[Code style] Improve code and implement spm package

Package.swift

@@ -0,0 +1,24 @@
+// swift-tools-version:5.3
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+import PackageDescription
+
+let package = Package(
+    name: "UPCarouselFlowLayout",
+    platforms: [
+        .iOS(.v11),
+        .watchOS(.v6)
+    ],
+    products: [
+        .library(
+            name: "UPCarouselFlowLayout",
+            type: .static,
+            targets: ["UPCarouselFlowLayout"]
+        )
+    ],
+    dependencies: [],
+    targets: [
+        .target(
+            name: "UPCarouselFlowLayout"
+        )
+    ]
+)

Sources/UPCarouselFlowLayout/UPCarouselFlowLayout.swift

@@ -0,0 +1,187 @@
+//
+//  UPCarouselFlowLayout.swift
+//  UPCarouselFlowLayoutDemo
+//
+//  Created by Paul Ulric on 23/06/2016.
+//  Copyright © 2016 Paul Ulric. All rights reserved.
+//
+
+import UIKit
+
+// MARK: - Enums
+
+public enum CarouselFlowLayoutSpacingMode {
+    case fixed(spacing: CGFloat)
+    case overlap(visibleOffset: CGFloat)
+}
+
+// MARK: - Class
+
+open class CarouselFlowLayout: UICollectionViewFlowLayout {
+
+    // MARK: - Properties
+
+    // Inspectable properties
+
+    @IBInspectable open var sideItemScale: CGFloat = 0.8
+    @IBInspectable open var sideItemAlpha: CGFloat = 0.6
+    @IBInspectable open var sideItemShift: CGFloat = 0.0
+
+    // Open properties
+
+    open var spacingMode = CarouselFlowLayoutSpacingMode.fixed(spacing: 8)
+
+    // File private properties
+
+    fileprivate var state = LayoutState(size: .zero, direction: .horizontal)
+
+    // MARK: - Methods
+
+    // Open methods
+
+    override open func prepare() {
+        super.prepare()
+        let currentState = LayoutState(
+            size: collectionView?.bounds.size ?? .init(),
+            direction: scrollDirection
+        )
+
+        if !self.state.isEqual(currentState) {
+            self.setupCollectionView()
+            self.updateLayout()
+            self.state = currentState
+        }
+    }
+
+    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) })
+    }
+
+    override open func targetContentOffset(
+        forProposedContentOffset proposedContentOffset: CGPoint,
+        withScrollingVelocity velocity: CGPoint
+    ) -> CGPoint {
+        guard
+            let collectionView = collectionView, !collectionView.isPagingEnabled,
+            let layoutAttributes = layoutAttributesForElements(in: collectionView.bounds)
+        else {
+            return super.targetContentOffset(forProposedContentOffset: proposedContentOffset)
+        }
+
+        let isHorizontal = 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 ?? .init()
+
+            targetContentOffset = .init(
+                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 ?? .init()
+
+            targetContentOffset = .init(
+                x: proposedContentOffset.x,
+                y: floor(closest.center.y - midSide)
+            )
+        }
+
+        return targetContentOffset
+    }
+
+    // File private methods
+
+    fileprivate func setupCollectionView() {
+        guard let collectionView = collectionView else { return }
+        if collectionView.decelerationRate != UIScrollView.DecelerationRate.fast {
+            collectionView.decelerationRate = UIScrollView.DecelerationRate.fast
+        }
+    }
+
+    fileprivate func updateLayout() {
+        guard let collectionView = collectionView else { return }
+
+        let collectionSize = collectionView.bounds.size
+        let isHorizontal = scrollDirection == .horizontal
+
+        let yInset = (collectionSize.height - itemSize.height) / 2
+        let xInset = (collectionSize.width - itemSize.width) / 2
+        sectionInset = .init(top: yInset, left: xInset, bottom: yInset, right: xInset)
+
+        let side = isHorizontal ? itemSize.width : itemSize.height
+        let scaledItemOffset =  (side - side*sideItemScale) / 2
+
+        switch self.spacingMode {
+        case .fixed(let spacing):
+            minimumLineSpacing = spacing - scaledItemOffset
+        case .overlap(let visibleOffset):
+            let fullSizeSideItemOverlap = visibleOffset + scaledItemOffset
+            let inset = isHorizontal ? xInset : yInset
+            minimumLineSpacing = inset - fullSizeSideItemOverlap
+        }
+    }
+
+    fileprivate func transformLayoutAttributes(_ attributes: UICollectionViewLayoutAttributes) -> UICollectionViewLayoutAttributes {
+
+        guard let collectionView = collectionView else { return attributes }
+
+        let isHorizontal = 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 ? itemSize.width : itemSize.height) + minimumLineSpacing
+        let distance = min(abs(collectionCenter - normalizedCenter), maxDistance)
+        let ratio = (maxDistance - distance)/maxDistance
+
+        let alpha = ratio * (1 - sideItemAlpha) + sideItemAlpha
+        let scale = ratio * (1 - sideItemScale) + sideItemScale
+        let shift = (1 - ratio) * sideItemShift
+
+        attributes.alpha = alpha
+        attributes.transform3D = CATransform3DScale(CATransform3DIdentity, scale, scale, 1)
+        attributes.zIndex = Int(alpha * 10)
+
+        if isHorizontal {
+            attributes.center.y += shift
+        } else {
+            attributes.center.x += shift
+        }
+
+        return attributes
+    }
+}
+
+// MARK: - Structs
+
+extension CarouselFlowLayout {
+    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
+        }
+    }
+}

UPCarouselFlowLayout.podspec

@@ -13,6 +13,6 @@ Pod::Spec.new do |s|
   s.ios.deployment_target = '8.1'
   s.swift_version	= '4.2'
 
-  s.source_files = 'UPCarouselFlowLayout/**/*.{h,swift}'
+  s.source_files = 'Sources/UPCarouselFlowLayout/**/*.{h,swift}'
 
 end