更改NSFetchedResultsController对SegmentedControl的更改

Question

我有一个UITableView，它使用核心数据NSFetchedResultsController填充。我现在已经向视图中添加了一个UISegmentedControl，当您更改当前段时，我希望更改表视图的内容。

我在网上读到，使用两种不同的NSFetchedResultsControllers是明智的，因为这样我就可以从内置的缓存中获益。唯一的问题是，我似乎找不到用于此操作的任何示例代码，也不知道从哪里开始。

有人能解释从哪里开始创建第二个NSFetchedResultsController，并根据分段的控件更改表视图的哪个源吗？

查看标题代码：

#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;
}

任何帮助都是非常感谢的。谢谢。

更新:更新的代码

查看标题代码：

@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;
} 

Answer 1

您可以添加另一个实例变量，它表示当前选定的NSFetchedResultsController。当UISegmentedControl发生变化时，更新这个ivar。

这可能是由段的valuechange事件触发的操作。

- (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];
}

并且必须确保只有当前的nsfrc触发NSFetchedResultsControllerDelegate方法中的tableview更新。所以你也得把它们都换掉。

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

编辑:不，你做错了。currentFetchedResultsController只是一个ivar，没有一个懒散的加载器。它“只是”指向当前使用的控制器的指针。

但是另外两个fetchedResultsControllers应该有这样一个懒散的加载getter。

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

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

然后切换到：

self.currentFetchedResultsController = self.keyphraseFetchedResultsController;

或

self.currentFetchedResultsController = self.competitorFetchedResultsController;