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class ViewModelTests: XCTestCase {

    var sut: ViewModel!
    var logger: MockLoggerProtocol!

    override func setUpWithError() throws {
    }

    override func tearDownWithError() throws {
    }

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    private var labelCache = [IndexPath: UILabel]()

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        ...
        if labelCache[indexPath] == nil {
            let label = UILabel()
            label.text = ...
            labelCache[indexPath] = label
        }

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extension UIView {
  
    func transparentPercentage() -> Double {
        let width = Int(bounds.width)
        let height = Int(bounds.height)

        // 1. Prepare a memory space where you write in RGBA data of the view
        var pixelData: [UInt8] = Array<UInt8>(repeating: 0, count: width * height * 4)
        let colorSpace = CGColorSpaceCreateDeviceRGB()
        let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.premultipliedLast.rawValue)

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override func draw(_ rect: CGRect) {
    super.draw(rect)
}

override func draw(_ layer: CALayer, in context: CGContext) {
    super.draw(layer, in: context)
    
    // 1. Let the subviews to render into the context 
    foregroundView?.layer.render(in: context)

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private var previousLocation: CGPoint?
// 0. Line/LineCollection class is a convenience class to store line(s)
private var lineCollection: LineCollection = LineCollection()

override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
    // 1. If there is no touch point or if the point did not move, ignore
    guard let touch = touches.first else {
        return
    }
    let location = touch.location(in: self)  

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func setupCatsSection() -> NSCollectionLayoutSection {
    // 1. Configuring Section. Item -> Group -> Section
    // Item
    let item = NSCollectionLayoutItem(
        layoutSize: NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0),
                                           heightDimension: .fractionalHeight(1.0)))
    item.contentInsets = NSDirectionalEdgeInsets(top: 6.7, leading: 10.0, bottom: 6.7, trailing: 10.0)

    // Group
    let group = NSCollectionLayoutGroup.vertical(