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实现: 自定义 UICollectionViewFlowLayout

//
//  AlignedCollectionViewFlowLayout.swift
//
//  Created by Mischa Hildebrand on 12/04/2017.
//  Copyright © 2017 Mischa Hildebrand.
//
//  Licensed under the terms of the MIT license:
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in
//  all copies or substantial portions of the Software.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
//  THE SOFTWARE.
//
import UIKit
// MARK: - 🦆 Type definitions
/// An abstract protocol that defines an alignment.
protocol Alignment {}

/// Defines an alignment for UI elements.
public enum HorizontalAlignment: Alignment {
    case left
    case justified
    case right
}

/// Defines a vertical alignment for UI elements.
public enum VerticalAlignment: Alignment {
    case top
    case center
    case bottom
}

/// Describes an axis with respect to which items can be aligned.
private struct AlignmentAxis<A: Alignment> {
    
    /// Determines how items are aligned relative to the axis.
    let alignment: A
    
    /// Defines the position of the axis.
    /// * If the `Alignment` is horizontal, the alignment axis is vertical and this is the position on the `x` axis.
    /// * If the `Alignment` is vertical, the alignment axis is horizontal and this is the position on the `y` axis.
    let position: CGFloat
}


/// A `UICollectionViewFlowLayout` subclass that gives you control
/// over the horizontal and vertical alignment of the cells.
/// You can use it to align the cells like words in a left- or right-aligned text
/// and you can specify how the cells are vertically aligned in their row.
public class AlignedCollectionViewFlowLayout: UICollectionViewFlowLayout {
    
    // MARK: - 🔶 Properties
    
    /// Determines how the cells are horizontally aligned in a row.
    /// - Note: The default is `.justified`.
    public var horizontalAlignment: HorizontalAlignment = .justified
    
    /// Determines how the cells are vertically aligned in a row.
    /// - Note: The default is `.center`.
    public var verticalAlignment: VerticalAlignment = .center
    
    /// The vertical axis with respect to which the cells are horizontally aligned.
    /// For a `justified` alignment the alignment axis is not defined and this value is `nil`.
    fileprivate var alignmentAxis: AlignmentAxis<HorizontalAlignment>? {
        switch horizontalAlignment {
        case .left:
            return AlignmentAxis(alignment: HorizontalAlignment.left, position: sectionInset.left)
        case .right:
            guard let collectionViewWidth = collectionView?.frame.size.width else {
                return nil
            }
            return AlignmentAxis(alignment: HorizontalAlignment.right, position: collectionViewWidth - sectionInset.right)
        default:
            return nil
        }
    }
    
    /// The width of the area inside the collection view that can be filled with cells.
    private var contentWidth: CGFloat? {
        guard let collectionViewWidth = collectionView?.frame.size.width else {
            return nil
        }
        return collectionViewWidth - sectionInset.left - sectionInset.right
    }
    
    
    // MARK: - 👶 Initialization
    
    /// The designated initializer.
    ///
    /// - Parameters:
    ///   - horizontalAlignment: Specifies how the cells are horizontally aligned in a row. --
    ///                          (Default: `.justified`)
    ///   - verticalAlignment:   Specified how the cells are vertically aligned in a row. --
    ///                          (Default: `.center`)
    public init(horizontalAlignment: HorizontalAlignment = .justified, verticalAlignment: VerticalAlignment = .center) {
        super.init()
        self.horizontalAlignment = horizontalAlignment
        self.verticalAlignment = verticalAlignment
    }
    
    required public init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
    
    
    // MARK: - 🅾️ Overrides
    
    override public func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        
        // 💡 IDEA:
        // The approach for computing a cell's frame is to create a rectangle that covers the current line.
        // Then we check if the preceding cell's frame intersects with this rectangle.
        // If it does, the current item is not the first item in the line. Otherwise it is.
        // (Vice-versa for right-aligned cells.)
        //
        // +---------+----------------------------------------------------------------+---------+
        // |         |                                                                |         |
        // |         |     +------------+                                             |         |
        // |         |     |            |                                             |         |
        // | section |- - -|- - - - - - |- - - - +---------------------+ - - - - - - -| section |
        // |  inset  |     |intersection|        |                     |   line rect  |  inset  |
        // |         |- - -|- - - - - - |- - - - +---------------------+ - - - - - - -|         |
        // | (left)  |     |            |             current item                    | (right) |
        // |         |     +------------+                                             |         |
        // |         |     previous item                                              |         |
        // +---------+----------------------------------------------------------------+---------+
        //
        // ℹ️ We need this rather complicated approach because the first item in a line
        //    is not always left-aligned and the last item in a line is not always right-aligned:
        //    If there is only one item in a line UICollectionViewFlowLayout will center it.
        
