的Xcode CollectionViewController scrollToItemAtIndexPath不工作

Question

我创建了一个CollectionView控制和处理图像填充它。现在我想在开始时滚动到特定索引处的项目。我试过scrollToItemAtIndexPath如下：

[self.myFullScreenCollectionView scrollToItemAtIndexPath:indexPath 
atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:YES]; 

但是，我得到以下异常。任何人都可以指导我在哪里出错。

2013-02-20 02:32:45.219 ControlViewCollection1[1727:c07] *** Assertion failure in 
-[UICollectionViewData layoutAttributesForItemAtIndexPath:], /SourceCache/UIKit_Sim/UIKit-2380.17 
/UICollectionViewData.m:485 2013-02-20 02:32:45.221 ControlViewCollection1[1727:c07] must return a 
UICollectionViewLayoutAttributes instance from -layoutAttributesForItemAtIndexPath: for path 
<NSIndexPath 0x800abe0> 2 indexes [0, 4] 

Answer 1

无论是错误或功能，UIKit中抛出这个错误，每当之前UICollectionView奠定了它的子视图scrollToItemAtIndexPath:atScrollPosition:Animated被调用。

作为一种变通方法，将您的滚动调用一个地方，在视图控制器生命周期中有你确定它已经计算出它的布局，像这样：

@implementation CollectionViewControllerSubclass 

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

    // scrolling here doesn't work (results in your assertion failure) 
} 

- (void)viewDidLayoutSubviews 
{ 
    [super viewDidLayoutSubviews]; 

    NSIndexPath *indexPath = // compute some index path 

    // scrolling here does work 
    [self.collectionView scrollToItemAtIndexPath:indexPath 
           atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally 
             animated:YES]; 
} 

@end 

最起码，错误消息应可能会更有帮助。我已经打开rdar://13416281;请欺骗。

Answer 2

还记得，你需要使用适当的UICollectionviewScrollPosition值。请看下面的代码说明：

typedef NS_OPTIONS(NSUInteger, UICollectionViewScrollPosition) { 
UICollectionViewScrollPositionNone     = 0, 

/* For a view with vertical scrolling */ 
// The vertical positions are mutually exclusive to each other, but are bitwise or-able with the horizontal scroll positions. 
// Combining positions from the same grouping (horizontal or vertical) will result in an NSInvalidArgumentException. 
UICollectionViewScrollPositionTop     = 1 << 0, 
UICollectionViewScrollPositionCenteredVertically = 1 << 1, 
UICollectionViewScrollPositionBottom    = 1 << 2, 

/* For a view with horizontal scrolling */ 
// Likewise, the horizontal positions are mutually exclusive to each other. 
UICollectionViewScrollPositionLeft     = 1 << 3, 
UICollectionViewScrollPositionCenteredHorizontally = 1 << 4, 
UICollectionViewScrollPositionRight    = 1 << 5 

};

Answer 3

U可以做到这一点，在viewDidLoad方法

只是拨打电话preformBatchUpdates

[self performBatchUpdates:^{ 
     if ([self.segmentedDelegate respondsToSelector:@selector(segmentedBar:selectedIndex:)]){ 
      [self.segmentedDelegate segmentedBar:self selectedIndex:_selectedPage]; 
     } 
    } completion:^(BOOL finished) { 
     if (finished){ 
      [self scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:_selectedPage inSection:0] atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:YES]; 
     } 

    }]; 

在我的情况，我的子类的CollectionView有一个属性selectedPage，并在此属性的setter方法我称之为

- (void)setSelectedPage:(NSInteger)selectedPage { 
    _selectedPage = selectedPage; 
    [self performBatchUpdates:^{ 
     if ([self.segmentedDelegate respondsToSelector:@selector(segmentedBar:selectedIndex:)]){ 
      [self.segmentedDelegate segmentedBar:self selectedIndex:_selectedPage]; 
     } 
    } completion:^(BOOL finished) { 
     if (finished){ 
      [self scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:_selectedPage inSection:0] atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:YES]; 
     } 

