectContext]];
NSSortDescriptor *sortDescriptorNameAscending = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptorNameAscending,nil];
[fetchRequest setSortDescriptors:sortDescriptors];
[sortDescriptorNameAscending release];
NSFetchedResultsController *frc = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"Lists"];
2) ManagedObjectContext
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:@"Item" inManagedObjectContext:managedObjectContext]];
NSError *error = nil;
NSArray *items = [ managedObjectContext executeFetchRequest:request error:&error];
What's the difference between get elements from Core Data with FetchResultController or ManagedObjectContext??
1) FetchResultController
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Item" inManagedObjectContext: managedObjectContext]];
NSSortDescriptor *sortDescriptorNameAscending = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptorNameAscending,nil];
[fetchRequest setSortDescriptors:sortDescriptors];
[sortDescriptorNameAscending release];
NSFetchedResultsController *frc = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"Lists"];
2) ManagedObjectContext
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:@"Item" inManagedObjectContext:managedObjectContext]];
NSError *error = nil;
NSArray *items = [ managedObjectContext executeFetchRequest:request error:&error];
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使用
NSFetchedResultsController而不是仅使用NSFetchRequest的目的是监视数据以及使用节时的便捷方法。当仅处理
NSFetchRequest时，您必须自己确定各个部分，并且需要在发生变化时重新获取数据。在处理 NSFetchedResultsController 时，它将确定您的部分，缓存结果（几乎瞬时对该数据发出第二次请求），并为您的 NSTableView 提供便捷的方法。最后，当您的数据发生更改时，
NSFetchedResultsController将通过其委托通知您。这两者的内部数据将是相同的。区别在于管理数据的状态。
The point behind using an
NSFetchedResultsControlleras opposed to just aNSFetchRequestis the monitoring of your data and the convenience methods when working with sections.When dealing with just a
NSFetchRequestyou have to determine the sections yourself and you need to refetch your data when something changes.When dealing with the
NSFetchedResultsController, it will determine your sections, cache the results (making a second request for that data near instantaneous), and provide convenience methods for your NSTableView. Finally, when your data changes, theNSFetchedResultsControllerwill notify you through its delegates.The data internal to both of these is going to be the same. It is managing the state of that data that is the difference.
NSFetchedResultsController对象会告诉您查询对象何时发生变化。只需提供一个委托对象来接收调用（请参阅 NSFetchedResultsControllerDelegate 协议的文档）。它还为您提供部分管理，如果您想在表视图中显示数据，这非常有用。
The
NSFetchedResultsControllerobject will tell you when the objects of your query change. Just provide a delegate object to receive the calls (see the doc forNSFetchedResultsControllerDelegateprotocol).It also provide you with section management which is useful if you want to display the data in a table view.