        // We may not change the original layout attributes or UICollectionViewFlowLayout might complain.
        guard let layoutAttributes = super.layoutAttributesForItem(at: indexPath)?.copy() as? UICollectionViewLayoutAttributes else {
            return nil
        }
        
        // For a justified layout there's nothing to do here
        // as UICollectionViewFlowLayout justifies the items in a line by default.
        if horizontalAlignment != .justified {
            layoutAttributes.alignHorizontally(collectionViewLayout: self)
        }
        
        // For a vertically centered layout there's nothing to do here
        // as UICollectionViewFlowLayout center-aligns the items in a line by default.
        if verticalAlignment != .center {
            layoutAttributes.alignVertically(collectionViewLayout: self)
        }
        
        return layoutAttributes
    }
    
    override public func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        // We may not change the original layout attributes or UICollectionViewFlowLayout might complain.
        let layoutAttributesObjects = copy(super.layoutAttributesForElements(in: rect))
        layoutAttributesObjects?.forEach({ (layoutAttributes) in
            setFrame(forLayoutAttributes: layoutAttributes)
        })
        return layoutAttributesObjects
    }
    
    
    // MARK: - 👷 Private layout helpers
    
    /// Sets the frame for the passed layout attributes object by calling the `layoutAttributesForItem(at:)` function.
    private func setFrame(forLayoutAttributes layoutAttributes: UICollectionViewLayoutAttributes) {
        if layoutAttributes.representedElementCategory == .cell { // Do not modify header views etc.
            let indexPath = layoutAttributes.indexPath
            if let newFrame = layoutAttributesForItem(at: indexPath)?.frame {
                layoutAttributes.frame = newFrame
            }
        }
    }
    
    /// A function to access the `super` implementation of `layoutAttributesForItem(at:)` externally.
    ///
    /// - Parameter indexPath: The index path of the item for which to return the layout attributes.
    /// - Returns: The unmodified layout attributes for the item at the specified index path
    ///            as computed by `UICollectionViewFlowLayout`.
    fileprivate func originalLayoutAttribute(forItemAt indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return super.layoutAttributesForItem(at: indexPath)
    }
    
    /// Determines if the `firstItemAttributes`' frame is in the same line
    /// as the `secondItemAttributes`' frame.
    ///
    /// - Parameters:
    ///   - firstItemAttributes:  The first layout attributes object to be compared.
    ///   - secondItemAttributes: The second layout attributes object to be compared.
    /// - Returns: `true` if the frames of the two layout attributes are in the same line, else `false`.
    ///            `false` is also returned when the layout's `collectionView` property is `nil`.
    fileprivate func isFrame(for firstItemAttributes: UICollectionViewLayoutAttributes, inSameLineAsFrameFor secondItemAttributes: UICollectionViewLayoutAttributes) -> Bool {
        guard let lineWidth = contentWidth else {
            return false
        }
        let firstItemFrame = firstItemAttributes.frame
        let lineFrame = CGRect(x: sectionInset.left,
                               y: firstItemFrame.origin.y,
                               width: lineWidth,
                               height: firstItemFrame.size.height)
        return lineFrame.intersects(secondItemAttributes.frame)
    }
    
    /// Determines the layout attributes objects for all items displayed in the same line as the item
    /// represented by the passed `layoutAttributes` object.
    ///
    /// - Parameter layoutAttributes: The layout attributed that represents the reference item.
    /// - Returns: The layout attributes objects representing all other items in the same line.
    ///            The passed `layoutAttributes` object itself is always contained in the returned array.
    fileprivate func layoutAttributes(forItemsInLineWith layoutAttributes: UICollectionViewLayoutAttributes) -> [UICollectionViewLayoutAttributes] {
        guard let lineWidth = contentWidth else {
            return [layoutAttributes]
        }
        var lineFrame = layoutAttributes.frame
        lineFrame.origin.x = sectionInset.left
        lineFrame.size.width = lineWidth
        return super.layoutAttributesForElements(in: lineFrame) ?? []
    }
    
