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ストレージに Core Data を使用している iOS アプリケーションに取り組んでいます。私のストアでは、すべてのエンティティが一意であり、既存のエンティティを渡す別のエンティティに置き換える関数を作成しています。渡すエンティティの例を次に示します。

    NSManagedObjectContext *context = [[MyDB sharedInstance] managedObjectContext];

    User *user = [NSEntityDescription insertNewObjectForEntityForName:@"User" inManagedObjectContext:context];

    NSNumber *userNumber = 12345;

    user.id = userNumber;
    user.name = @"John Doe";
    user.email = @"john@doe.net";
    user.createdDate = [NSDate date];

    [[MyDB sharedInstance] updateUser:user];

コア データ ストレージ内には、メール アドレスが「john@doe.com」であることを除いて、同じエンティティが既にあります。現時点での私の更新は次のようになります。

-(void)updateUser:(User *)user {

    NSError *error;
    NSManagedObjectContext *context = [[MyDB sharedInstance] managedObjectContext];

    // Create fetch request
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"User" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];

    // Create predicate
    NSPredicate *pred = [NSPredicate predicateWithFormat:@"id == %@", user.id];
    [fetchRequest setPredicate:pred];

    NSArray *results = [context executeFetchRequest:fetchRequest error:&error];

    if (error) {
        // handle fetch error
    } else {

        user = [[User alloc] initWithEntity:entity insertIntoManagedObjectContext:context];

        for (User *recordToDelete in results) {

            [context deleteObject:recordToDelete];//record gets deleted here, which is fine

        }

        [context save:&error]; //this doesn't save the new entity that I passed in

        if (error) {
            // handle save error
            NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
        }

    }


    [fetchRequest setEntity:entity];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"id=%@", 12345];
    [fetchRequest setPredicate:pred];

    NSArray *items = [context executeFetchRequest:fetchRequest error:&error];

    for (User *testObject in items) {

        NSLog(@"ID: %@, Name: %@, Email: %@, Created Date: %@", [testObject id], [testObject name], [testObject email], [testObject createdDate]);

    }

}

問題は、上記の関数がストア内の既存のレコードを削除することですが、それを置き換える新しいエンティティの追加に失敗することです。どうすればこれを修正できますか?

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2 に答える 2

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Also, i think you don't clearly understand what is NSManagedObjectContext. It's something like in-memory object cache. So, if you create object in context, it is tied to context. Object has reference to context, so passing context with object is not necessary - object's context can be obtained from it. Also, contexts and objects are not thread-safe - you cannot pass managed objects between threads and use same context in different threads. Instead, you have to:
1) Create context for every thread
2) If you need to pass something between threads, pass object.objectId from one thread, and in another thread do [context objectWithID:]. It's extremly fast, efficient and safe.

于 2013-09-26T21:09:34.330 に答える