Question

我目前有以下用于计算UICollectionViewCells尺寸的代码段：

- (CGSize)collectionView:(UICollectionView *)mainCollectionView
                  layout:(UICollectionViewLayout *)collectionViewLayout
  sizeForItemAtIndexPath:(NSIndexPath *)atIndexPath
{
    CGSize bounds = mainCollectionView.bounds.size;
    bounds.height /= 4;
    bounds.width /= 4;
    return bounds;
}

这很有效。但是，我现在在viewDidLoad中添加一个键盘观察器（它在UICollectionView出现之前触发了委托和数据源方法，并从故事板中调整自身大小）。因此，界限是错误的。我也想支持轮换。如果UICollectionView改变大小，处理这两个边缘情况并重新计算尺寸的好方法是什么？

Answer 1

当集合视图的边界发生更改时，使布局无效的解决方案是覆盖shouldInvalidateLayoutForBoundsChange:并返回YES。它也在文档中说明：https://developer.apple.com/documentation/uikit/uicollectionviewlayout/1617781-shouldinvalidatelayoutforboundsc

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds 
{
     return YES;
}

这也应该包括旋转支持。如果没有，请实施viewWillTransitionToSize:withTransitionCoordinator:

- (void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator
{
    [super viewWillTransitionToSize:size
          withTransitionCoordinator:coordinator];

    [coordinator animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext> context)
     {
         [self.collectionView.collectionViewLayout invalidateLayout];
     }
                                 completion:^(id<UIViewControllerTransitionCoordinatorContext> context)
     {
     }];
}

Answer 2

您应该在更改集合视图大小时处理这种情况。如果您更改方向或约束，将触发viewWillLayoutSubviews方法。
您应该使当前集合视图布局无效。使用invalidateLayout方法使布局无效后，将触发UICollectionViewDelegateFlowLayout方法。

以下是示例代码：

- (void)viewWillLayoutSubviews {
    [super viewWillLayoutSubviews];
    [mainCollectionView.collectionViewLayout invalidateLayout];
}

Answer 3

此方法允许您执行此操作而无需将布局子类化，而是将其添加到您可能已经存在的UICollectionViewController子类中，并且避免了递归调用viewWillLayoutSubviews的可能性，这是已接受的变体解决方案，略有简化，因为它不使用transitionCoordinator。在Swift中：

override func viewWillTransition(
    to size: CGSize,
    with coordinator: UIViewControllerTransitionCoordinator
) {
    super.viewWillTransition(to: size, with: coordinator)
    collectionViewLayout.invalidateLayout()
}

Answer 4

尽管@tubtub的答案是有效的，但有些人可能会遇到以下错误：The behaviour of the UICollectionViewFlowLayout is not defined。

首先，请记住在shouldInvalidateLayout类中覆盖CustomLayout。（下面的示例）

然后，您应该根据新布局考虑视图中所有元素的大小是否已更改（请参见示例代码中的可选步骤）。

以下是以下代码，可以帮助您入门。根据创建UI的方式，您可能必须尝试找到合适的视图来调用recalculate方法，但这应该可以指导您迈出第一步。

override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {

    super.viewWillTransition(to: size, with: coordinator)

    /// (Optional) Additional step 1. Depending on your layout, you may have to manually indicate that the content size of a visible cells has changed
    /// Use that step if you experience the `the behavior of the UICollectionViewFlowLayout is not defined` errors.

    collectionView.visibleCells.forEach { cell in
        guard let cell = cell as? CustomCell else {
            print("`viewWillTransition` failed. Wrong cell type")
            return
        }

        cell.recalculateFrame(newSize: size)

    }

    /// (Optional) Additional step 2. Recalculate layout if you've explicitly set the estimatedCellSize and you'll notice that layout changes aren't automatically visible after the #3

    (collectionView.collectionViewLayout as? CustomLayout)?.recalculateLayout(size: size)


