NSManagedObjectContext：从一个 UIViewController，如何在 iOS 中获取 ManagedObjectContext？

Question

您如何知道正确的 ManagedObjectContext 是什么？因为我不在 appDelegate 中（我认为代码所在的位置），并且我的应用程序不断崩溃 - 特别是在 MyTabBarViewController 的“ViewDidLoad”和 SecondViewController 的“sendPressed”方法中。

你能告诉我如何找到上下文吗？因为 alloc 初始化我自己的上下文不起作用。我想向 appDelegate 发送消息吗？我认为添加 appDelegate 头文件或调用它是错误的做法。

SecondViewController.m

#import "SecondViewController.h"

@implementation SecondViewController

- (IBAction) sendPressed:(UIButton *)sender
{
    for(UIViewController *controller in self.tabBarController.viewControllers)
    {
        if([controller isKindOfClass:[FirstViewController class]])
        {
            FirstViewController *fvc = (FirstViewController *)controller;
            [fvc realLabel];
        }
    }

    //add image to Core Data
    NSData *imageData = UIImageJPEGRepresentation(image, 1.0);
    NSManagedObjectContext *context = [[NSManagedObjectContext alloc] init];
    Photo *photo = [NSEntityDescription insertNewObjectForEntityForName:@"Photo" inManagedObjectContext:context];
    photo.photo = imageData;

    self.tabBarController.selectedIndex = 2;//switch over to the third view to see if it worked
}
...
@end

MyTabBarViewController.m

#import "MyTabBarViewController.h"


@implementation MyTabBarViewController
@synthesize pageControl,scroller;

-(IBAction)clickPageControl:(id)sender
{
    int page=pageControl.currentPage;
    CGRect frame=scroller.frame;
    frame.origin.x = frame.size.width * page;
    frame.origin.y = 0;
    [scroller scrollRectToVisible:frame animated:YES];
}

-(void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    int page = scrollView.contentOffset.x/scrollView.frame.size.width;
    pageControl.currentPage=page;
}

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

- (void)dealloc
{
    [super dealloc];
}

- (void)didReceiveMemoryWarning
{
    // Releases the view if it doesn't have a superview.
    [super didReceiveMemoryWarning];

    // Release any cached data, images, etc that aren't in use.
}

#pragma mark - View lifecycle

- (void)viewDidLoad
{
    [super viewDidLoad];
    scroller.delegate=self;
    scroller.pagingEnabled=YES;
    scroller.directionalLockEnabled=YES;
    scroller.showsHorizontalScrollIndicator=NO;
    scroller.showsVerticalScrollIndicator=NO;
    scroller.contentSize=CGSizeMake(pageControl.numberOfPages*scroller.frame.size.width, scroller.frame.size.height);
    CGFloat scrollWidth = 0;
    int pageNumber = 0;
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSManagedObjectContext *context = [[NSManagedObjectContext alloc] init];
    request.entity = [NSEntityDescription entityForName:@"Photo" inManagedObjectContext:context];
    NSError *error = nil;
    NSArray *fetchCount = [context executeFetchRequest:request error:&error];
    int pageCount = [fetchCount count];
    for (int i=0; i<pageCount; i++)
    {
        PhotoViewController *pvc = [[PhotoViewController alloc] initWithNibName:@"PhotoViewController" bundle:nil];
        CGRect rect = scroller.frame;
        rect.size.height = scroller.frame.size.height;
        rect.size.width = scroller.frame.size.width;
        rect.origin.x = scroller.frame.origin.x + scrollWidth;
        rect.origin.y = scroller.frame.origin.y;
        pvc.view.frame  = rect;
        [pvc view];
        pvc.label.text = [NSString stringWithFormat:@"%d", pageNumber];
        pvc.label.textColor = [UIColor redColor];
        Photo *photo = [[Photo alloc] init];
        photo = [fetchCount objectAtIndex:i];
        UIImage *fetchedImage = [UIImage imageWithData:photo.photo];
        pvc.imageView.image = fetchedImage;
        [scroller addSubview:pvc.view];
        [pvc release];
        pageNumber++;
        scrollWidth += scroller.frame.size.width;
    }
    pageControl.numberOfPages=pageCount;
    pageControl.currentPage=0;
    [self.view addSubview:scroller];
}

...

@end

如果我将我的 CoreDataProjAppDelegate 导入到这些文件中的每一个并使用NSManagedObjectContext *context = [(CoreDataProjAppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext];，那么它可以正常工作。但这是正确的方法吗？

Answer 1

您必须设置 PersistentStoreCoordinator 才能使上下文有效：

NSPersistentStoreCoordinator *psc = ((CoreDataProjAppDelegate *)[UIApplication sharedApplication].delegate).persistentStoreCoordinator;
    NSManagedObjectContext *context = [[NSManagedObjectContext alloc] init];
    [context setPersistentStoreCoordinator:psc];

这是假设您在应用程序委托上有一个名为 persistentStoreCoordinator 的属性，默认核心数据项目将拥有该属性。 psc 基本上是您的上下文（便笺簿）和实际持久存储（物理数据库）之间的链接。

Answer 2

您创建项目时是否勾选了“使用核心数据”框？如果你这样做了，你将自动拥有一个 managedObjectContext，然后你将它传递给任何其他需要它的类。