iOS開發學習之NSFetchedResultsController

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應用場合：配合Core Data和Table View使用，Core Data儲存資料，Table View顯示資料

 

1.在需要顯示資料的視圖控制器（包含有table view）中添加NSFetchedResultsController執行個體變數

@implementation LocationsViewController{

 NSFetchedResultsController *_fetchedResultsController;

} 

 

2.惰性執行個體化NSFetchedResultsController執行個體變數

-(NSFetchedResultsController *)fetchedResultsController

{

    if(_fetchedResultsController == nil)

    {

        NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];　　//建立一個請求

　　  //實體描述

        NSEntityDescription *entity = [NSEntityDescription entityForName:@"Location" inManagedObjectContext:self.managedObjectContext];

        [fetchRequest setEntity:entity];

        //設定排序　

        NSSortDescriptor *sortDescriptor1 = [NSSortDescriptor sortDescriptorWithKey:@"category" ascending:YES];

        NSSortDescriptor *sortDescriptor2 = [NSSortDescriptor sortDescriptorWithKey:@"date" ascending:YES];

        [fetchRequest setSortDescriptors:@[sortDescriptor1,sortDescriptor2]];

　　  //每次擷取20個資料

        [fetchRequest setFetchBatchSize:20];

        

  　　//按“category”關鍵字分組，cacheName參數的名字用於唯一標記擷取的資料結果，fetchedResultsController會將這些資料緩衝起來

        _fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:@"category" cacheName:@"Locations"];

        _fetchedResultsController.delegate = self;

    }

    return _fetchedResultsController;

}

 

3.讓視圖控制器實現NSFetchedResultsController協議

@interface LocationsViewController () <NSFetchedResultsControllerDelegate>

@end 

 

4.調用方法開始擷取Core Data儲存的資料

-  (void)viewDidLoad {

　　[super viewDidLoad];

　　[self performFetch];

}

 

-(void)performFetch

{

    NSError *error;

    if(![self.fetchedResultsController performFetch:&error])

    {

        //添加處理錯誤情況的代碼

        return;

    }

}

 

5.添加以下代碼

- (void)dealloc {

 　　_fetchedResultsController.delegate = nil;

} 

 

6.實現table view資料來源返回行數的方法

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

    id <NSFetchedResultsSectionInfo> sectionInfo = [self.fetchedResultsController sections][section];

    return [sectionInfo numberOfObjects];

}

 

7.擷取Core Data儲存的某一個資料（實體）時用以下語句，之後便可對該對象進行刪除，修改等操作，然後再同步到Core Data資料（參考Core Data博文）

Location *location = [self.fetchedResultsController objectAtIndexPath:indexPath];

 

8.實現NSFetchedResultsController協議方法

#pragma mark - NSFetchedResultsControllerDelegate

 

-(void)controllerWillChangeContent:(NSFetchedResultsController *)controller

{

    NSLog(@"*** controllerWillChangeContent");

    [self.tableView beginUpdates];

}

 

-(void)controller:(NSFetchedResultsController *)controller didChangeObject:(nonnull id)anObject atIndexPath:(nullable NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(nullable NSIndexPath *)newIndexPath

{

    switch (type) {

        case NSFetchedResultsChangeInsert:

            NSLog(@"*** NSFetchedResultsChangeInsert (object)");

            [self.tableView insertRowsAtIndexPaths:@[newIndexPath] withRowAnimation:UITableViewRowAnimationFade];

            break;

        case NSFetchedResultsChangeDelete:

            NSLog(@"*** NSFetchedResultsChangeDelete (object)");

            [self.tableView deleteRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];

            break;

        case NSFetchedResultsChangeUpdate:

            NSLog(@"*** NSFetchedResultsChangeUpdate (object)");

            //添加調用設定table view cell內容的方法，該方法需要另行建立

            break;

        case NSFetchedResultsChangeMove:

            NSLog(@"*** NSFetchedResultsChangeMove (object)");

            [self.tableView deleteRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];

            [self.tableView insertRowsAtIndexPaths:@[newIndexPath] withRowAnimation:UITableViewRowAnimationFade];

            break;

    }

}

 

-(void)controller:(NSFetchedResultsController *)controller didChangeSection:(nonnull id<NSFetchedResultsSectionInfo>)sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type

{

    switch (type) {

        case NSFetchedResultsChangeInsert:

            NSLog(@"*** NSFetchedResultsChangeInsert (section)");

            [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];

            break;

        case NSFetchedResultsChangeDelete:

            NSLog(@"*** NSFetchedResultsChangeDelete (section)");

            [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];

            break;

        case NSFetchedResultsChangeMove:    break;

        case NSFetchedResultsChangeUpdate:  break;

        

    }

}

 

-(void)controllerDidChangeContent:(NSFetchedResultsController *)controller

{

    NSLog(@"*** controllerDidChangeContent");

    [self.tableView endUpdates];

}

 

9.實現以下方法，當滑動刪除cell時，同時同步到Core Data資料庫

-(void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(nonnull NSIndexPath *)indexPath

{

    if(editingStyle == UITableViewCellEditingStyleDelete)

    {

        Location *location = [self.fetchedResultsController objectAtIndexPath:indexPath];

        [self.managedObjectContext deleteObject:location];

        

        NSError *error;

        if(![self.managedObjectContext save:&error])

        {

            //添加處理錯誤情況的代碼

            return;

        }

    }

}

 

10.實現table view分組相關的協議方法

-(NSInteger)numberOfSectionsInTableView:(UITableView *)tableView

{

    return [[self.fetchedResultsController sections] count];

}

 

-(NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section

{

    id <NSFetchedResultsSectionInfo> sectionInfo = [self.fetchedResultsController sections][section];

    return [sectionInfo name];

}

iOS開發學習之NSFetchedResultsController