    /// Step 3 (or 1 if none of the above is applicable)

    coordinator.animate(alongsideTransition: { context in
        self.collectionView.collectionViewLayout.invalidateLayout()
    }) { _ in
        // code to execute when the transition's finished.
    }

}

/// Example implementations of the `recalculateFrame` and `recalculateLayout` methods:

    /// Within the `CustomCell` class:
    func recalculateFrame(newSize: CGSize) {
        self.frame = CGRect(x: self.bounds.origin.x,
                            y: self.bounds.origin.y,
                            width: newSize.width - 14.0,
                            height: self.frame.size.height)
    }

    /// Within the `CustomLayout` class:
    func recalculateLayout(size: CGSize? = nil) {
        estimatedItemSize = CGSize(width: size.width - 14.0, height: 100)
    }

    /// IMPORTANT: Within the `CustomLayout` class.
    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {

        guard let collectionView = collectionView else {
            return super.shouldInvalidateLayout(forBoundsChange: newBounds)
        }

        if collectionView.bounds.width != newBounds.width || collectionView.bounds.height != newBounds.height {
            return true
        } else {
            return false
        }
    }

Answer 5

在UICollectionView的边界更改或方向更改时使用FlowLayout调整项目大小/布局的步骤。

子类UICollectionViewFlowLayout
覆盖shouldInvalidateLayout(forBoundsChange:)以返回true。您可以使用此方法来确定当特定范围发生更改时，此布局是否应自动调用invalidateLayout(with:)。基类UICollectionViewLayout总是返回false，也就是说，在边界改变时不会使任何内容失效。但是，UICollectionViewFlowLayout有时会返回true来完成自己的工作。 要违反现有功能，您一开始会叫super，只有在super返回false时，您才做额外的工作。
覆盖invalidateLayout(with:)，以控制要完全无效的内容。 context参数管理有关此布局的所有详细信息。对于FlowLayout，它提供了其他控件，包括invalidateFlowLayoutDelegateMetrics（是否要调整项目的大小，即通过调用collectionView(:layout:sizeForItemAt:)重新计算项目的大小）和invalidateFlowLayoutAttributes（是否要重新分发项目，例如，重新计算项目的位置）。您可以在调用super之前操纵此context。

示例代码（宽度更改时调整大小和重新布局项目）。

final class FlowLayout: UICollectionViewFlowLayout {
    
    /// A cache for `newBounds` parameter in `shouldInvalidateLayout(forBoundsChange:)`.
    var bounds: CGRect?
    
    /// Should we invalidate both size and layout information for next layout.
    var invalidatesAll = false
    
    override func invalidateLayout(with context: UICollectionViewLayoutInvalidationContext) {
        guard let flowContext = context as? UICollectionViewFlowLayoutInvalidationContext else {
            return
        }
        if invalidatesAll {
            // Tell layout to recompute the size of items
            flowContext.invalidateFlowLayoutDelegateMetrics = true
            // Tell layout to recompute the layout of items
            flowContext.invalidateFlowLayoutAttributes = true
            // Reset this flag, because we do this only if there is a width change.
            invalidatesAll = false
        }
        super.invalidateLayout(with: flowContext)
    }
    
    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        if super.shouldInvalidateLayout(forBoundsChange: newBounds) {
            // If super (flowlayout) has already determined an invalidation, we simply hand over.
            return true
        }
        
        // We make sure there is a width change to avoid unnecessory invalidations.
        // Note: for the first time, `bounds` is always nil and flow layout always do first layout automatically, so we want to avoid this too.
        let isWidthChanged = bounds != nil && newBounds.size.width != bounds?.size.width
        if isWidthChanged {
            // A width change happens!, and we want to recompute both size and layout on width change.
            invalidatesAll = true
        }
        bounds = newBounds
        return isWidthChanged
    }
}

UICollectionView使边界更改上的布局无效

5 个答案:

在UICollectionView的边界更改或方向更改时使用FlowLayout调整项目大小/布局的步骤。

示例代码（宽度更改时调整大小和重新布局项目）。