from NSFetchedResults objectAtIndex：0返回0

我一直在這一個特定的問題上敲了我的頭大約2個小時，現在閱讀文檔和示例，我只是無法環繞它。我擁有核心數據模型Person和Photo，並且它們已鏈接。我試圖顯示一個UITableView的內容和from NSFetchedResults objectAtIndex：0返回0

-(NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section

保持返回0.下面是相關的代碼。我知道我的核心數據中有數據，因爲我可以在終端中用sqlite查看它。

AppDelegate.h

@interface PaparazziAppDelegate : NSObject <UIApplicationDelegate> { 
    UIWindow *window; 

    UITabBarController *tabController; 
    UINavigationController *mainNavController; 
    UINavigationController *recentsNavController; 
    PersonListViewController *personListView; 
    PhotoListViewController *recentsList; 
    PhotoDetailViewController *photoDetail; 

    FlickrFetcher *fetcher; 
    NSManagedObjectContext *currentContext; 
} 

@property (nonatomic, retain) IBOutlet UIWindow *window; 

@property (nonatomic, retain) IBOutlet UITabBarController *tabController; 
@property (nonatomic, retain) IBOutlet UINavigationController *mainNavController, *recentsNavController; 

@end

AppDelegate.m

#import "PaparazziAppDelegate.h" 

@implementation PaparazziAppDelegate 

@synthesize window; 
@synthesize tabController, mainNavController, recentsNavController; 

#pragma mark - 
#pragma mark Application lifecycle 

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions { 

    // Setup the database instances for access. 
    fetcher = [FlickrFetcher sharedInstance]; 

    // Check to see if the database exists, if not, load the plist and populate. 
    if ([fetcher databaseExists]) 
     NSLog(@"I'm here!"); 
    else { 
     NSLog(@"No database yet!"); 

     currentContext = [fetcher managedObjectContext]; 

     /* -- REMOVED FOR READABILITY -- 
     A chunk of code for creating fresh data from a plist if the database doesn't yet exist. 
     I know this part works because I can see the data using the terminal */ 

     [currentContext release]; 
    } 
    // End of the plist loading and database creation. 

    personListView = [[PersonListViewController alloc] initWithStyle:UITableViewStylePlain]; 
    personListView.title = @"Contacts"; 
    [mainNavController pushViewController:personListView animated:NO]; 
    [personListView release]; 

    recentsList = [[PhotoListViewController alloc] initWithNibName:@"PhotoListViewController" bundle:[NSBundle mainBundle]]; 
    recentsList.title = @"Recents"; 
    [recentsNavController pushViewController:recentsList animated:NO]; 
    [recentsList release]; 

    // setting up view icons 
    UITabBarItem *contactsIcon = [[UITabBarItem alloc] 
          initWithTabBarSystemItem:UITabBarSystemItemContacts tag:0]; 
    mainNavController.tabBarItem = contactsIcon; 
    [contactsIcon release]; 
    UITabBarItem *recentsIcon = [[UITabBarItem alloc] 
            initWithTabBarSystemItem:UITabBarSystemItemRecents tag:0]; 
    recentsNavController.tabBarItem = recentsIcon; 
    [recentsIcon release]; 

    [self.window addSubview:tabController.view]; 

    // Override point for customization after application launch. 

    [self.window makeKeyAndVisible]; 

    return YES; 
} 

- (void)dealloc { 
    [window release]; 
    [tabController release]; 
    [mainNavController release]; 
    [recentsNavController release]; 

    [fetcher release]; 

    [super dealloc]; 
} 


@end

PersonListViewController.h

#import <UIKit/UIKit.h> 
#import "FlickrFetcher.h" 
#import "Person.h" 
#import "Photo.h" 

@interface PersonListViewController : UITableViewController { 
    NSArray *listOfPeople; 

    FlickrFetcher *fetcher; 
    NSManagedObjectContext *currentContext; 
    NSFetchedResultsController *fetchedResultsController; 
} 

@property (nonatomic, retain) NSArray *listOfPeople; 
@property (nonatomic, retain) NSManagedObjectContext *currentContext; 
@property (nonatomic, retain) NSFetchedResultsController *fetchedResultsController; 

@end

而且麻煩的文件，PersonListViewController.m

#import "PersonListViewController.h" 

@implementation PersonListViewController 

@synthesize listOfPeople, currentContext, fetchedResultsController; 

#pragma mark - 
#pragma mark Initialization 

- (id)initWithStyle:(UITableViewStyle)style { 
    // Override initWithStyle: if you create the controller programmatically and want to perform customization that is not appropriate for viewDidLoad. 
    self = [super initWithStyle:style]; 
    if (self) { 
     // Custom initialization. 
     fetcher = [FlickrFetcher sharedInstance]; 
     currentContext = [fetcher managedObjectContext]; 

     fetchedResultsController = [fetcher fetchedResultsControllerForEntity:@"Person" withPredicate:nil]; 
    } 
    return self; 
} 

#pragma mark - 
#pragma mark View lifecycle 


- (void)viewDidLoad { 
    [super viewDidLoad]; 

    self.title = @"Contacts"; 
} 

#pragma mark - 
#pragma mark Table view data source 

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView { 
    // Return the number of sections. 
    return 1; 
} 

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section { 
    // Return the number of rows in the section.  
    id <NSFetchedResultsSectionInfo> sectionInfo = [[fetchedResultsController sections] objectAtIndex:section]; 
    return [sectionInfo numberOfObjects]; 
} 

// Customize the appearance of table view cells. 
- (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]; 
    } 
    Person *person = [fetchedResultsController objectAtIndexPath:indexPath]; 
    cell.textLabel.text = [person name]; 

    // Configure the cell... 

    return cell; 
} 

#pragma mark - 
#pragma mark Memory management 

- (void)dealloc { 
    [super dealloc]; 
} 

@end

我有一種感覺，我錯過了一些簡單的東西，或者1個關閉，但我似乎無法找到它。 [FlickrFetcher sharedInstance]返回一個單例，如果我在每個.m文件中拋出幾個斷點，我可以看到* fetcher在每個區域中的對象ID是相同的。

來源

2010-11-30 Chuck

看起來你沒有使用performFetch:來執行提取結果控制器的請求。 viewWillAppear:通常是這樣做的好去處：

- (void)viewWillAppear:(BOOL)animated { 
    [super viewWillAppear:animated]; 

    NSError *error = nil; 
    if (![fetchedResultsController performFetch:&error]) { 
     NSLog(@"fetch error: %@", error); 
    } 
}

來源

2010-11-30 08:25:23

* facedesk * * facedesk * * facedesk *就是這樣，在試圖追蹤一個可能丟失的引用時，我忘記了performFetch。現在它可以工作，非常感謝你。另外，如果有人看到我發佈的任何愚蠢的錯誤，隨時可以對我大吼一聲。 – Chuck 2010-11-30 08:41:44

順便說一句 - 你可能會考慮在viewDidLoad而不是initWithStyle:初始化你fetchedResultsController，那麼您可以在viewDidUnload下降，並因此釋放一些內存，如果操作系統的需求卸載視圖控制器。

來源

2010-11-30 10:49:08

from NSFetchedResults objectAtIndex：0返回0

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