iOS UICollectionView 卡片效果传送带效果 Carousel FlowLayout

雨点打透心脏的1/2处 2022-03-02 10:12 376阅读 0赞

# 场景 #

在我们的app中需要一个类似影院传送带式选择电影场次的控件，效果如下：

![Image 1][]

CarouselCenter.gif

# 实现思路 #

1.  控件选择  
    看控件特征，是一个可滚动的长列表，在iOS中一般都使用`UICollectionView`来展现，这里我们也选择它。
2.  布局选择  
    `UICollectionView`中每个`item`的显示样式都通过`UICollectionViewLayout`来控制，这里明显是一个“流式布局”，我们可以选择`UICollectionViewFlowLayout`来定制样式。
3.  布局控制  
    在上面的显示效果中，我们需要控制两个点：
    
     *  缩放效果
        
            // 该方法指定UICollectionView的每个item滚动到相应rect的显示效果（UICollectionViewLayoutAttributes，包含尺寸、透明度等信息）
            open func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]?
     *  滚动结束，定位到中间位置
        
            // 该方法指定UICollectionView滚动到的目标位置
            // a layout can return the content offset to be applied during transition or update animations
            open func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint) -> CGPoint

# 实现细节 #

*  缩放效果

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
        attributes.alpha = alpha
        attributes.transform3D = CATransform3DScale(CATransform3DIdentity, scale, scale, 1)
        attributes.zIndex = Int(alpha * 10)
    
        let scrollDirectionItemHeight = isHorizontal ? itemSize.height : itemSize.width
        var sideItemFixedOffset: CGFloat = 0
        switch sideItemBaselineType {
        case .top:
            sideItemFixedOffset = -(scrollDirectionItemHeight - scrollDirectionItemHeight * self.sideItemScale) / 2
        case .center:
            sideItemFixedOffset = 0
        case .bottom:
            sideItemFixedOffset = (scrollDirectionItemHeight - scrollDirectionItemHeight * self.sideItemScale) / 2
        }
        let shift = (1 - ratio) * (sideItemOffset + sideItemFixedOffset)
        if isHorizontal {
            attributes.center.y += shift
        } else {
            attributes.center.x += shift
        }
        
        return attributes
    }

*  滚动结束，定位到中间位置

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

# 封装 #

[Demo][]

拓展实现

*  `WTCarouselFlowLayoutBaselineType.top`
    
    ![Image 1][]
    
    CarouselTop.gif
    
     
 *  `WTCarouselFlowLayoutBaselineType.center`
    
    ![Image 1][]
    
    CarouselCenter.gif
    
     
 *  `WTCarouselFlowLayoutBaselineType.bottom`
    
    ![Image 1][]
    
    CarouselBottom.gif
    
     
 *  `layout.itemSpacing = -15`
    
    ![Image 1][]
    
    CarouselOverlap.gif

# 集成使用 #

*  Requirements  
    iOS 8.0+
 *  CocoaPods
    
        pod "WTCarouselFlowLayout"
 *  Example
    
        import WTCarouselFlowLayout
    
        let layout = self.collectionView.collectionViewLayout as! WTCarouselFlowLayout
        layout.itemSize = CGSize(width: 70, height: 100)
        layout.scrollDirection = .horizontal
        layout.spacingMode = WTCarouselFlowLayoutSpacingMode.between(spacing: 50)
        //  layout.spacingMode = WTCarouselFlowLayoutSpacingMode.overlap(overlapSpacing: 15)
        layout.sideItemScale = 0.7
        layout.sideItemAlpha = 0.7
        layout.sideItemBaselineType = .center
        layout.sideItemOffset = 0.0

# 参考 #

[UPCarouselFlowLayout][]

作者：wentianen  
链接：https://www.jianshu.com/p/1a45ecdc2d5c  
来源：简书  
简书著作权归作者所有，任何形式的转载都请联系作者获得授权并注明出处。

[Image 1]: 
[Demo]: https://link.jianshu.com?t=https://github.com/wusuowei/WTCarouselFlowLayout
[UPCarouselFlowLayout]: https://link.jianshu.com?t=https://github.com/ink-spot/UPCarouselFlowLayout