    /// Copmutes the alignment axis with which to align the items represented by the `layoutAttributes` objects vertically.
    ///
    /// - Parameter layoutAttributes: The layout attributes objects to be vertically aligned.
    /// - Returns: The axis with respect to which the layout attributes can be aligned
    ///            or `nil` if the `layoutAttributes` array is empty.
    private func verticalAlignmentAxisForLine(with layoutAttributes: [UICollectionViewLayoutAttributes]) -> AlignmentAxis<VerticalAlignment>? {
        
        guard let firstAttribute = layoutAttributes.first else {
            return nil
        }
        
        switch verticalAlignment {
        case .top:
            let minY = layoutAttributes.reduce(CGFloat.greatestFiniteMagnitude) { min($0, $1.frame.minY) }
            return AlignmentAxis(alignment: .top, position: minY)
            
        case .bottom:
            let maxY = layoutAttributes.reduce(0) { max($0, $1.frame.maxY) }
            return AlignmentAxis(alignment: .bottom, position: maxY)
            
        default:
            let centerY = firstAttribute.center.y
            return AlignmentAxis(alignment: .center, position: centerY)
        }
    }
    
    /// Computes the axis with which to align the item represented by the `currentLayoutAttributes` vertically.
    ///
    /// - Parameter currentLayoutAttributes: The layout attributes representing the item to be vertically aligned.
    /// - Returns: The axis with respect to which the item can be aligned.
    fileprivate func verticalAlignmentAxis(for currentLayoutAttributes: UICollectionViewLayoutAttributes) -> AlignmentAxis<VerticalAlignment> {
        let layoutAttributesInLine = layoutAttributes(forItemsInLineWith: currentLayoutAttributes)
        // It's okay to force-unwrap here because we pass a non-empty array.
        return verticalAlignmentAxisForLine(with: layoutAttributesInLine)!
    }
    
    /// Creates a deep copy of the passed array by copying all its items.
    ///
    /// - Parameter layoutAttributesArray: The array to be copied.
    /// - Returns: A deep copy of the passed array.
    private func copy(_ layoutAttributesArray: [UICollectionViewLayoutAttributes]?) -> [UICollectionViewLayoutAttributes]? {
        return layoutAttributesArray?.map{ $0.copy() } as? [UICollectionViewLayoutAttributes]
    }
    
}


// MARK: - 👷 Layout attributes helpers
fileprivate extension UICollectionViewLayoutAttributes {
    
    private var currentSection: Int {
        return indexPath.section
    }
    
    private var currentItem: Int {
        return indexPath.item
    }
    
    /// The index path for the item preceding the item represented by this layout attributes object.
    private var precedingIndexPath: IndexPath {
        return IndexPath(item: currentItem - 1, section: currentSection)
    }
    
    /// The index path for the item following the item represented by this layout attributes object.
    private var followingIndexPath: IndexPath {
        return IndexPath(item: currentItem + 1, section: currentSection)
    }
    
    /// Checks if the item represetend by this layout attributes object is the first item in the line.
    ///
    /// - Parameter collectionViewLayout: The layout for which to perform the check.
    /// - Returns: `true` if the represented item is the first item in the line, else `false`.
    func isRepresentingFirstItemInLine(collectionViewLayout: AlignedCollectionViewFlowLayout) -> Bool {
        if currentItem <= 0 {
            return true
        }
        else {
            if let layoutAttributesForPrecedingItem = collectionViewLayout.originalLayoutAttribute(forItemAt: precedingIndexPath) {
                return !collectionViewLayout.isFrame(for: self, inSameLineAsFrameFor: layoutAttributesForPrecedingItem)
            }
            else {
                return true
            }
        }
    }
    
