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 <NSFetchedResultsControllerDelegate> {
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 <NSFetchedResultsSectionInfo>)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];
}
