coredata的使用

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NSInvalidArgumentException', reason: '+entityForName: nil is not a legal NSManagedObjectContext parameter searching for entity name 'StudentEntity'

错误原因，向entityForName：方法发送了一个空消息，本来以为是StudentEntity这个名字有误，实际上没错。最后发现了一个低级错误，

NSManagedObjectContext对象是nil。原来是managedObjectContext的get方法，方法名写错了。

</pre><pre code_snippet_id="397922" snippet_file_name="blog_20140618_4_9870731" name="code" class="objc"><pre name="code" class="objc">"The operation couldn’t be completed. (Cocoa error 256.)" UserInfo=0x1092a2680 {NSSQLiteErrorDomain=14, NSUnderlyingException=unable to open database file},{    NSSQLiteErrorDomain = 14;    NSUnderlyingException = "unable to open database file";}

程序中止原因：不能打开数据库，原来是忘了创建数据库文件

写了 CoreDataManager类 代码如下：<pre name="code" class="objc">#import <Foundation/Foundation.h>#import <CoreData/CoreData.h>@interface CoreDataManager : NSObject@property (readonly, strong, nonatomic) NSManagedObjectContext *managedObjectContext;@property (readonly, strong, nonatomic) NSManagedObjectModel *managedObjectModel;@property (readonly, strong, nonatomic) NSPersistentStoreCoordinator *persistentStoreCoordinator;+ (CoreDataManager *)defaultManager;- (NSManagedObject *)managerObjectWithEntityName:(NSString *)entityName managerObjectClass:(Class)aclass;@end

<pre name="code" class="objc">#import "CoreDataManager.h"@implementation CoreDataManager@synthesize managedObjectContext = _managedObjectContext;@synthesize managedObjectModel = _managedObjectModel;@synthesize persistentStoreCoordinator = _persistentStoreCoordinator;+ (CoreDataManager *)defaultManager{    static CoreDataManager *manager = nil;    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        manager = [[CoreDataManager alloc] init];    });    return manager;}-(NSManagedObjectContext *)managedObjectContext{    if (_managedObjectContext != nil) {        return _managedObjectContext;    }        NSPersistentStoreCoordinator *coodinator = [self persistentStoreCoodinator];    if (coodinator) {        //  创建对象        _managedObjectContext = [[NSManagedObjectContext alloc] init];        //  设置一个持久化存储协调器        [_managedObjectContext setPersistentStoreCoordinator:coodinator];            }    return _managedObjectContext;}-(NSPersistentStoreCoordinator *)persistentStoreCoodinator{    if (_persistentStoreCoordinator != nil) {        return _persistentStoreCoordinator;    }    NSError *error = nil;    //  根据托管对象创建一个协调器    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];    //  设置持久化存储的文件夹路径 存储类型    NSURL *fileUrl = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"sutdent.sqlit"];    NSLog(@"%s__%d__|%@",__FUNCTION__,__LINE__,[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject]);

    NSLog(@"%s__%d__|%@",__FUNCTION__,__LINE__,fileUrl);

//  <span style="color:#ff0000;">附加一个问题：以上两句话打印出的结果不一样，用url打印得到的路径在finder中找不到。</span>    if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:fileUrl options:nil error:&error]) {        NSLog(@"unresolved error %@,%@",error,[error userInfo]);        abort();  //  中止    }        return _persistentStoreCoordinator;}- (NSManagedObjectModel *)managedObjectModel{    if (_managedObjectModel) {        return _managedObjectModel;    }    NSURL *modelUrl = [[NSBundle mainBundle] URLForResource:@"Model" withExtension:@"momd"];    _managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelUrl];        return _managedObjectModel;}- (NSURL *)applicationDocumentsDirectory{    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];}-(NSManagedObject *)managerObjectWithEntityName:(NSString *)entityName managerObjectClass:(Class)aclass{    //  创建实体描述    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:entityName inManagedObjectContext:[CoreDataManager defaultManager].managedObjectContext];    //  通过实体描述创建托管对象    NSManagedObject *managerObject = [[[aclass class] alloc] initWithEntity:entityDescription insertIntoManagedObjectContext:self.managedObjectContext];    return managerObject;}@end