    /// Checks if the item represetend by this layout attributes object is the last item in the line.
    ///
    /// - Parameter collectionViewLayout: The layout for which to perform the check.
    /// - Returns: `true` if the represented item is the last item in the line, else `false`.
    func isRepresentingLastItemInLine(collectionViewLayout: AlignedCollectionViewFlowLayout) -> Bool {
        guard let itemCount = collectionViewLayout.collectionView?.numberOfItems(inSection: currentSection) else {
            return false
        }
        
        if currentItem >= itemCount - 1 {
            return true
        }
        else {
            if let layoutAttributesForFollowingItem = collectionViewLayout.originalLayoutAttribute(forItemAt: followingIndexPath) {
                return !collectionViewLayout.isFrame(for: self, inSameLineAsFrameFor: layoutAttributesForFollowingItem)
            }
            else {
                return true
            }
        }
    }
    
    /// Moves the layout attributes object's frame so that it is aligned horizontally with the alignment axis.
    func align(toAlignmentAxis alignmentAxis: AlignmentAxis<HorizontalAlignment>) {
        switch alignmentAxis.alignment {
        case .left:
            frame.origin.x = alignmentAxis.position
        case .right:
            frame.origin.x = alignmentAxis.position - frame.size.width
        default:
            break
        }
    }
    
    /// Moves the layout attributes object's frame so that it is aligned vertically with the alignment axis.
    func align(toAlignmentAxis alignmentAxis: AlignmentAxis<VerticalAlignment>) {
        switch alignmentAxis.alignment {
        case .top:
            frame.origin.y = alignmentAxis.position
        case .bottom:
            frame.origin.y = alignmentAxis.position - frame.size.height
        default:
            center.y = alignmentAxis.position
        }
    }
    
    /// Positions the frame right of the preceding item's frame, leaving a spacing between the frames
    /// as defined by the collection view layout's `minimumInteritemSpacing`.
    ///
    /// - Parameter collectionViewLayout: The layout on which to perfom the calculations.
    private func alignToPrecedingItem(collectionViewLayout: AlignedCollectionViewFlowLayout) {
        let itemSpacing = collectionViewLayout.minimumInteritemSpacing
        
        if let precedingItemAttributes = collectionViewLayout.layoutAttributesForItem(at: precedingIndexPath) {
            frame.origin.x = precedingItemAttributes.frame.maxX + itemSpacing
        }
    }
    
    /// Positions the frame left of the following item's frame, leaving a spacing between the frames
    /// as defined by the collection view layout's `minimumInteritemSpacing`.
    ///
    /// - Parameter collectionViewLayout: The layout on which to perfom the calculations.
    private func alignToFollowingItem(collectionViewLayout: AlignedCollectionViewFlowLayout) {
        let itemSpacing = collectionViewLayout.minimumInteritemSpacing
        
        if let followingItemAttributes = collectionViewLayout.layoutAttributesForItem(at: followingIndexPath) {
            frame.origin.x = followingItemAttributes.frame.minX - itemSpacing - frame.size.width
        }
    }
    
    /// Aligns the frame horizontally as specified by the collection view layout's `horizontalAlignment`.
    ///
    /// - Parameters:
    ///   - collectionViewLayout: The layout providing the alignment information.
    func alignHorizontally(collectionViewLayout: AlignedCollectionViewFlowLayout) {
        
        guard let alignmentAxis = collectionViewLayout.alignmentAxis else {
            return
        }
        
        switch collectionViewLayout.horizontalAlignment {
            
        case .left:
            if isRepresentingFirstItemInLine(collectionViewLayout: collectionViewLayout) {
                align(toAlignmentAxis: alignmentAxis)
            } else {
                alignToPrecedingItem(collectionViewLayout: collectionViewLayout)
            }
            
        case .right:
            if isRepresentingLastItemInLine(collectionViewLayout: collectionViewLayout) {
                align(toAlignmentAxis: alignmentAxis)
            } else {
                alignToFollowingItem(collectionViewLayout: collectionViewLayout)
            }
            
        default:
            return
        }
    }
    
    /// Aligns the frame vertically as specified by the collection view layout's `verticalAlignment`.
    ///
    /// - Parameter collectionViewLayout: The layout providing the alignment information.
    func alignVertically(collectionViewLayout: AlignedCollectionViewFlowLayout) {
        let alignmentAxis = collectionViewLayout.verticalAlignmentAxis(for: self)
        align(toAlignmentAxis: alignmentAxis)
    }
    
}