iOS学习爬坑记录12：学习Core data简单的CURD

这是参考了前人工作“Core Data浅谈系列之二 ： 简单的CURD”：http://blog.csdn.net/jasonblog/article/details/8526148

CURD指creat、update、retrieve和delete。把他的代码在Xcode 6.1下重现了一遍，中间遇到点错误，耽搁了两天，主要是这个项目在建工程的时候不添加core data支持（否则core data相关代码会出现在AppDelegate文件中），而且建立工程后不添加xcdatamodeld文件，都怪我理解不深。

相关代码如下：

ViewController.h文件

#import <UIKit/UIKit.h>#import <CoreData/CoreData.h>@interface ViewController : UIViewController@property (nonatomic, retain) NSManagedObjectModel *managedObjectModel;@property (nonatomic, retain) NSManagedObjectContext *managedObjectContext;@property (nonatomic, retain) NSPersistentStoreCoordinator *persistentStoreCoordinator;@end

ViewController.m文件

#import "ViewController.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidLoad{    [super viewDidLoad];    // 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);        }    }}- (void)didReceiveMemoryWarning {    [super didReceiveMemoryWarning];    // Dispose of any resources that can be recreated.}#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 = nil;        return array;}#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();    }}#pragma mark -#pragma mark Core Data Stack- (NSManagedObjectModel *)managedObjectModel{    if (nil != _managedObjectModel) {        return _managedObjectModel;    }        _managedObjectModel = [NSManagedObjectModel mergedModelFromBundles:nil];    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:nil URL: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;}@end

中间过程爬坑的时候，犯了两个错误，一次是添加了core data支持又添加了xcdatamodeld文件（这里写的core data相关代码和系统自动生成的core data代码还是有差别的），还有一次是添加了xcdatamodeld文件，只要你添加了一次，后面怎么调试都会报错“"NSInternalInconsistencyException', reason: 'This NSPersistentStoreCoordinator has no persistent stores. It cannot perform a save operation.'"”，最后我参考了http://www.cnblogs.com/mybkn/archive/2012/09/03/2669412.html，把我手机上（真机调试）的调试应用删除，再编译运行就好了，记住，这里不能添加xcdatamodeld文件。

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这里更正一下，创建工程时添加core data支持也是可以的，而且可以减少代码量，只是需要稍作修改。而且，以后调试代码要注意，如果有本地文件，在真机上运行出错的时候会不妨把app删掉重新运行Xcode。在有core data支持后，ViewController.m代码全部如下：

#import "ViewController.h"#import "AppDelegate.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidLoad {    [super viewDidLoad];    // Do any additional setup after loading the view, typically from a nib.    AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;        [self createTeamWithName:@"Heat"city:@"Miami"];    [self createTeamWithName:@"Lakers"city:@"LA"];    [appDelegate 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);        }    }}- (void)didReceiveMemoryWarning {    [super didReceiveMemoryWarning];    // Dispose of any resources that can be recreated.}#pragma mark -#pragma mark CURD Operations- (BOOL)createTeamWithName:(NSString *)teamName city:(NSString *)teamCity{    //managedObjectContext 定义在AppDelegate.h文件    AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;    NSManagedObjectContext *context = [appDelegate managedObjectContext];        if (!teamName || !teamCity) {        return NO;    }        NSManagedObject *teamObject = [NSEntityDescription insertNewObjectForEntityForName:@"Team" inManagedObjectContext:context];    [teamObject setValue:teamName forKey:@"name"];    [teamObject setValue:teamCity forKey:@"city"];        return YES;}- (NSArray *)fetchTeamList{    //managedObjectContext 定义在AppDelegate.h文件    AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;    NSManagedObjectContext *context = [appDelegate managedObjectContext];        NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];    [fetchRequest setEntity:[NSEntityDescription entityForName:@"Team" inManagedObjectContext:context]];        NSError *error = NULL;    NSArray *array = [context executeFetchRequest:fetchRequest error:&error];    if (error) {        NSLog(@"Error : %@\n", [error localizedDescription]);    }        //[fetchRequest release], fetchRequest = nil;        return array;}@end