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Core Data淺談係列之二 ： 簡單的CURD
在上一篇中簡單介紹了Core Data Stack，自上而下地對Core Data應用結構有個基本的認識，不過都是理論上的。這裏就以上一篇的理論構建起一個可運行的Demo，執行一些簡單的增刪改查操作。
（圖片來自Apple）
首先，我們需要建立如上圖的棧結構。因此，在ViewController裏添加3個屬性：
@interface ViewController : UIViewController @property (nonatomic, retain) NSManagedObjectModel *managedObjectModel; @property (nonatomic, retain) NSManagedObjectContext *managedObjectContext; @property (nonatomic, retain) NSPersistentStoreCoordinator *persistentStoreCoordinator; @end
根據前文討論的順序，我們首先創建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;
}
接著創建NSManagedObject的生存環境 —— NSManagedObjectContext：
- (NSManagedObjectContext *)managedObjectContext
{
if (nil != _managedObjectContext) {
return _managedObjectContext;
}
_managedObjectContext = [[NSManagedObjectContext alloc] init];
return _managedObjectContext;
}由於Demo會將數據存儲到本地文件，所以還需要NSPersistentStoreCoordinator和NSManagedObjectContext配合。
下麵是創建Core Data棧結構的完整代碼：
#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;
}
在搭建完基礎的Core Data棧結構後，就可以寫一些簡單的CURD代碼了：
#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;
}
我們可以把讀寫的代碼放到viewDidLoad函數中：
- (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);
}
}
}
由於在context中創建的對象隻是存在於內存中，所以我們還需要顯式地把數據保存到sqlite文件裏：
#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();
}
}最後，就是跑一下Demo，檢驗一下結果。
執行過一次程序後，我們可以搜索下cdNBA.sqlite文件，然後查看裏麵的數據：
同時，控製台也輸出了數據：
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
最後更新：2017-04-04 07:03:38