vds_ios/VDS/Components/Pagination/Pagination.swift

//
//  Pagination.swift
//  VDS
//
//  Created by Bandaru, Krishna Kishore on 01/03/24.
//

import Foundation
import VDSColorTokens
import Combine

///Pagination is a control that enables customers to navigate multiple pages of content by selecting either a specific page or the next or previous set of four pages.
@objc(VDSPagination)
open class Pagination: View {
    
    //--------------------------------------------------
    // MARK: - Private Properties
    //--------------------------------------------------
    ///Collectionview width anchor
    private var collectionViewWidthAnchor: NSLayoutConstraint?
    ///Selected page index
    private var _selectedPageIndex: Int = 0
    ///Custom flow layout defined for the Pagination
    private let flowLayout = PaginationFlowLayout()
    ///A root view for the pagination
    private let containerView: View = View().with {
        $0.translatesAutoresizingMaskIntoConstraints = false
    }
    ///Collectionview to render pagination indexes
    private lazy var collectionView: UICollectionView = {
        let collectionView = UICollectionView(frame: .zero, collectionViewLayout: flowLayout)
        collectionView.isScrollEnabled = false
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        collectionView.delegate = self
        collectionView.dataSource = self
        collectionView.showsHorizontalScrollIndicator = false
        collectionView.showsVerticalScrollIndicator = false
        collectionView.register(PaginationCellItem.self, forCellWithReuseIdentifier: PaginationCellItem.identifier)
        collectionView.backgroundColor = .clear
        return collectionView
    }()
    
    //--------------------------------------------------
    // MARK: - Public Properties
    //--------------------------------------------------
    ///Previous button to select previous page
    public let previousButton: PaginationButton = .init(type: .previous)
    ///Next button to select next page
    public let nextButton: PaginationButton = .init(type: .next)
    /// A callback when the page changes. Passes parameters (selectedPage).
    public var onPageDidSelect: ((Int) -> Void)?
    /// Total number of pages, allows limit ranging from 0 to 9999.
    @Clamping(range: 0...9999)
    public var total: Int {
        didSet {
            previousButton.isHidden = true
            nextButton.isHidden = total <= 1
            _selectedPageIndex = 0
            setNeedsUpdate()
            updateSelection()
        }
    }
    ///Selected active page number and clips to total pages if selected index is greater than the total pages.
    public var selectedPage: Int {
        set {
            if newValue >= total {
                _selectedPageIndex = total - 1
            } else if newValue < 0 {
                _selectedPageIndex = 0
            } else {
                _selectedPageIndex = max(newValue - 1, 0)
            }
            setNeedsUpdate()
            updateSelection()
        }
        get {
            _selectedPageIndex + 1 //Returns selected page value not index
        }
    }
    
    //--------------------------------------------------
    // MARK: - Overrides
    //--------------------------------------------------
    /// Executed on initialization for this View.
    open override func initialSetup() {
        super.initialSetup()
        
        addSubview(containerView)
        containerView.pinToSuperView()
        containerView.widthAnchor.constraint(greaterThanOrEqualToConstant: 288).activate()
        containerView.heightAnchor.constraint(equalToConstant: 44).activate()
        containerView.addSubview(previousButton)
        containerView.addSubview(collectionView)
        containerView.addSubview(nextButton)
        
        previousButton
            .pinTop()
            .pinBottom()
            .pinLeading()
        previousButton.trailingAnchor.constraint(greaterThanOrEqualTo: collectionView.leadingAnchor).activate()
        collectionView.heightAnchor.constraint(equalToConstant: VDSLayout.Spacing.space4X.value).activate()
        collectionView.centerYAnchor.constraint(equalTo: centerYAnchor).activate()
        collectionView.centerXAnchor.constraint(equalTo: centerXAnchor).activate()
        collectionView.trailingAnchor.constraint(greaterThanOrEqualTo: nextButton.leadingAnchor).activate()
        collectionViewWidthAnchor = collectionView.widthAnchor.constraint(equalToConstant: 92)
        collectionViewWidthAnchor?.activate()
        
        nextButton
            .pinTop()
            .pinBottom()
            .pinTrailing()
        
        nextButton.onClick = onbuttonTapped
        previousButton.onClick = onbuttonTapped
        previousButton.isHidden = true
        
        flowLayout.$collectionViewWidth
            .receive(on: RunLoop.main)
            .sink { [weak self] value in
            self?.collectionViewWidthAnchor?.constant = value
        }.store(in: &subscribers)
    }
    
    /// Used to make changes to the View based off a change events or from local properties.
    open override func updateView() {
        super.updateView()
        nextButton.surface = surface
        previousButton.surface = surface
        collectionView.reloadData()
    }
    
    //--------------------------------------------------
    // MARK: - Private Methods
    //--------------------------------------------------
    ///When previous/next button is tapped
    private func onbuttonTapped(_ sender: UIButton) {
        let isNextAction = sender == nextButton
        _selectedPageIndex = if isNextAction { _selectedPageIndex + 1 } else { _selectedPageIndex - 1 }
        updateSelection()
    }
    
    ///Refreshing the UI based on the selected page
    private func updateSelection() {
        guard _selectedPageIndex < total else { return }
        //Need to make selected page as second element so scrolling previous index of the selected page to left
        collectionView.scrollToItem(at: IndexPath(row: max(_selectedPageIndex - 1, 0), section: 0), at: .left, animated: false)
        previousButton.isHidden = _selectedPageIndex == 0
        nextButton.isHidden = _selectedPageIndex == total - 1
        collectionView.reloadData()
        verifyIfMaxDigitChanged()
    }
    
    ///Identifying if there is any change in the digits of upcoming page
    func verifyIfMaxDigitChanged() {
        let upperLimitPage = _selectedPageIndex + flowLayout.maxNumberOfColumns
        let upperLimitDigits = upperLimitPage.digitCount //future value digits
        switch (flowLayout.numberOfColumns, upperLimitDigits) {
        case (_, 3), (_, 4):
            flowLayout.numberOfColumns = 3
        default:
            flowLayout.numberOfColumns = 4
        }
        if upperLimitDigits != flowLayout.upperLimitDigits {
            flowLayout.upperLimitDigits = upperLimitDigits
            flowLayout.invalidateLayout()
            collectionView.reloadData()
            //Need to make selected page as second element so scrolling previous index of the selected page to left
            collectionView.scrollToItem(at: IndexPath(row: max(_selectedPageIndex - 1, 0), section: 0), at: .left, animated: false)
        }
    }
}

extension Pagination: UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
    //--------------------------------------------------
    // MARK: - UICollectionView Delegate & Datasource
    //--------------------------------------------------
    public func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int { total }

    public func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        guard let cell = collectionView.dequeueReusableCell(withReuseIdentifier: PaginationCellItem.identifier, for: indexPath) as? PaginationCellItem else { return UICollectionViewCell() }
        cell.update(_selectedPageIndex, currentIndex: indexPath.row, surface: surface)
        return cell
    }
    
    public func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        guard _selectedPageIndex != indexPath.row else { return }
        _selectedPageIndex = indexPath.row
        updateSelection()
        onPageDidSelect?(selectedPage)
    }
}

fileprivate extension Int {
    //--------------------------------------------------
    // MARK: - Extension on Int to identify number of digits in given number.
    //--------------------------------------------------
    var digitCount: Int {
        numberOfDigits(in: self)
    }
    
    private func numberOfDigits(in number: Int) -> Int {
        number < 10 && number >= 0 ? 1 : 1 + numberOfDigits(in: number/10)
    }
}