import UIKit

class ViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
    
    let headerViewHeight: CGFloat = 500
    
    private lazy var headerView: UIView = {
        let view = UIView(frame: CGRect(x: 0, y: -headerViewHeight, width: UIScreen.main.bounds.width, height: headerViewHeight))
        view.clipsToBounds = true
        return view
    }()
    
    private lazy var imageView: UIImageView = {
        let view = UIImageView(image: UIImage(named: "image"))
        view.frame = headerView.bounds
        return view
    }()
    
    private lazy var collectionView: UICollectionView = {
        let view = UICollectionView(frame: CGRect(x: 0, y: 0, width: UIScreen.main.bounds.width, height: UIScreen.main.bounds.height), collectionViewLayout: UICollectionViewFlowLayout())
        view.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "UICollectionViewCell")
        view.delegate = self
        view.dataSource = self
        view.contentInset = UIEdgeInsets(top: headerViewHeight, left: 0, bottom: 0, right: 0)
        view.backgroundColor = .white
        view.contentInsetAdjustmentBehavior = .never
        return view
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.addSubview(collectionView)
        collectionView.addSubview(headerView)
        headerView.addSubview(imageView)
    }
    
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        let offset = scrollView.contentOffset.y
        
        // 重新赋值，防止用力拖拽时的回弹
        imageView.frame.origin.y = 0
        if (offset >= -headerViewHeight && offset <= headerViewHeight) {
            imageView.frame.origin.y = (offset + headerViewHeight) * 0.5
        }
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 600
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "UICollectionViewCell", for: indexPath)
        cell.backgroundColor = UIColor(red: CGFloat(arc4random() % 256) / 255.0, green: CGFloat(arc4random() % 256) / 255.0, blue: CGFloat(arc4random() % 256) / 255.0, alpha: 1.0)
        return cell
    }
}