UICollectionView+Extension

import UIKit

// MARK: - IndexPaths
extension UICollectionView {
    
    var lastIndexPath: IndexPath? {
        return lastIndexPath(inSection: lastSection)
    }

    var lastSection: Int {
        return numberOfSections > 0 ? numberOfSections - 1 : 0
    }
    
    func numberOfItems() -> Int {
        var section = 0
        var itemsCount = 0
        while section < numberOfSections {
            itemsCount += numberOfItems(inSection: section)
            section += 1
        }
        return itemsCount
    }
    
    func lastIndexPath(inSection section: Int) -> IndexPath? {
        guard section >= 0 else {
            return nil
        }
        guard section < numberOfSections else {
            return nil
        }
        guard numberOfItems(inSection: section) > 0 else {
            return IndexPath(item: 0, section: section)
        }
        return IndexPath(item: numberOfItems(inSection: section) - 1, section: section)
    }
}


// MARK: - Registeration
extension UICollectionView {

    func register<T: UICollectionReusableView>(supplementaryView kind: String, className: T.Type) {
        register(T.self, forSupplementaryViewOfKind: kind, withReuseIdentifier: String(describing: className))
    }
    
    func registerHeader<T: UICollectionReusableView>(supplementaryView className: T.Type) {
        register(T.self, forSupplementaryViewOfKind: UICollectionView.elementKindSectionHeader,
                 withReuseIdentifier: String(describing: className))
    }
    
    func registerFooter<T: UICollectionReusableView>(supplementaryView className: T.Type) {
        register(T.self, forSupplementaryViewOfKind: UICollectionView.elementKindSectionFooter,
                 withReuseIdentifier: String(describing: className))
    }

    func register<T: UICollectionViewCell>(_ classType: T.Type) {
        register(T.self, forCellWithReuseIdentifier: String(describing: classType))
    }
}


// MARK: - Dequeuing
extension UICollectionView {
    
    func dequeueCell<T: UICollectionViewCell>(withClass name: T.Type, for indexPath: IndexPath) -> T {
        guard let cell = dequeueReusableCell(withReuseIdentifier: String(describing: name),
                                             for: indexPath) as? T else {
            fatalError("Couldn't find UICollectionViewCell for \(String(describing: name)), make sure the cell is registered with collection view")
        }
        return cell
    }

    func dequeueHeader<T: UICollectionReusableView>(className: T.Type, for indexPath: IndexPath) -> T {
        guard let cell = dequeueReusableSupplementaryView(ofKind: UICollectionView.elementKindSectionHeader, withReuseIdentifier: String(describing: className), for: indexPath) as? T else {
            fatalError("Couldn't find header for \(String(describing: className)), make sure the view is registered with collection view")
        }
        return cell
    }
    
    func dequeueFooter<T: UICollectionReusableView>(className: T.Type, for indexPath: IndexPath) -> T {
        guard let cell = dequeueReusableSupplementaryView(ofKind: UICollectionView.elementKindSectionFooter, withReuseIdentifier: String(describing: className), for: indexPath) as? T else {
            fatalError("Couldn't find footer for \(String(describing: className)), make sure the view is registered with collection view")
        }
        return cell
    }
}


// MARK: - Scrolling
extension UICollectionView {
    
    func safeScrollToItem(indexPath: IndexPath,
                          position: UICollectionView.ScrollPosition,
                          animated: Bool) {
        
        guard indexPath.item >= 0 &&
            indexPath.section >= 0 &&
            indexPath.section < numberOfSections &&
            indexPath.item < numberOfItems(inSection: indexPath.section) else {
                return
        }
        scrollToItem(at: indexPath, at: position, animated: animated)
    }
}