的Xcode CollectionViewController scrollToItemAtIndexPath不工作

Question

我創建了一個CollectionView控制和處理圖像填充它。現在我想在開始時滾動到特定索引處的項目。我試過scrollToItemAtIndexPath如下：

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

但是，我得到以下異常。任何人都可以指導我在哪裏出錯。

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

無論是錯誤或功能，UIKit中拋出這個錯誤，每當之前UICollectionView奠定了它的子視圖scrollToItemAtIndexPath:atScrollPosition:Animated被調用。

作爲一種變通方法，將您的滾動調用一個地方，在視圖控制器生命週期中有你確定它已經計算出它的佈局，像這樣：

@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 

最起碼，錯誤消息應可能會更有幫助。我已經打開rdar://13416281;請欺騙。

Answer 2

還記得，你需要使用適當的UICollectionviewScrollPosition值。請看下面的代碼說明：

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可以做到這一點，在viewDidLoad方法

只是撥打電話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]; 
     } 

    }]; 

在我的情況，我的子類的CollectionView有一個屬性selectedPage，並在此屬性的setter方法我稱之爲

- (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]; 
     } 

    }]; 
} 

在視圖中，我通過代碼調用此代碼

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

Answer 4

如果您嘗試在視圖控制器加載時進行滾動，請確保在致電scrollToItemAtIndexPath之前致電layoutIfNeeded,UICollectionView。這比將scroll邏輯放在viewDidLayoutSubviews中要好，因爲每次佈置父視圖的子視圖時都不會執行滾動操作。

Answer 5

添加滾動邏輯viewDidAppear工作對我來說：

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，除非你要滾動邏輯調用任何時間有什麼變化。在我的情況下，從手動滾動項目

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

作爲iOS的9.3時，Xcode 8.2的防止用戶。1，Swift 3：

從viewWillAppear()調用scrollToItem(at:)仍然有問題，特別是如果您使用節頁眉/頁腳，自動佈局，區域插頁。

即使您在collectionView上調用setNeedsLayout()和layoutIfNeeded()，行爲仍然存在。將滾動代碼放入動畫塊中並不可靠。

正如其他答案所示，解決方案只有在您確定所有內容都已經佈置完畢後才致電scrollToItem(at:)。即在viewDidLayoutSubviews()中。

但是，您需要有選擇性;每次調用viewWillLayoutSubviews()時，都不想執行滾動。因此，解決方案是在viewWillAppear()中設置一個標誌，並在viewDidLayoutSubviews()中對其執行操作。

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

有時collectionView(_:didSelectItemAt:)要麼不叫主線程，或阻止它，造成scrollToItem(at:at:animated:)不要做任何事情。解決此

工作做：

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

Answer 9

這是基於@旺布爾的回答，都歸功於他們：

viewDidLayoutSubviews()被反覆調用的方法。對我來說（iOS 11.2）第一次被稱爲collectionView.contentSize的是{0,0}。第二次，contentSize是正確的。因此，我不得不添加一個支票本：

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) 
     } 
    } 
} 

增加額外的&& collectionView.contentSize.width > 0之後，它精美的作品。