Serhii Liamtsev SergLam

1. The Layer Tree

Core Animation's original name is Layer Kit

Core Animation is a compositing engine; its job is to compose different pieces of visual content on the screen, and to do so as fast as possible. The content in question is divided into individual layers stored in a hierarchy known as the layer tree. This tree forms the underpinning for all of UIKit, and for everything that you see on the screen in an iOS application.

In UIView, tasks such as rendering, layout and animation are all managed by a Core Animation class called CALayer. The only major feature of UIView that isn’t handled by CALayer is user interaction.

There are four hierarchies, each performing a different role:

	import UIKit

	final class CustomVC: UIViewController {

	private lazy var contentView: CustomView = CustomView()

	// MARK: - Life cycle
	deinit {
	NotificationCenter.default.removeObserver(self)
	}

	import UIKit

	protocol WordsFlexViewDelegate: class {
	func didSelectWord(word: String, intId: Int, phraseView: WordsFlexView)
	}

	class WordsFlexView: UIView {

	enum WordsFlexViewType {
	case selectable

	import UIKit

	class HitTestUIView: UIView {
	override func hitTest(_ point: CGPoint, with event: UIEvent?) -> UIView? {
	if clipsToBounds \|\| isHidden \|\| alpha == 0 {
	return nil
	}

	for subview in subviews.reversed() {
	let subPoint = subview.convert(point, from: self)

	import Foundation

	// https://en.wikipedia.org/wiki/List_of_U.S._state_and_territory_abbreviations
	// https://en.wikipedia.org/wiki/ISO_3166-2

	// NOTE: - Last two symbols from ISO 3166-2 state code
	enum USAState: Int, CaseIterable {

	case alabama = 1
	case alaska = 2

	Practical code snippet that works:
	https://stackoverflow.com/questions/44187881/uicollectionview-full-width-cells-allow-autolayout-dynamic-height

	Theoretical material:
	https://www.vadimbulavin.com/collection-view-cells-self-sizing/

	import Moya
	import SwiftyJSON

	enum FileWebService {
	case download(url: String, fileName: String?)

	var localLocation: URL {
	switch self {
	case .download(let url, let fileName):
	let fileKey: String = url.MD5 // use url's md5 as local file name

	extension SocketClient {

	enum Event: String, CaseIterable {
	// Chat:

	case sendMessage = "speak"
	case typing = "typing"
	case lastSeen = "last_seen"
	case unreadConversations = "unread_conversations"
	}

	import Foundation

	/// The WeakSet is weak: References to objects in the collection are held weakly.
	/// If there is no other reference to an object stored in the WeakSet, they can be garbage collected.
	public class WeakSet<T>: Sequence, ExpressibleByArrayLiteral, CustomStringConvertible, CustomDebugStringConvertible {

	private var objects = NSHashTable<AnyObject>.weakObjects()

	public init(_ objects: [T]) {
	for object in objects {

	import Foundation
	import UIKit

	// MARK: - Localized
	public protocol Localized {
	var localized: String { get }
	}

	extension String: Localized {
	public var localized: String {