UICollectionView 总是在当前之前显示上一个选择

Question

我似乎遇到了一整天都在努力解决的问题。我有一个 UICollectionView。在此视图中，显示了来自核心数据的项目（图像）。您单击图像，它会进入详细视图，其中包含通过相机拍摄图像时输入的更多信息。图像在集合视图中显示良好。我遇到的问题是，总是在显示当前选择之前，首先显示最后一个选择。我不知道该怎么办，这是代码和一些输出。

{
NSInteger selectedFavoriteIndex;
}


- (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.
}
- (NSManagedObjectContext *)managedObjectContext
{
NSManagedObjectContext *context = nil;
id delegate = [[UIApplication sharedApplication]delegate];
if ([delegate performSelector:@selector(managedObjectContext)]) {
    context = [delegate managedObjectContext];
}
return context;

}

- (void)viewDidAppear:(BOOL)animated{
[super viewDidAppear:animated];


//Fetch all the stuff from data store.
NSManagedObjectContext *managedObjectContext = [self managedObjectContext];

NSFetchRequest *fetchRequest =[[NSFetchRequest alloc]initWithEntityName:@"Favorites"];
self.favorites = [[managedObjectContext executeFetchRequest:fetchRequest error:nil]mutableCopy];
[self.collectionView reloadData];
[self.collectionViewLayout invalidateLayout];


}



-(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    return 1;
}

-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:    (NSInteger)section
{
return [self.favorites count];
}

-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = @"Cell";
CustomCollectionCell *customCell = [self.collectionView dequeueReusableCellWithReuseIdentifier:CellIdentifier forIndexPath:indexPath];

NSManagedObject *favorite = [self.favorites objectAtIndex:indexPath.row];
[customCell.collectionImageView setImage:[UIImage imageWithData:[favorite valueForKey:@"image"]]];
return customCell;



}
 - (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
selectedFavoriteIndex = indexPath.item;
NSLog(@"Index path: %@", indexPath);
NSLog(@"Selected Favorite Index: %ld", (long)selectedFavoriteIndex);
NSLog(@"IndexPath.row: %ld", (long)indexPath.row);
NSLog(@"IndexPath.item: %ld", (long)indexPath.item);


}  



-(void)collectionView:(UICollectionView *)collectionView didDeselectItemAtIndexPath:(NSIndexPath *)indexPath
{

}

-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{

if ([[segue identifier]isEqualToString:@"moment"]) {
    NSManagedObject *selectedFavorite = [self.favorites objectAtIndex:selectedFavoriteIndex];
    ViewMomentViewController *viewMomentViewController = segue.destinationViewController;
    viewMomentViewController.favorite = selectedFavorite;
    viewMomentViewController.hidesBottomBarWhenPushed = YES;



}


}

在此处粘贴时，格式有点混乱。不过，Xcode 中的格式是正确的。这是一些输出。

索引路径：{length = 2, path = 0 - 1}

我也很困惑为什么路径总是减去，无论是 0-0 还是 0-2。谁能给我解释一下？

Answer 1

听起来prepareForSegue 在didSelectItemAtIndexPath 之前被调用。所以prepareForSegue 会从上一次调用didSelectItemAtIndexPath 中看到selectedFavoriteIndex 的值。

这很容易确认，只需在两种方法中放置NSLog 语句即可查看日志消息在控制台中出现的顺序。

有几种方法可以解决这个问题：

1. 不要使用故事板定义的转场，而是在didDeselectItemAtIndexPath 之后使用[self.storyboard instantiateViewControllerWithIdentifier:]设置selectedFavoriteIndex进行程序化转场。

2. 不要依赖didSelectItemAtIndexPath 来设置选定的索引，而是通过调用[self.collectionView indexPathForCell:sender] 来确定@​​987654331@ 中的选定索引（当您使用来自集合视图单元格的故事板segue 时，@987654333 的值@ in prepareForSegue 将是单元格本身，您可以使用它来确定索引路径）。

方法 (2) 如下所示：

NSIndexPath *indexPath = [self.collectionView indexPathForCell:sender];
NSManagedObject *selectedFavorite = [self.favorites objectAtIndex:indexPath.item];