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Question

所以我创建了一个社交媒体应用程序的应用程序。我正在使用教程发现here作为跳板，因为我仍然试图围绕核心数据包装我的大脑。我偏离了教程，增加了一个注册按钮，让用户访问一个新的视图控制器，并创建一个.h和一个.m文件，并将新成员屏幕设置为引用.h和.m文件。他们建立了.H如下：

#import <UIKit/UIKit.h> 

@interface NewMemberViewController : UIViewController 

@property (strong, nonatomic) NSManagedObjectContext *managedObjectContext; 

@property (strong, nonatomic) IBOutlet UITextField *nameTF; 

@property (strong, nonatomic) IBOutlet UITextField *ageTF; 

@property (strong, nonatomic) IBOutlet UITextField *usernameTF; 

@property (strong, nonatomic) IBOutlet UITextField *passwordTF; 

- (IBAction)alreadyMember:(id)sender; 
- (IBAction)checkAndLogin:(id)sender; 


@end 

，为.M：

#import "NewMemberViewController.h" 
#import "CoreDataHelper.h" 

@interface NewMemberViewController() 

@end 

@implementation NewMemberViewController 

@synthesize usernameTF, ageTF, passwordTF, nameTF, managedObjectContext; 

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil 
{ 
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil]; 
    if (self) { 
    // Custom initialization 
} 
    return self; 
} 

- (void)viewDidLoad 
{ 
    [super viewDidLoad]; 
// Do any additional setup after loading the view. 
} 

- (void)didReceiveMemoryWarning 
{ 
    [super didReceiveMemoryWarning]; 
    // Dispose of any resources that can be recreated. 
} 
//If the user is already a member simply dismiss the VC 
- (IBAction)alreadyMember:(id)sender { 
    [self dismissViewControllerAnimated:YES completion:nil]; 
} 

//When done editing keyboard 
- (IBAction)checkAndLogin:(id)sender { 
managedObjectContext =self.managedObjectContext; 

//Textfield Reference 
UITextField *tf = (UITextField *)sender; 

//Check tag numbers If its equal to 1 or 2(nameTF or ageTF) then 
if (tf.tag==1||tf.tag==2) 
{ 
    [sender resignFirstResponder]; 
    NSLog(@"This is working"); 
} 
//If its equal to 3 then this means the username text field is active 
else if (tf.tag == 3) 
{ 
    [sender resignFirstResponder]; 
    //do a quick search to see if username is availible 
    NSPredicate *pred = [NSPredicate predicateWithFormat:@"(username == %@)", [usernameTF text]]; 

    //Run the query to check if user exists 
    if([CoreDataHelper countForEntity:@"Users" withPredicate:pred andContext:managedObjectContext] > 0) 
    { 
     //we found a user 
     NSLog(@"oh no..."); 
    } 
    } 
} 
@end 

有了，如果我跑我的应用程序并进入注册画面和测试，看看这样说如果用户存在，通过管理输入，因为它已经存在，我得到以下错误在Xcode：

画布[779：11603] *的WebKit丢弃在web视图未捕获的异常：shouldInsertText ：substitutionDOMRange：givenAction：delegate：+ entityForName：nil不是合法的NSManagedObjectContext参数搜索实体名称'用户'

这是什么意思？为什么它是由什么引起的？

Answer 1

您是否将上下文传递给视图控制器？试用：

YourAppDelegate *appDelegate = (YourAppDelegate *)[[UIApplication sharedApplication]delegate]; 
yourVontext = [appDelegate managedObjectContext]; 

取前。

Answer 2

下方有问题。

managedObjectContext = self.managedObjectContext; 

self.managedObjectContext将返回零到managedObjectContext。除了self.managedObjectContext，您应该在应用程序委托中创建Core Data Stack（方法实现），然后将ManagedObjectContext的对象引用从AppDelegate传递到您的调用者类。