获取关系并在UITableView中显示 [英] Fetch relationships and show in a UITableView

问题描述

我的核心数据模型在我的购物车应用中设置了以下关系。

I have the following relationship set up in my Core Data model, in my shopping cart app.

菜单 - >> 产品<< - 购物车（见下图）。

Menu ->> Product <<- Cart (See picture below).

以及具有以下代码的Objective-C类别：

And a Objective-C category with the following code:

+ (Cart *)addProductToCartWithProduct:(Product *)product inManagedObjectContext:(NSManagedObjectContext *)context
{
    Cart *cart = [NSEntityDescription insertNewObjectForEntityForName:@"Cart" inManagedObjectContext:context];

    NSManagedObjectID *retID = [product objectID];
    [(Product *)[context objectWithID:retID] setInCart:cart];
    [cart addProductsObject:(Product *)[context objectWithID:retID]];    

    return cart;
}

当用户按下应用中的添加到购物车按钮时，方法被触发（上面的方法）。我现在想要获取这些产品（由用户添加到购物车中），在我的其他类中，购物车所在的位置。我希望这些产品显示在该类的 UITableView 中，因此我使用了FRC。 但我无法弄清楚如何获取关系，从而显示产品。我将如何设法做到这一点？

When the user pushes the "add to cart" button in the app, this method is fired (the method above). I now want to fetch these products (which is added to the cart by the user), in my other class, where the "cart" is located. I want these products to be shown in my UITableView in that class, I therefore use an FRC. But I can't figure out how to fetch the relationship and thereby show the products. How would I manage to do this?

我现在有以下代码。

- (void)loadCart
{
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    fetchRequest.entity = [NSEntityDescription entityForName:@"Cart" inManagedObjectContext:_theManagedObjectContext];

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"id" ascending:YES];
    fetchRequest.sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

    [NSFetchedResultsController deleteCacheWithName:@"cartproducts"];
    self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:_theManagedObjectContext sectionNameKeyPath:nil cacheName:@"cartproducts"];
    _fetchedResultsController.delegate = self;

    NSError *error = nil;
    if (![_fetchedResultsController performFetch:&error]) {
        NSLog(@"Fetch Failed");        
    }
}

我的 UITableview cellForRowAtIndexPath：方法。

- (UITableViewCell *)tableView:(UITableView *)aTableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *plainCellIdentifier = @"OrderCell";

    ProductsCell *cell = (ProductsCell *)[aTableView dequeueReusableCellWithIdentifier:plainCellIdentifier];

    if (!cell) {
        cell = [[ProductsCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:plainCellIdentifier];
    }

    Cart *ret = (Cart *)[self.fetchedResultsController objectAtIndexPath:indexPath];

    // How to show the product names using the relationship? 
    cell.title = ret.products ??;  

    [cell setNeedsDisplay];

    return cell;
}

推荐答案

显示特定产品购物车，您将创建获取结果控制器
，如下所示（未编译器检查，可能存在语法错误！）：

To display product for one specific cart, you would create the fetched results controller as follows (not compiler checked, there may be syntax errors!) :

Cart *theCart = ... the cart that you want display products for ...

// Fetch request for "Product":
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Product"];

// Fetch only products for the cart:
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"inCart = %@", theCart];
[fetchRequest setPredicate] = predicate;

// Assuming that you want to sort by "navn":
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"navn" ascending:YES];
[fetchRequest setSortDescriptors:@[sortDescriptor]];

self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:_theManagedObjectContext sectionNameKeyPath:nil cacheName:nil];
_fetchedResultsController.delegate = self;

获取的对象现在是 Product 对象，因此，在 cellForRowAtIndexPath：中你会这样做：

The fetched objects are now Product objects, therefore in cellForRowAtIndexPath: you would do for example:

...
Product *product = [self.fetchedResultsController objectAtIndexPath:indexPath];
cell.title = product.navn;  

备注：获取的结果控制器使用缓存（就我而言）知道）仅用于部分信息，因此无需在此处指定 cacheName：。

Remark: The fetched results controller uses a cache (as far as I know) only for section information, so there is no need to specify a cacheName: here.

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