我有一个 UITableView，它使用 Core Data NSFetchedResultsController 填充。我现在已将 UISegmentedControl 添加到 View 中，当您更改当前段时，我希望更改 tableview 的内容。

我在网上了解到，使用两个不同的 NSFetchedResultsControllers 是明智的，因为这样我就可以从内置缓存中获益。唯一的问题是我似乎找不到执行此操作的任何示例代码，也不知道从哪里开始。

谁能解释从哪里开始创建第二个 NSFetchedResultsController 并根据分段控件更改 tableview 的来源？

查看头部代码:

#import <CoreData/CoreData.h>

@interface DomainViewController : UIViewController <NSFetchedResultsControllerDelegate, UITableViewDataSource, UITableViewDelegate> {

    UITableView *domainView;
    UISegmentedControl *segmentedControl;
    NSString *domain;

}

@property (nonatomic, retain) NSFetchedResultsController *fetchedResultsController;
@property (nonatomic, retain) NSManagedObjectContext *managedObjectContext;
@property (nonatomic, retain) NSString *domain;

@property (nonatomic, retain) IBOutlet UITableView *domainView;
@property (nonatomic, retain) IBOutlet UISegmentedControl *segmentedControl;

- (IBAction)segmentedControlIndexChanged;

查看实现代码:

@interface DomainViewController ()
- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath;
@end

@implementation DomainViewController

@synthesize fetchedResultsController = __fetchedResultsController;
@synthesize managedObjectContext = __managedObjectContext;
@synthesize domain;
@synthesize domainView;
@synthesize segmentedControl;

- (void)viewDidLoad
{
    [super viewDidLoad];

    if (self.managedObjectContext == nil) 
    { 
        self.managedObjectContext = [(GARankingsAppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext]; 
    }

}

- (IBAction)segmentedControlIndexChanged
{
    switch(self.segmentedControl.selectedSegmentIndex){
        case 0:
            break;
        case 1:
            break;
        default:
            break;
    }
}

- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath
{
    NSManagedObject *managedObject = [self.fetchedResultsController objectAtIndexPath:indexPath];
    cell.textLabel.text = [[managedObject valueForKey:@"Keyphrase"] description];
}

#pragma mark - Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return [[self.fetchedResultsController sections] count];
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
    return [sectionInfo numberOfObjects];
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"Cell";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
    }

    // Configure the cell.
    [self configureCell:cell atIndexPath:indexPath];
    return cell;
}

- (NSFetchedResultsController *)fetchedResultsController
{
    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:@"Result" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    // Set the batch size to a suitable number.
    [fetchRequest setFetchBatchSize:20];

    // Edit the sort key as appropriate.
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"Keyphrase" ascending:NO];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

    [fetchRequest setSortDescriptors:sortDescriptors];

    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;

    [aFetchedResultsController release];
    [fetchRequest release];
    [sortDescriptor release];
    [sortDescriptors release];

    NSError *error = nil;
    if (![self.fetchedResultsController performFetch:&error])
    {
        /*
         Replace this implementation with code to handle the error appropriately.

         abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.
         */
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return __fetchedResultsController;
}

非常感谢任何帮助。谢谢。

更新:更新代码

查看头部代码:

@property (nonatomic, retain) NSFetchedResultsController *currentFetchedResultsController;
@property (nonatomic, retain) NSFetchedResultsController *competitorFetchedResultsController;
@property (nonatomic, retain) NSFetchedResultsController *keyphraseFetchedResultsController;

查看实现代码:

@synthesize currentFetchedResultsController = __fetchedResultsController;
@synthesize competitorFetchedResultsController;
@synthesize keyphraseFetchedResultsController;

- (NSFetchedResultsController *)fetchedResultsController
{
    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:@"Result" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    // Set the batch size to a suitable number.
    [fetchRequest setFetchBatchSize:20];

    // Edit the sort key as appropriate.
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"Keyphrase" ascending:NO];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

    [fetchRequest setSortDescriptors:sortDescriptors];

    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
    aFetchedResultsController.delegate = self;
    //self.fetchedResultsController = aFetchedResultsController;
    self.currentFetchedResultsController = aFetchedResultsController;

    [aFetchedResultsController release];
    [fetchRequest release];
    [sortDescriptor release];
    [sortDescriptors release];

    NSError *error = nil;
    //if (![self.fetchedResultsController performFetch:&error])
    if (![self.currentFetchedResultsController performFetch:&error])
    {
        /*
         Replace this implementation with code to handle the error appropriately.

         abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.
         */
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return __fetchedResultsController;
} 

最佳答案

你可以添加另一个代表当前选择的 NSFetchedResultsController 的实例变量。并且当 UISegmentedControl 更改时更新此 ivar。

这可能是由段的 valuechange 事件触发的 Action

- (IBAction *)segmentChanged:(UISegmentedControl *)sender {
    if ([sender selectedSegmentIndex] == 0) {
        self.currentFetchedResultsController = self.nsfrc1;
        [self.tableView reloadData];
    }
    else if ([sender selectedSegmentIndex] == 1) {
        self.currentFetchedResultsController = self.nsfrc2;     
        [self.tableView reloadData];
    }
}

以一个 UITableViewDataSource 方法为例:

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
    return [[self.currentFetchedResultsController sections] count];
}

并且您必须确保只有当前的 nsfrc 会触发 NSFetchedResultsControllerDelegate 方法中的 TableView 更新。所以你也必须改变所有这些。

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
    if (controller == self.currentFetchedResultsController) {
        [self.tableView beginUpdates];
    } 
}

编辑:不，你做错了。 currentFetchedResultsController 只是一个ivar，没有延迟加载getter。它“只是”指向当前使用的 Controller 的指针。

但是另外两个fetchedResultsControllers应该有这么一个懒加载的getter。

- (NSFetchedResultsController *)competitorFetchedResultsController {
    if (!myCompetitorFetchedResultsController) {
        // create competitorFetchedResultsController
    }
    return myCompetitorFetchedResultsController;
}

- (NSFetchedResultsController *)keyphraseFetchedResultsController {
    if (!myKeyphraseFetchedResultsController) {
        // create keyphraseFetchedResultsController
    }
    return myKeyphraseFetchedResultsController;
}

然后切换为:

self.currentFetchedResultsController = self.keyphraseFetchedResultsController;

或

self.currentFetchedResultsController = self.competitorFetchedResultsController;

关于iphone - 在 SegmentedControl 更改时更改 NSFetchedResultsController，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/5686938/