OrderedCV_Step1.swift

//: A UIKit based Playground for presenting user interface
  
import UIKit
import PlaygroundSupport

class MyViewController : UIViewController, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout, UICollectionViewDataSource {
    
  ///1.
    var baseCollectionViewArray = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]
    var secondCollectionViewArray = [String]()
    
    let baseCollectionView = UICollectionView(frame: CGRect(x: 25, y: 50, width: 300, height: 200), collectionViewLayout: UICollectionViewFlowLayout())
    let secondCollectionView = UICollectionView(frame: CGRect(x: 25, y: 300, width: 300, height: 200), collectionViewLayout: UICollectionViewFlowLayout())
    
    override func loadView() {
        let view = UIView()
        view.backgroundColor = .white
        
      ///2.
        baseCollectionView.register(AlphabetCollectionCell.self, forCellWithReuseIdentifier: "Cell")
        baseCollectionView.delegate = self
        baseCollectionView.dataSource = self
        
        
        secondCollectionView.register(AlphabetCollectionCell.self, forCellWithReuseIdentifier: "Cell")
        secondCollectionView.delegate = self
        secondCollectionView.dataSource = self
       
      ///3.
        view.addSubview(baseCollectionView)
        view.addSubview(secondCollectionView)
        
        self.view = view
    }
    
  ///4.
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! AlphabetCollectionCell
        
        if collectionView == baseCollectionView {
            ///We have 2 collectionviews, so we must differentiate.
            cell.backgroundColor = .green
            cell.nameLabel.text = baseCollectionViewArray[indexPath.item]
        } else {
            cell.backgroundColor = .red
            cell.nameLabel.text = secondCollectionViewArray[indexPath.item]
        }
        
        
        return cell
    }
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        if collectionView == baseCollectionView {
            ///If this collectionview is the second one
            return baseCollectionViewArray.count
        } else { ///If not, this must be the first collection view.
            return secondCollectionViewArray.count
        }
    }
  ///5.
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        if collectionView == baseCollectionView {
            let alphabetLetter = baseCollectionViewArray[indexPath.item]
            secondCollectionViewArray.append(alphabetLetter)
            let newIndexPath = IndexPath(item: secondCollectionViewArray.count - 1, section: 0)
            secondCollectionView.insertItems(at: [newIndexPath])
            
            baseCollectionViewArray.remove(at: indexPath.item)
            baseCollectionView.deleteItems(at: [indexPath])
            
        } else {
            ///We'll do this later
        }
    }

}
///6.
class AlphabetCollectionCell: UICollectionViewCell {

    var nameLabel:  UILabel = UILabel(frame: CGRect.zero)

    override init(frame : CGRect) {
        super.init(frame : frame)
        ///Set up the label that shows the letter
        nameLabel.frame.size = CGSize(width: self.frame.width, height: self.frame.height)
        nameLabel.textAlignment = .center
        contentView.addSubview(nameLabel)
        
    }
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}
// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()