Core Data浅谈系列之二 ： 简单的CURD

在 上一篇中简单介绍了Core Data Stack，自上而下地对Core Data应用结构有个基本的认识，不过都是理论上的。这里就以上一篇的理论构建起一个可运行的Demo，执行一些简单的增删改查操作。

（图片来自Apple）

首先，我们需要建立如上图的栈结构。因此，在ViewController里添加3个属性：

@interface ViewController : UIViewController

@property (nonatomic, retain) NSManagedObjectModel *managedObjectModel;
@property (nonatomic, retain) NSManagedObjectContext *managedObjectContext;
@property (nonatomic, retain) NSPersistentStoreCoordinator *persistentStoreCoordinator;

@end

根据前文讨论的顺序，我们首先创建managedObjectModel（在那之前需要引入CoreData Framework）：

- (NSManagedObjectModel *)managedObjectModel
{
    if (nil != _managedObjectModel) {
        return _managedObjectModel;
    }
    
    _managedObjectModel = [[NSManagedObjectModel mergedModelFromBundles:nil] retain];
    return _managedObjectModel;
}

接着创建NSManagedObject的生存环境 —— NSManagedObjectContext：

- (NSManagedObjectContext *)managedObjectContext
{
    if (nil != _managedObjectContext) {
        return _managedObjectContext;
    }
    
    _managedObjectContext = [[NSManagedObjectContext alloc] init];
    return _managedObjectContext;
}

由于Demo会将数据存储到本地文件，所以还需要NSPersistentStoreCoordinator和NSManagedObjectContext配合。

下面是 创建Core Data栈结构的完整代码：

#pragma mark - 
#pragma mark - Core Data Stack

- (NSManagedObjectModel *)managedObjectModel
{
    if (nil != _managedObjectModel) {
        return _managedObjectModel;
    }
    
    _managedObjectModel = [[NSManagedObjectModel mergedModelFromBundles:nil] retain];
    return _managedObjectModel;
}

- (NSManagedObjectContext *)managedObjectContext
{
    if (nil != _managedObjectContext) {
        return _managedObjectContext;
    }
    
    _managedObjectContext = [[NSManagedObjectContext alloc] init];
    
    if (self.persistentStoreCoordinator) {
        [_managedObjectContext setPersistentStoreCoordinator:self.persistentStoreCoordinator];
    }
    
    return _managedObjectContext;
}

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator
{
    if (nil != _persistentStoreCoordinator) {
        return _persistentStoreCoordinator;
    }
    
    NSString *storeType = NSSQLiteStoreType;
    NSString *storeName = @"cdNBA.sqlite";
    
    NSError *error = NULL;
    NSURL *storeURL = [NSURL fileURLWithPath:[[self applicationDocumentsDirectory] stringByAppendingPathComponent:storeName]];
    
    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:self.managedObjectModel];
    if (![_persistentStoreCoordinator addPersistentStoreWithType:storeType configuration:nilURL:storeURL options:nil error:&error]) {
        NSLog(@"Error : %@\n", [error localizedDescription]);
        NSAssert1(YES, @"Failed to create store %@ with NSSQLiteStoreType", [storeURL path]);
    }
    
    return _persistentStoreCoordinator;
}

#pragma mark -
#pragma mark Application's Documents Directory

- (NSString *)applicationDocumentsDirectory
{
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *basePath = ([paths count] > 0) ? [paths objectAtIndex:0] : nil;
    return basePath;
}

在搭建完基础的Core Data栈结构后，就可以写一些简单的CURD代码了：

#pragma mark - 
#pragma mark - CURD Operations

- (BOOL)createTeamWithName:(NSString *)teamName city:(NSString *)teamCity
{
    if (!teamName || !teamCity) {
        return NO;
    }
    
    NSManagedObject *teamObject = [NSEntityDescription insertNewObjectForEntityForName:@"Team" inManagedObjectContext:self.managedObjectContext];
    [teamObject setValue:teamName forKey:@"name"];
    [teamObject setValue:teamCity forKey:@"city"];
    
    return YES;
}

- (NSArray *)fetchTeamList
{
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    [fetchRequest setEntity:[NSEntityDescription entityForName:@"Team" inManagedObjectContext:self.managedObjectContext]];
    
    NSError *error = NULL;
    NSArray *array = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
    if (error) {
        NSLog(@"Error : %@\n", [error localizedDescription]);
    }
    
    [fetchRequest release], fetchRequest = nil;
    
    return array;
}

我们可以把读写的代码放到viewDidLoad函数中：

- (void)viewDidLoad
{
    [superviewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
    
    [self createTeamWithName:@"Heat"city:@"Miami"];
    [self createTeamWithName:@"Lakers"city:@"LA"];
    [self saveContext];
    
    NSArray *teamArray = [self fetchTeamList];
    if (teamArray) {
        for (NSManagedObject *teamObject in teamArray) {
            NSString *teamName = [teamObject valueForKey:@"name"];
            NSString *teamCity = [teamObject valueForKey:@"city"];
            NSLog(@"Team info : %@, %@\n", teamName, teamCity);
        }
    }
}

由于在context中创建的对象只是存在于内存中，所以我们还需要显式地把数据保存到sqlite文件里：

#pragma mark -
#pragma mark - Save Context

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

最后，就是跑一下Demo，检验一下结果。

执行过一次程序后，我们可以搜索下cdNBA.sqlite文件，然后查看里面的数据： 

同时，控制台也输出了数据：

2013-01-16 14:32:52.638 cdNBA[4717:c07] Team info : Heat, Miami
2013-01-16 14:32:52.639 cdNBA[4717:c07] Team info : Lakers, LA

Brief Talk About Core Data Series, Part 2 : Simple CURD 

Jason Lee @ Hangzhou

Blog : http://blog.youkuaiyun.com/jasonblog

Weibo : http://weibo.com/jasonmblog