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UPCarouselFlowLayout.swift
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UPCarouselFlowLayout.swift
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//
// UPCarouselFlowLayout.swift
// UPCarouselFlowLayoutDemo
//
// Created by Paul Ulric on 23/06/2016.
// Copyright © 2016 Paul Ulric. All rights reserved.
//
import UIKit
public enum UPCarouselFlowLayoutSpacingMode {
case fixed(spacing: CGFloat)
case overlap(visibleOffset: CGFloat)
}
open class UPCarouselFlowLayout: UICollectionViewFlowLayout {
fileprivate struct LayoutState {
var size: CGSize
var direction: UICollectionView.ScrollDirection
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 = 0.6
@IBInspectable open var sideItemShift: CGFloat = 0.0
open var spacingMode = UPCarouselFlowLayoutSpacingMode.fixed(spacing: 40)
fileprivate var state = LayoutState(size: CGSize.zero, direction: .horizontal)
override open func prepare() {
super.prepare()
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 != UIScrollView.DecelerationRate.fast {
collectionView.decelerationRate = UIScrollView.DecelerationRate.fast
}
}
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 = UIEdgeInsets.init(top: yInset, left: xInset, bottom: yInset, right: 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
attributes.alpha = alpha
attributes.transform3D = CATransform3DScale(CATransform3DIdentity, scale, scale, 1)
attributes.zIndex = Int(alpha * 10)
if isHorizontal {
attributes.center.y = attributes.center.y + shift
} 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
}
}