iPhone开发教程之Core Data 常见问题的总结

一、如果想把模拟其中保存的数据在真机中显示出来，可以在 AppDelegate 里用下面方法：

　　模拟器保存完数据，然后进入目录：“用户名/Library/Application Support/iPhone Simulator/4.2/Applications/" 目录下，找到 p.sqlite，复制到软件的工程目录里。然后导入到工程里面，这样可以用模拟器里面保存的数据了。

　　- (NSManagedObjectModel *)managedObjectModel {

　　if (managedObjectModel != nil) {

　　return managedObjectModel;

　　}

　　managedObjectModel = [[NSManagedObjectModel mergedModelFromBundles:nil] retain];

　　return managedObjectModel;

　　}

　　/**

　　Returns the persistent store coordinator for the application.

　　If the coordinator doesn't already exist, it is created and the application's store added to it.

　　*/

　　- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {

　　if (persistentStoreCoordinator != nil) {

　　return persistentStoreCoordinator;

　　}

　　NSString *storePath = [[self applicationDocumentsDirectory] stringByAppendingPathComponent: @"p.sqlite"];

　　/*

　　Set up the store.

　　For the sake of illustration, provide a pre-populated default store.

　　*/

　　NSFileManager *fileManager = [NSFileManager defaultManager];

　　// If the expected store doesn't exist, copy the default store.

　　if (![fileManager fileExistsAtPath:storePath]) {

　　NSString *defaultStorePath = [[NSBundle mainBundle] pathForResource:@"p" ofType:@"sqlite"];

　　if (defaultStorePath) {

　　[fileManager copyItemAtPath:defaultStorePath toPath:storePath error:NULL];

　　}

　　}

　　NSURL *storeUrl = [NSURL fileURLWithPath:storePath];

　　NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption, [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];

　　persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel: [self managedObjectModel]];

　　NSError *error;

　　if (![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeUrl options:options error:&error]) {

　　// Update to handle the error appropriately.

　　NSLog(@"Unresolved error %@, %@", error, [error userInfo]);

　　exit(-1); // Fail

　　}

　　return persistentStoreCoordinator;

　　}

　　//Returns the path to the application's documents directory.

　　- (NSString *)applicationDocumentsDirectory {

　　NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);

　　NSString *basePath = ([paths count] > 0) ? [paths objectAtIndex:0] : nil;

　　return basePath;

　　}

　　二、常见错误 entityForName: could not locate an NSManagedObjectModel for entity name "Entity" 的解决方法：

　　如果是如果是 AppDelegate.m 里面出现错误，需要这么写

　　FavoriteViewController *favoriteVC = [[FavoriteViewController alloc] initWithNibName:nil bundle:nil];

　　favoriteVC.managedObjectContext = self.managedObjectContext;

　　如果是两个界面跳转，主要是出现在 iOS 4.0 以下版本，正确写法是：

　　Test.h

　　NSManagedObjectContext *managedObjectContext;

　　NextTest *next;

　　Test.m

　　next.managedObjectContext = self.managedObjectContext;

　　[self.navigationController pushViewController: next animated:YES];

　　错误写法:

　　NextTest *next = [[NextTest alloc] init];

　　next.managedObjectContext = self.managedObjectContext;

　　[self.navigationController pushViewController: next animated:YES];

　　[next release];

　　三、条件过滤：

　　选择 cid=1 的数据

　　- (NSFetchedResultsController *)fetchedResultsController {

　　NSPredicate *predicate = [NSPredicate predicateWithFormat:@"cid=1"];

　　[fetchRequest setPredicate:predicate];

　　}

　　四、通用代码，在 viewcontroller 里面会用到的。

　　TestViewController.h

　　@interface TestViewController : UITableViewController {

　　NSFetchedResultsController *fetchedResultsController;

　　NSManagedObjectContext *managedObjectContext;

　　}

　　@property (nonatomic, retain) NSFetchedResultsController *fetchedResultsController;

　　@property (nonatomic, retain) NSManagedObjectContext *managedObjectContext;

　　- (void)configureCell:(UITableViewCell *)cell withCategory:(Category *)category;

　　@end

　　TestViewController.m

　　#pragma mark -

　　#pragma mark fetchedResultsController

　　- (NSFetchedResultsController *)fetchedResultsController {

　　if (fetchedResultsController != nil) {

　　return fetchedResultsController;

　　}

　　NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

　　NSEntityDescription *entity = [NSEntityDescription entityForName:@"Category" inManagedObjectContext:managedObjectContext];

　　[fetchRequest setEntity:entity];

　　NSSortDescriptor *nameDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];

　　NSArray *arraySortDescriptor = [[NSArray alloc] initWithObjects:nameDescriptor, nil];

　　[fetchRequest setSortDescriptors:arraySortDescriptor];

　　NSPredicate *predicate = [NSPredicate predicateWithFormat:cidString];

　　[fetchRequest setPredicate:predicate];

　　NSFetchedResultsController *frController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest

　　managedObjectContext:managedObjectContext

　　sectionNameKeyPath:@"name"

　　cacheName:@"Root"];

　　frController.delegate = self;

　　self.fetchedResultsController = frController;

　　[frController release];

　　[fetchRequest release];

　　[nameDescriptor release];

　　return fetchedResultsController;

　　}

　　- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {

　　// The fetch controller is about to start sending change notifications, so prepare the table view for updates.

　　[self.tableView beginUpdates];

　　}

　　- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {

　　UITableView *tableView = self.tableView;

　　switch (type) {

　　case NSFetchedResultsChangeInsert:

　　[tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];

　　break;

　　case NSFetchedResultsChangeDelete:

　　[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];

　　break;

　　case NSFetchedResultsChangeMove:

　　[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];

　　[tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];

　　break;

　　case NSFetchedResultsChangeUpdate:

　　[self configureCell:[tableView cellForRowAtIndexPath:indexPath] withCategory:anObject];

　　break;

　　default:

　　break;

　　}

　　}

　　- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id )sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {

　　switch(type) {

　　case NSFetchedResultsChangeInsert:

　　[self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];

　　break;

　　case NSFetchedResultsChangeDelete:

　　[self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];

　　break;

　　}

　　}

　　- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {

　　// The fetch controller has sent all current change notifications, so tell the table view to process all updates.

　　[self.tableView endUpdates];

　　}