Core Data学习

1.在创建Project时如果选择上Core Data的话，在AppDelegate中会出现几个方法用于设置Core Data的栈

一个创建管理对象上下文，一个创建管理对象模型，还有一个创建持久性存储协调器

#pragma mark - Core Data stack@synthesize managedObjectContext = _managedObjectContext;@synthesize managedObjectModel = _managedObjectModel;@synthesize persistentStoreCoordinator = _persistentStoreCoordinator;- (NSURL *)applicationDocumentsDirectory {    // The directory the application uses to store the Core Data store file. This code uses a directory named "cc.linkmob.FailedBankCD" in the application's documents directory.    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];}- (NSManagedObjectModel *)managedObjectModel {    // The managed object model for the application. It is a fatal error for the application not to be able to find and load its model.    if (_managedObjectModel != nil) {        return _managedObjectModel;    }    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"FailedBankCD" withExtension:@"momd"];    _managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];    return _managedObjectModel;}- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {    // The persistent store coordinator for the application. This implementation creates and return a coordinator, having added the store for the application to it.    if (_persistentStoreCoordinator != nil) {        return _persistentStoreCoordinator;    }        // Create the coordinator and store        _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"FailedBankCD.sqlite"];    NSError *error = nil;    NSString *failureReason = @"There was an error creating or loading the application's saved data.";    if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {        // Report any error we got.        NSMutableDictionary *dict = [NSMutableDictionary dictionary];        dict[NSLocalizedDescriptionKey] = @"Failed to initialize the application's saved data";        dict[NSLocalizedFailureReasonErrorKey] = failureReason;        dict[NSUnderlyingErrorKey] = error;        error = [NSError errorWithDomain:@"YOUR_ERROR_DOMAIN" code:9999 userInfo:dict];        // Replace this with code to handle the error appropriately.        // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);        abort();    }        return _persistentStoreCoordinator;}- (NSManagedObjectContext *)managedObjectContext {    // Returns the managed object context for the application (which is already bound to the persistent store coordinator for the application.)    if (_managedObjectContext != nil) {        return _managedObjectContext;    }        NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];    if (!coordinator) {        return nil;    }    _managedObjectContext = [[NSManagedObjectContext alloc] init];    [_managedObjectContext setPersistentStoreCoordinator:coordinator];    return _managedObjectContext;}#pragma mark - Core Data Saving support- (void)saveContext {    NSManagedObjectContext *managedObjectContext = self.managedObjectContext;    if (managedObjectContext != nil) {        NSError *error = nil;        if ([managedObjectContext hasChanges] && ![managedObjectContext save:&error]) {            // Replace this implementation with code to handle the error appropriately.            // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);            abort();        }    }}

管理对象模型（Managed Object Model）：可以认为是数据库的架构。管理对象模型包含了每个保存在数据库中对象（实体）的定义信息。一般情况下，可以使用Xcode的编辑器来设置数据库中的对象，比如说对象的属性，数据之间的关系。也可以用代码进行设置。

持久性存储协调器（Persistent Store Coordinator）：可以认为是数据库的连接。在这里，将要设置数据存储的名称和位置，以及数据存储的时间。

管理对象上下文（Managed Object Context）：可以认为是数据库中对象的实际内容。这是三个类中最重要的。在获取，插入或者删除对象时都要调用到管理对象上下文。

2.两个实体中，如果创建了实体A有一个对应关系到实体B，则也应该创建一个实体B到实体A的关系。

3.将Delete Rule设置为Cascade(级联)的话，意味着在Core Data删除一个对象的话，与之相关的对象也会被删除。

4.Fetch Request ：可以认为是SQL中的SELECT。

5.因为NSManagedObject不是一个强类型的类，所以在编码过程中比较容易出错，所以最好的方法是位每个Entity创建一个Model：

注意选择的是NSManagedObject subclass

完成以后要注意关系对象的类名，有可能为NSManagedObject，要对其更正

6.在第二篇教程中http://www.raywenderlich.com/12170/core-data-tutorial-how-to-preloadimport-existing-data-updated

由于Xcode的更新，所以在创建新项目中OS X命令行没有了使用Core Data的选项，所以只能手动添加以下方法

static NSManagedObjectModel *managedObjectModel() {        static NSManagedObjectModel *model = nil;    if (model != nil) {        return model;    }        NSString *path = @"FailedBankCD";    NSURL *modelURL = [NSURL fileURLWithPath:[path stringByAppendingPathExtension:@"momd"]];    model = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];        return model;}static NSManagedObjectContext *managedObjectContext() {        static NSManagedObjectContext *context = nil;    if (context != nil) {        return context;    }        @autoreleasepool {        context = [[NSManagedObjectContext alloc] init];                NSPersistentStoreCoordinator *coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:managedObjectModel()];        [context setPersistentStoreCoordinator:coordinator];                NSString *STORE_TYPE = NSSQLiteStoreType;                NSString *path = [[[NSProcessInfo processInfo] arguments] objectAtIndex:0];        path = [path stringByDeletingPathExtension];        NSURL *url = [NSURL fileURLWithPath:[path stringByAppendingPathExtension:@"sqlite"]];                NSError *error;        NSPersistentStore *newStore = [coordinator addPersistentStoreWithType:STORE_TYPE                                                                configuration:nil                                                                          URL:url                                                                      options:nil                                                                        error:&error];        if (newStore == nil) {            NSLog(@"Store Configuration Failure %@", ([error localizedDescription] != nil) ? [error  localizedDescription] : @"Unknown Error");        }    }    return context;}

记得导入CoreData.framework，并在main.m文件中#import <CoreData/CoreData.h>

7.教程三（http://www.raywenderlich.com/999/core-data-tutorial-for-ios-how-to-use-nsfetchedresultscontroller）

1)使用NSFetchedResultsController的时候，需要在提取请求中设置一个排序描述（NSSortDescriptor）。排序描述不仅可以排序所返回的对象的特性，还能排序相关对象的特性。

2）需要在提取请求中将批量大小设置成较小的。

3）在完成匹配后，需要创建一个NSFetchedRequestController