在UICollectionView中设置特定单元格的动画

Question

有没有办法在启动视图控制器时为特定单元格设置动画？

上面是一个collectionView单元格，在屏幕启动时需要显示第二个项目，同时需要显示左右单元格，并且动画略有淡入淡出。实现此动画的最佳方法是什么？我已经在自定义轮播动画中使用自定义flowLayout动画了。我的主要问题是如何在viewDidLoad上制作第1和第3个单元格动画？

Answer 1

我可以通过玩 UICollectionViewFlowLayout 来制作相同类型的动画。

您可以查看最终结果：https://imgur.com/a/lNoD2

基本上我使用UPCarouselFlowLayout Library来实现这个动画。

我将 sideItemOffsetShift 添加到自定义flowLayout：

//  UPCarouselFlowLayout.swift
//  UPCarouselFlowLayoutDemo
//
//  Created by Paul Ulric on 23/06/2016.
//  Copyright © 2016 Paul Ulric. All rights reserved.

public enum UPCarouselFlowLayoutSpacingMode {
  case fixed(spacing: CGFloat)
  case overlap(visibleOffset: CGFloat)
}

class CustomLayout: UICollectionViewFlowLayout {
  fileprivate struct LayoutState {
    var size: CGSize
    var direction: UICollectionViewScrollDirection
    func isEqual(_ otherState: LayoutState) -> Bool {
      return self.size.equalTo(otherState.size) && self.direction == otherState.direction
    }
  }

  @IBInspectable open var sideItemScale: CGFloat = 0.6
  @IBInspectable open var sideItemAlpha: CGFloat = 0.0
  @IBInspectable open var sideItemShift: CGFloat = 0.0
  @IBInspectable open var sideItemOffsetShift: CGFloat = 50.0
  open var spacingMode = UPCarouselFlowLayoutSpacingMode.fixed(spacing: 10)

  fileprivate var state = LayoutState(size: CGSize.zero, direction: .horizontal)


  override open func prepare() {
    super.prepare()

    self.scrollDirection = .horizontal
    let currentState = LayoutState(size: self.collectionView!.bounds.size, direction: self.scrollDirection)

    if !self.state.isEqual(currentState) {
      self.setupCollectionView()
      self.updateLayout()
      self.state = currentState
    }
  }

  fileprivate func setupCollectionView() {
    guard let collectionView = self.collectionView else { return }
    if collectionView.decelerationRate != UIScrollViewDecelerationRateFast {
      collectionView.decelerationRate = UIScrollViewDecelerationRateFast
    }
  }

  fileprivate func updateLayout() {
    guard let collectionView = self.collectionView else { return }

    let collectionSize = collectionView.bounds.size
    let isHorizontal = (self.scrollDirection == .horizontal)

    let yInset = (collectionSize.height - self.itemSize.height) / 2
    let xInset = (collectionSize.width - self.itemSize.width) / 2
    self.sectionInset = UIEdgeInsetsMake(yInset, xInset, yInset, xInset)

    let side = isHorizontal ? self.itemSize.width : self.itemSize.height
    let scaledItemOffset =  (side - side*self.sideItemScale) / 2
    switch self.spacingMode {
    case .fixed(let spacing):
      self.minimumLineSpacing = spacing - scaledItemOffset
    case .overlap(let visibleOffset):
      let fullSizeSideItemOverlap = visibleOffset + scaledItemOffset
      let inset = isHorizontal ? xInset : yInset
      self.minimumLineSpacing = inset - fullSizeSideItemOverlap
    }
  }

  override open func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
    return true
  }

  override open func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
    guard let superAttributes = super.layoutAttributesForElements(in: rect),
      let attributes = NSArray(array: superAttributes, copyItems: true) as? [UICollectionViewLayoutAttributes]
      else { return nil }
    return attributes.map({ self.transformLayoutAttributes($0) })
  }

