swift - collectionView中如何从第二个item开始？

Question

现在我有一个总是从第一个元素开始的 Collection View ，但我希望用户完成关卡，一旦他们完成， Collection View 应该从第二个元素开始，如下图所示。

一段时间后，当关卡完成时，从第二个元素开始，如下所示:

你知道怎么做吗？到目前为止，我已经做到了这一点。

  public protocol SnapLikeDataDelegate: class {
func cellSelected(_ index: Int)
}

 public class SnapLikeDataSource<Cell: UICollectionViewCell>: NSObject, UICollectionViewDelegate, UICollectionViewDataSource where Cell: SnapLikeCell {

enum NearestPointDirection: Int {
    case any
    case left
    case right
}
private var scrollVelocity: CGFloat = 0.0
private var selectedItem: Int = 0

public weak var delegate: SnapLikeDataDelegate?
public var items: [Cell.Item] = []
public var images: [Cell.Image] = []

private let selectionFB = UISelectionFeedbackGenerator()

private weak var collectionView: UICollectionView?
private var collectionViewCenter: CGFloat
private let cellSize: SnapLikeCellSize

public init(collectionView: UICollectionView, cellSize: SnapLikeCellSize) {
    self.collectionView = collectionView
    self.collectionViewCenter = collectionView.bounds.width / 2
    self.cellSize = cellSize
}

// MARK: - Animation:

public func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
    if self.selectedItem == Int.max { return }

    let previousSelectedIndex: Int = selectedItem
    // add a placeholder value for selectedItem while scrolling
    selectedItem = Int.max

    reloadCell(at: IndexPath(item: previousSelectedIndex, section: 0), withSelectedState: false)
}

public func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
    scrollVelocity = velocity.x

    if scrollVelocity == 0 {
        targetContentOffset.pointee = offset(forCenterX: targetContentOffset.pointee.x + collectionViewCenter, with: .any)
    }
    if scrollVelocity < 0 {
        targetContentOffset.pointee = offset(forCenterX: targetContentOffset.pointee.x + collectionViewCenter, with: .left)
    } else if scrollVelocity > 0 {
        targetContentOffset.pointee = offset(forCenterX: targetContentOffset.pointee.x + collectionViewCenter, with: .right)
    }

}

public func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    reloadCell(at: IndexPath(item: selectedItem, section: 0), withSelectedState: true)
    selectionFB.selectionChanged()
    delegate?.cellSelected(selectedItem)
}

/// Reload cell so it becomes selected or unselected
public func reloadCell(at indexPath: IndexPath, withSelectedState selected: Bool) {
    if let cell = collectionView?.cellForItem(at: indexPath) {
        cell.isSelected = selected
    }
}

public func selectItem(_ indexPath: IndexPath) {
    perform(#selector(collectionView(_:didSelectItemAt:)),
            with: collectionView,
            with: indexPath)
}

// Calculate the offset to the center from the nearest cell
func offset(forCenterX centerX: CGFloat, with direction: NearestPointDirection) -> CGPoint {
    let leftNearestCenters = nearestLeftCenter(forCenterX: centerX)
    let leftCenterIndex: Int = leftNearestCenters.index
    let leftCenter: CGFloat = leftNearestCenters.value
    let rightNearestCenters = nearestRightCenter(forCenterX: centerX)
    let rightCenterIndex: Int = rightNearestCenters.index
    let rightCenter: CGFloat = rightNearestCenters.value
    var nearestItemIndex: Int = Int.max
    switch direction {
    case .any:
        if leftCenter > rightCenter {
            nearestItemIndex = rightCenterIndex
        } else {
            nearestItemIndex = leftCenterIndex
        }
    case .left:
        nearestItemIndex = leftCenterIndex
    case .right:
        nearestItemIndex = rightCenterIndex
    }
    selectedItem = nearestItemIndex
    return CGPoint(x: CGFloat(nearestItemIndex) * cellSize.normalWidth, y: 0.0)
}

/// Getting the nearest cell attributes on the left
func nearestLeftCenter(forCenterX centerX: CGFloat) -> (index: Int, value: CGFloat) {
    let nearestLeftElementIndex: CGFloat = (centerX - collectionViewCenter - cellSize.centerWidth + cellSize.normalWidth) / cellSize.normalWidth
    let minimumLeftDistance: CGFloat = centerX - nearestLeftElementIndex * cellSize.normalWidth - collectionViewCenter - cellSize.centerWidth + cellSize.normalWidth
    return (Int(nearestLeftElementIndex), minimumLeftDistance)
}

/// Getting the nearest cell attributes on the right
func nearestRightCenter(forCenterX centerX: CGFloat) -> (index: Int, value: CGFloat) {
    let nearestRightElementIndex: Int = Int(ceilf(Float((centerX - collectionViewCenter - cellSize.centerWidth + cellSize.normalWidth) / cellSize.normalWidth)))
    let minimumRightDistance: CGFloat = CGFloat(nearestRightElementIndex) * cellSize.normalWidth + collectionViewCenter - centerX - cellSize.centerWidth + cellSize.normalWidth
    return (nearestRightElementIndex, minimumRightDistance)
}

// MARK : - Delegate

@objc public func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    collectionView.deselectItem(at: indexPath, animated: false)

    scrollViewWillBeginDragging(collectionView)
    selectedItem = indexPath.item

    let layout = collectionView.collectionViewLayout as! SnapLikeCollectionViewFlowLayout
    let x: CGFloat = CGFloat(selectedItem) * cellSize.normalWidth
    layout.ignoringBoundsChange = true
    collectionView.setContentOffset(CGPoint(x: x, y: 0), animated: true)
    layout.ignoringBoundsChange = false

    perform(#selector(self.scrollViewDidEndDecelerating), with: collectionView, afterDelay: 0.3)
}


  public func numberOfSections(in collectionView: UICollectionView) -> Int {
    return 1
}

public func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return items.count
}

public func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: String(describing: Cell.self), for: indexPath) as! Cell
    cell.item = items[indexPath.item]
    cell.image = images[indexPath.item]
    return cell
}

在我的 View Controller 中，我这样做:

         var logoImage: [UIImage] = [
         UIImage(named: "1") ?? UIImage(named: "1")!,
         UIImage(named: "2") ?? UIImage(named: "1")!,
         UIImage(named: "3") ?? UIImage(named: "1")!,
         UIImage(named: "2") ?? UIImage(named: "1")!,
         UIImage(named: "1") ?? UIImage(named: "1")!
]

    dataSource?.items = ["1", "2", "3", "4", "F"]
    dataSource?.images = logoImage

Answer 1

我使用了这个函数:

 collectionView.scrollToItem(at: IndexPath(item: 3, section: 0), at: [], animated: false)

它起作用了，一旦应用程序启动，CollectionView 就会显示项目集中的第三个元素。