Core Data Archives

Deleting objects from Core Data should not be recognized as deleting rows from SQLite table. It is like deleting objects from Object Graph. This is an example of deleting all rows for the current file number. I retrieve all NSManagedObject objects and delete them in cycle.

Source code

+ (BOOL)deleteScrollPositionsForFileNumber:(NSInteger)fileNumber {
    BOOL success;
    useDelegate
    NSManagedObjectContext *context = appDelegate.managedObjectContext;
    if (context == nil) {
        NSLog(@"Error: No context");
        success = NO;
        return success;
    }
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"fileNumber == %d", fileNumber];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"ScrollPosition" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];
    [fetchRequest setPredicate:predicate];
    NSError *error = nil;
    NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
    NSLog(@"Number of deleted objects: %d", [fetchedObjects count]);
    for (NSManagedObject *object in fetchedObjects) {
        [context deleteObject:object];
    }
    success = YES;
    return success;
}

Core Data – это объектноориентированная оболочка над базой данных приложения в iOS/Mac OS. Позволяет создавать таблицы визуально, сама следит за Commitment Control, управлением версиями и т.д.

Persistent Store – это “надежное” хранилище данных. (NSPersistenStoreCoordinator). Данные хранятся в applicationDocumentsDirectory/App.sqlite. Где App.sqlite – это база SQLite приложения.
Context – это instance базы. Промежуточное хранилище, в котором производятся изменения и затем утверждаются (Commitment). Также запрос производится из Context. У одной базы может быть много Context. В каждом Context есть Managed Objects.
Managed Object – объект, с которым мы работаем, но изменения, которые мы в него вносим не вносятся в Persistent Store до тех пор, пока мы не сделаем Save.

CoreData – Core Data Programming Guide

CoreDataSnippets – Core Data Code Snippets

Базовый запрос без предиката всех строк из таблицы:

+ (BOOL)basicFetch {
    // Test Fetch of 1 item 
    
    useDelegate
    NSManagedObjectContext *context = appDelegate.managedObjectContext;
    if (context == nil) {
        NSLog(@"Error: No context");
        return NO;
    }
    
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription 
entityForName:@"FLCatRanking" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];
    
    NSError *error = nil;
    NSArray *fetchedObjects = [context 
executeFetchRequest:fetchRequest error:&error];
    if (fetchedObjects == nil) {
        // Handle the error
        NSLog(@"Error: No fetched objects.");
        return NO;
    }
    
    // Take first product name
    FLCatRanking *flCatRanking = [fetchedObjects objectAtIndex:0];
    NSLog(@"First fetched item: %@", flCatRanking.prodID);
    return YES;
}

+ (BOOL)basicFetch {

// Test Fetch of 1 item

useDelegate

NSManagedObjectContext *context = appDelegate.managedObjectContext;

if (context == nil) {

NSLog(@"Error: No context");

return NO;

}

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

NSEntityDescription *entity = [NSEntityDescription

entityForName:@"FLCatRanking" inManagedObjectContext:context];

[fetchRequest setEntity:entity];

NSError *error = nil;

NSArray *fetchedObjects = [context

executeFetchRequest:fetchRequest error:&error];

if (fetchedObjects == nil) {

// Handle the error

NSLog(@"Error: No fetched objects.");

return NO;

}

// Take first product name

FLCatRanking *flCatRanking = [fetchedObjects objectAtIndex:0];

NSLog(@"First fetched item: %@", flCatRanking.prodID);

return YES;

}

Запрос с фильтрацией – аналог WHERE в SQL :

+ (NSMutableDictionary*) retrieveProductsForCategory:
(NSString*) catName {
    
    NSMutableDictionary *dic = [[NSMutableDictionary alloc] init];
    
    useDelegate
    NSManagedObjectContext *context = appDelegate.managedObjectContext;
    if (context == nil) {
        NSLog(@"Error: No context");
        return nil;
    }
    
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSPredicate *predicate = [NSPredicate 
predicateWithFormat:@"prodCatName == %@", catName];
    NSEntityDescription *entity = [NSEntityDescription 
entityForName:@"FLCatRanking" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];
    [fetchRequest setPredicate:predicate];
    
    NSError *error = nil;
    NSArray *fetchedObjects = [context 
executeFetchRequest:fetchRequest error:&error];
    if (fetchedObjects == nil) {
        // Handle the error
        NSLog(@"Error: No fetched objects.");
        return nil;
    }
    
    // Take first product name
    for (FLCatRanking *flCatRanking in fetchedObjects) {
        NSLog(@"FLCatRanking item: %@ - %@", flCatRanking.prodID, 
flCatRanking.prodPos);
        [dic addEntriesFromDictionary:[NSDictionary 
dictionaryWithObject:flCatRanking.prodID 
forKey:flCatRanking.prodPos]];
    }
    
    return dic; 
}

+ (NSMutableDictionary*) retrieveProductsForCategory:

(NSString*) catName {

NSMutableDictionary *dic = [[NSMutableDictionary alloc] init];

useDelegate

NSManagedObjectContext *context = appDelegate.managedObjectContext;

if (context == nil) {

NSLog(@"Error: No context");

return nil;

}

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

NSPredicate *predicate = [NSPredicate

predicateWithFormat:@"prodCatName == %@", catName];

NSEntityDescription *entity = [NSEntityDescription

entityForName:@"FLCatRanking" inManagedObjectContext:context];

[fetchRequest setEntity:entity];

[fetchRequest setPredicate:predicate];

NSError *error = nil;

NSArray *fetchedObjects = [context

executeFetchRequest:fetchRequest error:&error];

if (fetchedObjects == nil) {

// Handle the error

NSLog(@"Error: No fetched objects.");

return nil;

}

// Take first product name

for (FLCatRanking *flCatRanking in fetchedObjects) {

NSLog(@"FLCatRanking item: %@ - %@", flCatRanking.prodID,

flCatRanking.prodPos);

[dic addEntriesFromDictionary:[NSDictionary

dictionaryWithObject:flCatRanking.prodID

forKey:flCatRanking.prodPos]];

}

return dic;

}

Alwawee

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