无法从另一个视图控制器访问核心数据中的实体

Question

我是一个新手目标-C程序员和核心数据用户。这是我第一次使用核心数据。我要做的是把实体放在一个核心数据数据库中，然后在另一个中检索和编辑它们。

以下是我的ViewController.m中的代码：

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSURL *documentsDirectory = [[fileManager URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] firstObject];

    NSString *documentName = @"myDocument";
    NSURL *url = [documentsDirectory URLByAppendingPathComponent:documentName];
    UIManagedDocument *document = [[UIManagedDocument alloc]initWithFileURL:url];
    BOOL fileExists = [[NSFileManager defaultManager] fileExistsAtPath:[url path] ];
    if(fileExists)
    {
        [document openWithCompletionHandler:^(BOOL success){
            if(!success) NSLog(@"Error (this isn't printed)");
        }];
    }
    else
    {
        [document saveToURL:url forSaveOperation:UIDocumentSaveForCreating completionHandler:^(BOOL success){
            if(!success) NSLog(@"Error (this isn't printed)");
        }];

        NSManagedObjectContext *context = document.managedObjectContext;

        QuestionResponse *question1 = [NSEntityDescription insertNewObjectForEntityForName: @"QuestionResponse" inManagedObjectContext: context];
        QuestionResponse *question2 = [NSEntityDescription insertNewObjectForEntityForName: @"QuestionResponse" inManagedObjectContext: context];
        QuestionResponse *question3 = [NSEntityDescription insertNewObjectForEntityForName: @"QuestionResponse" inManagedObjectContext: context];
        QuestionResponse *question4 = [NSEntityDescription insertNewObjectForEntityForName: @"QuestionResponse" inManagedObjectContext: context];

        question1.question = @"What is your favorite food?";
        question2.question = @"Who is your favorite fictional character?";
        question3.question = @"Name someone who loves you:";
        question4.question = @"What is your favorite place (real or fictional)?";

        question1.order = [NSNumber numberWithInt:1];
        question2.order = [NSNumber numberWithInt:2];
        question3.order = [NSNumber numberWithInt:3];
        question4.order = [NSNumber numberWithInt:4];
        question1.group = [NSNumber numberWithInt:1];
        question2.group = [NSNumber numberWithInt:1];
        question3.group = [NSNumber numberWithInt:2];
        question3.group = [NSNumber numberWithInt:2];
        question1.display = [NSNumber numberWithBool:NO];
        question2.display = [NSNumber numberWithBool:NO];
        question3.display = [NSNumber numberWithBool:NO];
        question4.display = [NSNumber numberWithBool:NO];

        NSFetchRequest* request2 = [NSFetchRequest fetchRequestWithEntityName:@ "QuestionResponse"];
        NSError* error2 = nil;
        NSArray* results = [context executeFetchRequest:request2 error:&error2];
        if (!results || !results.count){
            NSLog(@"No elements (this isn't printed)");
        }
        if(results.count == 4)
        {
            NSLog(@"We have four entities (this is what is printed)");
        }        
    }
}

M在我的答疑视图控制器中，这是代码：

 - (IBAction)answerQuestion1:(id)sender {    
    //File creation begins here (same as in View Controller)
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSURL *documentsDirectory = [[fileManager URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] firstObject];
    NSString *documentName = @"myDocument";    
    NSURL *url = [documentsDirectory URLByAppendingPathComponent:documentName];
    UIManagedDocument *document = [[UIManagedDocument alloc]initWithFileURL:url];
    BOOL fileExists = [[NSFileManager defaultManager] fileExistsAtPath:[url path] ];
    if(fileExists)
    {
        [document openWithCompletionHandler:^(BOOL success){
            if(!success) NSLog(@"Error");
        }];
        NSLog(@"File Exists (this is printed)");
    }
    else
    {
        [document saveToURL:url forSaveOperation:UIDocumentSaveForCreating completionHandler:^(BOOL success){
            if(!success)NSLog(@"There has been a problem (this isn't printed)");
        }];
    }
    NSManagedObjectContext *context = document.managedObjectContext;
    //File Creation ends here

    NSError *error = nil;

    NSFetchRequest* request2 = [NSFetchRequest fetchRequestWithEntityName:@ "QuestionResponse"];
    NSError* error2 = nil;
    NSArray* results = [context executeFetchRequest:request2 error:&error2];
    if (!results || !results.count){
        NSLog(@"No entities (this is what is printed");
    }
    else
    {
        NSLog(@"Some entities (this isn't printed)");
    }
}

在我的ViewController.m中，核心数据有四个元素，但是当我试图在AnswereViewController.m中访问它们时，核心数据中没有任何内容。为什么会发生这种情况，我如何访问AnswereViewController.m中的元素？

Answer 1

为了成功地使用CoreData，您必须首先了解它的流程。本质上，CoreData可以分为两个主要部分：

1. 数据库(所以是.sqlite文件本身)
2. 持久存储协调器(此后称为psc)

到目前为止，您在ViewController.m中所做的是告诉编译器将4个对象插入到managedObjectContext中。这是在psc级别上完成的。psc的作用就像所有数据库更改的便签。在您真正告诉它保存它之前，它将只存在于psc中，而不是在您的.sqlite文件中。那么你需要做的就是打电话给：

[context save:&error2]

这将告诉编译器将psc的当前状态推入数据库，以使其保持不变。

希望这能帮到你。