iOS-Assessment-V1/LiveLikeiOSAssessmentV1/ViewControllers/Views/GifListViewController.swift

//
//  GifListViewController.swift

import UIKit

class GifListViewController: UIViewController {

    private var presenter: GifListPresenter = GifListPresenter()
    
    private lazy var collectionView: UICollectionView = {
        let collectionView = UICollectionView(frame: .zero, collectionViewLayout: GifListCollectionViewFlowLayout())
        collectionView.register(GifCollectionViewCell.self, forCellWithReuseIdentifier: GifCollectionViewCell.reuseIdentifier)
        collectionView.keyboardDismissMode = .onDrag
        return collectionView
    }()
    
    private lazy var searchBar: UISearchBar = {
        let sb = UISearchBar()
        sb.delegate = self
        sb.placeholder = "search library..."
        return sb
    }()
    
    /// Indicator view that we expect to show when no data is present
    private lazy var activityIndicator: UIActivityIndicatorView = {
        let aIndicator = UIActivityIndicatorView()
        aIndicator.hidesWhenStopped = true
        return aIndicator
    }()
    
    /// Delay the search by couple of ms so the web devs will like us
    private var delayedWorkItem: DispatchWorkItem?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        self.setupUI()
        self.setupSelf()
        self.presenter.getData(query: "")
    }
    
    private func setupUI() {
        
        self.view.addSubview(self.collectionView)
        self.view.addSubview(self.activityIndicator)
        
        self.initialConstraintSetup()
        
    }
    
    private func setupSelf() {
        
        self.navigationItem.titleView = self.searchBar
                
        presenter.delegate = self
        
        collectionView.delegate = self
        collectionView.dataSource = self
        collectionView.prefetchDataSource = self
        
    }

}

// MARK: - Constraints

extension GifListViewController {
    
    /*!
        Initial constraints setup
     */
    private func initialConstraintSetup() {
        
        self.collectionView.translatesAutoresizingMaskIntoConstraints = false
        self.activityIndicator.translatesAutoresizingMaskIntoConstraints = false

        NSLayoutConstraint.activate([
            
            self.collectionView.topAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.topAnchor),
            self.collectionView.leadingAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.leadingAnchor),
            self.collectionView.trailingAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.trailingAnchor),
            self.collectionView.bottomAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.bottomAnchor),
            
            self.activityIndicator.centerXAnchor.constraint(equalTo: self.view.centerXAnchor),
            self.activityIndicator.centerYAnchor.constraint(equalTo: self.view.centerYAnchor)
            
        ])

    }
}

// MARK: - Presenter delegate

extension GifListViewController: GifListPresenterDelegate {
    
    func presenterDidReceiveInitialData() {
        
        DispatchQueue.main.async {
            
            self.collectionView.setContentOffset(CGPoint(x: 0, y: self.collectionView.safeAreaInsets.top), animated: false)
            self.collectionView.reloadData()
            
            if self.activityIndicator.isAnimating == true {
                self.activityIndicator.stopAnimating()
            }
            
        }

    }
    
    
    func presenterDidReceiveAdditionalData(at indexPaths: [IndexPath]) {
        DispatchQueue.main.async {
            self.collectionView.insertItems(at: indexPaths)
        }
    }
    
    
    func presenterWillStartLoadingForEmptyData() {
        DispatchQueue.main.async {
            self.activityIndicator.startAnimating()
        }
    }
    
}


// MARK: - SearchBar delegate

extension GifListViewController: UISearchBarDelegate {
    func searchBar(_ searchBar: UISearchBar, textDidChange searchText: String) {
        
        self.delayedWorkItem?.cancel()
        
        delayedWorkItem = DispatchWorkItem { [weak self] in
            self?.presenter.getData(query: searchText)
        }
        
        DispatchQueue.global(qos: .userInitiated).asyncAfter(deadline: .now() + 0.2, execute: delayedWorkItem!)
    }
}


// MARK: - CollectionView delegate and data source

extension GifListViewController: UICollectionViewDataSource, UICollectionViewDelegate {
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return presenter.gifsList.count
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: GifCollectionViewCell.reuseIdentifier, for: indexPath) as! GifCollectionViewCell
        cell.gif = presenter.gifsList[indexPath.row]
        return cell
    }
        
}

extension GifListViewController: UICollectionViewDataSourcePrefetching {
    
    func collectionView(_ collectionView: UICollectionView, prefetchItemsAt indexPaths: [IndexPath]) {
        
        guard presenter.isLoadingNewPage == false else { return }
        
        for indexPath in indexPaths {
            if indexPath.row > presenter.gifsList.count - Constants.GiphyPaginationLimit/2 {
                delayedWorkItem?.cancel()
                presenter.getData(query: self.searchBar.text ?? "")
                break
            }
        }
        
    }
    
    
}