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
