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, Miami2013-01-16 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.csdn.net/jasonblog

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