  fileprivate func transformLayoutAttributes(_ attributes: UICollectionViewLayoutAttributes) -> UICollectionViewLayoutAttributes {
    guard let collectionView = self.collectionView else { return attributes }
    let isHorizontal = (self.scrollDirection == .horizontal)

    let collectionCenter = isHorizontal ? collectionView.frame.size.width/2 : collectionView.frame.size.height/2
    let offset = isHorizontal ? collectionView.contentOffset.x : collectionView.contentOffset.y
    let normalizedCenter = (isHorizontal ? attributes.center.x : attributes.center.y) - offset

    let maxDistance = (isHorizontal ? self.itemSize.width : self.itemSize.height) + self.minimumLineSpacing
    let distance = min(abs(collectionCenter - normalizedCenter), maxDistance)
    let ratio = (maxDistance - distance)/maxDistance

    let alpha = ratio * (1 - self.sideItemAlpha) + self.sideItemAlpha
    let scale = ratio * (1 - self.sideItemScale) + self.sideItemScale
    let shift = (1 - ratio) * self.sideItemShift
    let offsetShift = (1 - ratio) * self.sideItemOffsetShift
    attributes.alpha = alpha
    attributes.transform3D = CATransform3DScale(CATransform3DIdentity, scale, scale, 1)
    attributes.zIndex = Int(alpha * 10)

    if isHorizontal {
      attributes.center.y = attributes.center.y + shift

      if normalizedCenter < collectionCenter {
        attributes.center.x = attributes.center.x - offsetShift
      } else {
        attributes.center.x = attributes.center.x + offsetShift
      }
    } else {
      attributes.center.x = attributes.center.x + shift
    }

    return attributes
  }

  override open func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
    guard let collectionView = collectionView , !collectionView.isPagingEnabled,
      let layoutAttributes = self.layoutAttributesForElements(in: collectionView.bounds)
      else { return super.targetContentOffset(forProposedContentOffset: proposedContentOffset) }

    let isHorizontal = (self.scrollDirection == .horizontal)

    let midSide = (isHorizontal ? collectionView.bounds.size.width : collectionView.bounds.size.height) / 2
    let proposedContentOffsetCenterOrigin = (isHorizontal ? proposedContentOffset.x : proposedContentOffset.y) + midSide

    var targetContentOffset: CGPoint
    if isHorizontal {
      let closest = layoutAttributes.sorted { abs($0.center.x - proposedContentOffsetCenterOrigin) < abs($1.center.x - proposedContentOffsetCenterOrigin) }.first ?? UICollectionViewLayoutAttributes()
      targetContentOffset = CGPoint(x: floor(closest.center.x - midSide), y: proposedContentOffset.y)
    }
    else {
      let closest = layoutAttributes.sorted { abs($0.center.y - proposedContentOffsetCenterOrigin) < abs($1.center.y - proposedContentOffsetCenterOrigin) }.first ?? UICollectionViewLayoutAttributes()
      targetContentOffset = CGPoint(x: proposedContentOffset.x, y: floor(closest.center.y - midSide))
    }

    return targetContentOffset
  }
}

在我的viewController中，我有：

override func viewDidLoad() {
    super.viewDidLoad()

    // scroll to index 1 
    let indexPath = IndexPath(item: 1, section: 0)
    collectionView?.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: false)

    DispatchQueue.main.async {
      self.updateCollectionView()
    }
  }

  func updateCollectionView() {
    if let layout = collectionView?.collectionViewLayout as? CustomLayout {

      UIView.animate(withDuration: 0.5, animations: {
        layout.sideItemAlpha = 0.6
        layout.sideItemOffsetShift = 0
        self.collectionView?.collectionViewLayout.invalidateLayout()
      })
    }
  }

因此，默认情况下，加载集合时，会隐藏旁边项目。然后我们调用 updateCollectionView（）来更新 sideItemOffsetShift 和 sideItemAlpha 。