Objective-C 错误：使用 NSArray 的单元格上的 IndexPath 错误

Question

我在我的代码中通过 Swift Bridging 为我的 CollectionView 使用 CardsCollectionViewLayout。

Objective-C（不工作）

我的问题是 IndexPath 返回错误的Item 索引。这是最小的代码：

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.collectionViewEvents.collectionViewLayout = [[CardsCollectionViewLayout alloc] init];
    self.collectionViewEvents.dataSource = self;
    self.collectionViewEvents.delegate = self;
    self.collectionViewEvents.pagingEnabled = YES;
    self.collectionViewEvents.showsHorizontalScrollIndicator = NO;

    [self load];
}

// Issue visible here, the indexPath.item is not correct
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    for (UICollectionViewCell *cell in self.collectionViewEvents.visibleCells) {
        NSIndexPath *indexPath = [[self collectionViewEvents] indexPathForCell:cell];
        NSLog(@"Visible Cell IndexPath Item %ld", indexPath.item);
        return;
    }
}

- (void)load
{

    // self.eventData is declares as: @property (nonatomic) NSArray *eventData;
    self.eventData = [[NSArray alloc] initWithObjects:UIColor.blackColor, UIColor.whiteColor, UIColor.brownColor, nil];
    [[self collectionViewEvents] reloadData];
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"CollectionViewCellReuseIdentifier"
                                                                           forIndexPath:indexPath];

    cell.layer.cornerRadius = 7.0;
    cell.backgroundColor = UIColor.blackColor;

    return cell;
}

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

结果（注意第一个和第二个单元格如何具有相同的 IndexPath 项）：

2020-03-19 14:48:05.334905+0100 App[7422:2617858] Visible Cell IndexPath Item 2 # => 3rd cell
2020-03-19 14:48:05.741805+0100 App[7422:2617858] Visible Cell IndexPath Item 1 # => 2nd cell
2020-03-19 14:48:06.184932+0100 App[7422:2617858] Visible Cell IndexPath Item 1 # => 1st cell

---

斯威夫特（工作）

我从他的代码库中尝试了this Example，它静态声明了colors：

  var colors: [UIColor]  = [
    UIColor(red: 237, green: 37, blue: 78),
    UIColor(red: 249, green: 220, blue: 92),
    UIColor(red: 194, green: 234, blue: 189),
    UIColor(red: 1, green: 25, blue: 54),
    UIColor(red: 255, green: 184, blue: 209)
  ]
...

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "CollectionViewCellReuseIdentifier", for: indexPath)

        cell.layer.cornerRadius = 7.0
        cell.backgroundColor = .black

        return cell
    }

在执行下面这段代码后，它会返回正确的 IndexPath。

    // Issue NOT visible here, the indexPath.item IS correct
    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {

        for cell in collectionView.visibleCells {
            let indexPath = collectionView.indexPath(for: cell)
            print(indexPath?.item)
            return
        }
    }

结果：

2020-03-19 14:48:05.334905+0100 App[7422:2617858] Visible Cell IndexPath Item 2 # => 3rd cell
2020-03-19 14:48:05.741805+0100 App[7422:2617858] Visible Cell IndexPath Item 1 # => 2nd cell
2020-03-19 14:48:06.184932+0100 App[7422:2617858] Visible Cell IndexPath Item 0 # => 1st cell

我在Objective-C 代码中做错了什么？

---

我尝试过的事情

// 1st cell has index 1, instead of 0
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    for (UICollectionViewCell *cell in [[self collectionViewEvents] visibleCells]) {
        NSIndexPath *indexPath = [[self collectionViewEvents] indexPathForCell:cell];
        NSLog(@"Visible Cell IndexPath Item %ld", indexPath.item);
        return;
    }

// 1st cell has index 1, instead of 0
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    NSArray *visible = [self.collectionViewEvents indexPathsForVisibleItems];
    NSIndexPath *indexPath = [visible firstObject];
    NSLog(@"Visible Cell IndexPath Item %ld", indexPath.item);
    return;

// Calling it in main thread, same result
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    dispatch_async(dispatch_get_main_queue(), ^{
        for (UICollectionViewCell *cell in [[self collectionViewEvents] visibleCells]) {
            NSIndexPath *indexPath = [[self collectionViewEvents] indexPathForCell:cell];
            NSLog(@"Visible Cell IndexPath Item %ld", indexPath.item);
            return;
        }
    });

Answer 1

问题是你在这个循环中有一个返回。

for (UICollectionViewCell *cell in [[self collectionViewEvents] visibleCells]) {
    NSIndexPath *indexPath = [[self collectionViewEvents] indexPathForCell:cell];
    NSLog(@"Visible Cell IndexPath Item %ld", indexPath.item);
    return; // <- This here
}

visibleCells 返回集合中所有可见单元格的数组。您只 NSLog 该数组中的第一项，因此这取决于您滚动到哪个位置将是第一个。