动态高度的分页效果.swift

var currentSection = 0

extension MoviesSubViewController: UIScrollViewDelegate {

    public func scrollViewDidScroll(_ scrollView: UIScrollView) {
        guard collectionView == scrollView as? UICollectionView else { return }
        let touches = scrollView.panGestureRecognizer.numberOfTouches
        guard scrollView.isDecelerating && touches == 0 else { return }
        guard collectionView.numberOfSections > currentSection else { return }
        let cellCount = collectionView.numberOfItems(inSection: currentSection)
        let minY = collectionView.cellForItem(at: IndexPath(row: 0, section: currentSection))?.frame.minY ?? -300
        let maxY = collectionView.cellForItem(
            at: IndexPath(row: cellCount - 1, section: currentSection)
        )?.frame.maxY ?? (minY + view.height)
        if scrollView.contentOffset.y < minY {
            scrollView.contentOffset.y = minY
        }
        let maxOffset = maxY - view.height
        if maxOffset > minY, scrollView.contentOffset.y > maxOffset {
            scrollView.contentOffset.y = maxOffset
        }
    }
    
    public func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        guard !decelerate else { return }
        scrollToNearestSection(scrollView)
    }

    public func scrollViewWillBeginDecelerating(_ scrollView: UIScrollView) {
        let velocity = scrollView.panGestureRecognizer.velocity(in: self.view)
        if velocity.y > 100 {
            scrollToPreviousSection(scrollView)
        }
        else if velocity.y < -100 {
            scrollToNextSection(scrollView)
        }
        else {
            scrollToNearestSection(scrollView)
        }
    }
    
    public func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {

    }

    private func scrollToNextSection(_ scrollView: UIScrollView) {
        scrollToNearestSection(scrollView, offset: 1)
    }

    private func scrollToPreviousSection(_ scrollView: UIScrollView) {
        scrollToNearestSection(scrollView, offset: -1)
    }

    private func scrollToNearestSection(_ scrollView: UIScrollView, offset: Int = 0) {
        let visibleSections = Set(collectionView.indexPathsForVisibleItems.map { $0.section })
        guard visibleSections.count > 1 else { return }
        guard collectionView == scrollView as? UICollectionView else { return }
        // 以屏幕中间的位置为准，找到最近的cell 对应的 section
        let point = CGPoint(x: 100, y: view.height * 0.5)
        let centerPoint = collectionView.convert(point, from: view)
        guard let indexPath = collectionView.indexPathForItem(at: centerPoint) else { return }
        guard indexPath.section + offset < collectionView.numberOfSections else {
            return
        }
        var targetSection = indexPath.section
        if (offset != 0) {
            targetSection = max(currentSection + offset, 0)
        }
        let tragetIndexPath = IndexPath(row: 0, section: targetSection)
        currentSection = tragetIndexPath.section
        collectionView.scrollToItem(at: tragetIndexPath, at: .top, animated: true)
    }
}