Core Data

•   Core Data   是 iOS SDK   里的一个很强大的框架,允许程序员 以面向对象 的方式储存和管理数据 。使用 Core Data 框架,程序员可以很轻松有效 地通过面向对象的接口管理数据  
•   Core Data 框架提供了 对象 - 关系映射 (ORM) 的功能,即能够将 OC 对象转 化成数据,保存在 SQLite3 数据库文件中,也能够将保存在数据库中的 数据还原成 OC 对象  
•   在数据操作过程中,无需编写任何 SQL 语句  
•   要使用 Core Data , 需要导入 CoreData 框架

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Core Data 主要对象
•   NSManagedObjectContext :负责应用和数据库之间的交互 (CRUD)
•   NSPersistentStoreCoordinator :添加持久化存储库(如 SQLite 数据 库), 是物理数据存储的物理文件和程序之间的联系的桥梁 ,负责管 理 不同对象上下文
•   NSManagedObjectModel :被管理的对象模型 

三者关系 参考：

#pragma mark - Core Data stack

@synthesize managedObjectContext = _managedObjectContext;
@synthesize managedObjectModel = _managedObjectModel;
@synthesize persistentStoreCoordinator = _persistentStoreCoordinator;

- (NSURL *)applicationDocumentsDirectory {
    // The directory the application uses to store the Core Data store file. This code uses a directory named "nyhx.Core_Data" in the application's documents directory.
    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
}

- (NSManagedObjectModel *)managedObjectModel {
    // The managed object model for the application. It is a fatal error for the application not to be able to find and load its model.
    if (_managedObjectModel != nil) {
        return _managedObjectModel;
    }
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"Core_Data" withExtension:@"momd"];
    _managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
    return _managedObjectModel;
}

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
    // The persistent store coordinator for the application. This implementation creates and returns a coordinator, having added the store for the application to it.
    if (_persistentStoreCoordinator != nil) {
        return _persistentStoreCoordinator;
    }
    
    // Create the coordinator and store
    
    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"Core_Data.sqlite"];
    NSError *error = nil;
    NSString *failureReason = @"There was an error creating or loading the application's saved data.";
    if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {
        // Report any error we got.
        NSMutableDictionary *dict = [NSMutableDictionary dictionary];
        dict[NSLocalizedDescriptionKey] = @"Failed to initialize the application's saved data";
        dict[NSLocalizedFailureReasonErrorKey] = failureReason;
        dict[NSUnderlyingErrorKey] = error;
        error = [NSError errorWithDomain:@"YOUR_ERROR_DOMAIN" code:9999 userInfo:dict];
        // Replace this 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.
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }
    
    return _persistentStoreCoordinator;
}


- (NSManagedObjectContext *)managedObjectContext {
    // Returns the managed object context for the application (which is already bound to the persistent store coordinator for the application.)
    if (_managedObjectContext != nil) {
        return _managedObjectContext;
    }
    
    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (!coordinator) {
        return nil;
    }
    _managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
    [_managedObjectContext setPersistentStoreCoordinator:coordinator];
    return _managedObjectContext;
}

#pragma mark - Core Data Saving support

- (void)saveContext {
    NSManagedObjectContext *managedObjectContext = self.managedObjectContext;
    if (managedObjectContext != nil) {
        NSError *error = nil;
        if ([managedObjectContext hasChanges] && ![managedObjectContext save:&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.
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
            abort();
        }
    }
}

•   NSEntityDescription :实体描述

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新增记录（NSEntityDescription）

//_context--->NSManagedObjectContext
Person   *p = [ NSEntityDescription insertNewObjectForEntityForName : @"Person" inManagedObjectContext : _context ];
p. name   =   @" 张三 " ; p. age   =   @18 ;
[ _context   save : nil ];  

查询数据（NSFetchRequest）
NSFetchRequest   *request = [ NSFetchRequest fetchRequestWithEntityName : @"Person" ];
request. predicate   = [ NSPredicate predicateWithFormat : @"%K LIKE '*52*' OR name CONTAINS ' 三 '" ,   @"phoneNo" ];
NSArray   *array = [ _context executeFetchRequest :request   error : nil ];

删除数据

NSArray * mutablFetchResult = [_context executeFetchRequest:request error:&error];

for (User*user in mutableFetchResult) {

     [_context deleteObject:user];

}

//判断是否删除成功

BOOL isDeleteSuccess = [_context save:&error];//保存（容易忘）

修改

NSArray * mutablFetchResult = [_context executeFetchRequest:request error:&error];

for (User*user in mutablFetchResult) {

    [user setAge:[NSNumber numberWithInteger:999]];

}

//判断是否修改成功

BOOL isSaveSuccess = [_context save:&error];//保存（容易忘）

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创建好实体后，可以通过添加NSManagedObject subclass文件，系统可以自动添加实体对应的数据模型类，如图所示：