我在iOS中创建了一个Single View应用程序,该应用程序还包含Core Data。我将.xcdatamodel文件从另一个应用程序移到了我正在处理的文件中,但出现了问题。我所做的是剪切并粘贴来自先前应用程序的代码,并将其放置在我的AppDelegate.h / m文件中:
@interface DBAppDelegate : UIResponder <UIApplicationDelegate>
@property (strong, nonatomic) UIWindow *window;
@property (readonly, strong, nonatomic) NSManagedObjectContext *managedObjectContext;
@property (readonly, strong, nonatomic) NSManagedObjectModel *managedObjectModel;
@property (readonly, strong, nonatomic) NSPersistentStoreCoordinator *persistentStoreCoordinator;
- (void)saveContext;
- (NSURL *)applicationDocumentsDirectory;
@end
和我的.m文件:
@implementation DBAppDelegate
@synthesize managedObjectContext = _managedObjectContext;
@synthesize managedObjectModel = _managedObjectModel;
@synthesize persistentStoreCoordinator = _persistentStoreCoordinator;
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
// Override point for customization after application launch.
UINavigationController *navigationController = (UINavigationController *)self.window.rootViewController;
//the line below is what is causing an error
DBViewController *controller = (DBViewController *)navigationController.topViewController;
controller.managedObjectContext = self.managedObjectContext;
return YES;
}
在我的.m文件中,我还包含Core Data的样板代码,该代码也在我以前的应用程序中未发布。在我的新应用程序中,我正在做的是创建一个访问层,该访问层还提供了一个Singleton实例来访问该层。我在该类中进行CRUD操作,并在.h文件中声明了以下属性:
@property (strong, nonatomic) NSManagedObjectContext *managedObjectContext;
@property (strong, nonatomic) NSManagedObjectModel *managedObjectModel;
@property (strong, nonatomic) NSPersistentStoreCoordinator *persistentStoreCoordinator;
@property (nonatomic, strong) NSFetchedResultsController *fetchedResultsController;
此刻,我收到以下错误:
“在类型” DBViewController“的对象上找不到属性'managedObjectContext'。我想做的是在我的方法中初始化ManagedObjectContext,该方法允许创建Singleton实例:
static DB *sharedSingleton = nil;
+ (DB *) sharedInstance {
if (sharedSingleton == nil) {
sharedSingleton = [[super alloc] init];
}
return sharedSingleton;
}
我在做什么错?我意识到我没有在DBViewController中声明一个ManagedObjectContext对象,但是该替换该行什么呢?我认为这与我的Singleton班有关,但老实说,我这里没有任何线索。
您试图做的事情通常被称为“数据存储”单例,这是一个很好的设计模式。我在应用程序中所做的是有一个名为DataStore
的单例类,您可以使用类方法随意调用它:
+ (id)sharedStore{
static DataStore *sharedStore = nil;
if (!sharedStore) {
sharedStore = [[self alloc] init];
}
return sharedStore;
}
[每当需要访问数据存储提供的资源时,我就会:
Datastore *ds = [Datastore sharedStore];
为了提供对Core Data的访问,我有一个数据存储方法:
+ (NSManagedObjectContext*)managedObjectContext{
static NSManagedObjectContext *context = nil;
if(context){
return context;
}
NSPersistentStoreCoordinator *coordinator = nil;
NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"Model" withExtension:@"momd"];
NSManagedObjectModel *objectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
if (!coordinator) {
coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:objectModel];
}
if(!coordinator){
return nil;
}
NSString *storePath = [[self documentsDirectoryPath] stringByAppendingPathComponent:@"datastore.sqlite"];
NSURL *storeURL = [NSURL URLWithString:storePath];
NSError *error;
if (![coordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:NULL error:&error])
{
NSLog(@"Database error: %@", error);
// if you make changes to your model and a database already exists in the app
// you'll get a NSInternalInconsistencyException exception. When the model is updated
// the databasefile must be removed. Remove the database here because it's easy.
NSFileManager *fileManager = [NSFileManager defaultManager];
[fileManager removeItemAtURL:storeURL error:nil];
//try to add the persistant store one more time. If it still fails then abort
if (![coordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:NULL error:&error])
return nil;
}
context = [[NSManagedObjectContext alloc] init];
[context setPersistentStoreCoordinator:coordinator];
[context setUndoManager:nil];
return context;
}
如果无法创建nil
,则此方法返回NSManagedObjectContext
。这样,您只需要执行以下操作:
NSManagedObjectContext *context = [[DataStore sharedStore] managedObjectContext];
无论何时需要使用核心数据。可以在viewDidLoad
中完成一次。
编辑:
managedObjectContext
方法使用以下方法找到文档目录:
+ (NSString *)documentsDirectoryPath
{
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *basePath = ([paths count] > 0) ? [paths objectAtIndex:0] : nil;
return basePath;
}