    }]; 
} 

在视图中，我通过代码调用此代码

- (void)viewDidLoad { 
    [super viewDidLoad]; 
    self.navigationBar.segmentedPages.selectedPage = 1; 
} 

Answer 4

如果您尝试在视图控制器加载时进行滚动，请确保在致电scrollToItemAtIndexPath之前致电layoutIfNeeded,UICollectionView。这比将scroll逻辑放在viewDidLayoutSubviews中要好，因为每次布置父视图的子视图时都不会执行滚动操作。

Answer 5

添加滚动逻辑viewDidAppear工作对我来说：

override func viewDidAppear(animated: Bool) { 
    super.viewDidAppear(animated) 
    self.collectionView?.scrollToItemAtIndexPath(
     someIndexPath, 
     atScrollPosition: UICollectionViewScrollPosition.None, 
     animated: animated) 
} 

它添加到viewDidLoad不起作用：它被忽略。

它添加到viewDidLayoutSubviews不起作用，除非你要滚动逻辑调用任何时间有什么变化。在我的情况下，从手动滚动项目

Answer 6

夫特3

UIView.animate(withDuration: 0.4, animations: { 
    myCollectionview.scrollToItem(at: myIndexPath, at: .centeredHorizontally, animated: false) 
}, completion: { 
    (value: Bool) in 
    // put your completion stuff here 
}) 

Answer 7

作为iOS的9.3时，Xcode 8.2的防止用户。1，Swift 3：

从viewWillAppear()调用scrollToItem(at:)仍然有问题，特别是如果您使用节页眉/页脚，自动布局，区域插页。

即使您在collectionView上调用setNeedsLayout()和layoutIfNeeded()，行为仍然存在。将滚动代码放入动画块中并不可靠。

正如其他答案所示，解决方案只有在您确定所有内容都已经布置完毕后才致电scrollToItem(at:)。即在viewDidLayoutSubviews()中。

但是，您需要有选择性;每次调用viewWillLayoutSubviews()时，都不想执行滚动。因此，解决方案是在viewWillAppear()中设置一个标志，并在viewDidLayoutSubviews()中对其执行操作。

即

fileprivate var needsDelayedScrolling = false 

override func viewWillAppear(_ animated: Bool) 
{ 
    super.viewWillAppear(animated) 
    self.needsDelayedScrolling = true 
    // ... 
} 

override func viewDidLayoutSubviews() 
{ 
    super.viewDidLayoutSubviews() 

    if self.needsDelayedScrolling { 
     self.needsDelayedScrolling = false 
     self.collectionView!.scrollToItem(at: someIndexPath, 
       at: .centeredVertically, 
       animated: false) 
     } 
    } 
} 

Answer 8

有时collectionView(_:didSelectItemAt:)要么不叫主线程，或阻止它，造成scrollToItem(at:at:animated:)不要做任何事情。解决此

工作做：

DispatchQueue.main.async { 
    collectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true) 
} 

Answer 9

这是基于@旺布尔的回答，都归功于他们：

viewDidLayoutSubviews()被反复调用的方法。对我来说（iOS 11.2）第一次被称为collectionView.contentSize的是{0,0}。第二次，contentSize是正确的。因此，我不得不添加一个支票本：

var needsDelayedScrolling = false 

override func viewWillAppear(_ animated: Bool) { 
    super.viewWillAppear(animated) 
    self.needsDelayedScrolling = true 
    // ... 
} 

override func viewDidLayoutSubviews() { 
    super.viewDidLayoutSubviews() 
    if self.needsDelayedScrolling && collectionView.contentSize.width > 0 { 
     self.needsDelayedScrolling = false 
     self.collectionView!.scrollToItem(at: someIndexPath, 
       at: .centeredVertically, 
       animated: false) 
     } 
    } 
} 

增加额外的&& collectionView.contentSize.width > 0之后，它精美的作品。