full_gesture_code.swift

//
//  GestureCode.swift
//  Blending
//
//  Created by Riccardo Cipolleschi on 14/11/2020.
//

import Foundation
import UIKit

// MARK: - View Model
struct VM {
  var offset: CGPoint
}

// MARK: - View
class View: UIView {
  var model: VM? {
    didSet {
      self.update(old: oldValue)
    }
  }

  // MARK: Variable Declaration
  private var ball = UIView()

  // MARK: Interactions
  var userDragged: ((CGPoint) -> ())?

  // MARK: Initializers
  override init(frame: CGRect) {
    super.init(frame: frame)
    self.setup()
    self.style()
  }

  required init?(coder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }

  // MARK: Setup
  private func setup() {
    self.addSubview(self.ball)

    let gestureRecognizer = UIPanGestureRecognizer(target: self, action: #selector(self.panGestureRecognized))
    self.ball.addGestureRecognizer(gestureRecognizer)
  }

  @objc private func panGestureRecognized(_ gestureRecognizer: UIPanGestureRecognizer) {
    switch gestureRecognizer.state {
    case .began:
      let currentOffset = model?.offset ?? .zero
      let translation = gestureRecognizer.translation(in: self)
      let initialTranslation = currentOffset + translation
      gestureRecognizer.setTranslation(initialTranslation, in: self)
      self.userDragged?(initialTranslation)
      break
    case .changed:
      let translation = gestureRecognizer.translation(in: self)
      self.userDragged?(translation)
      break
    case .ended:
      let translation = gestureRecognizer.translation(in: self)
      self.userDragged?(translation)
      break
    default:
      break
    }
  }

  // MARK: Style
  private func style() {
    self.backgroundColor = .black
    self.ball.backgroundColor = .red
    self.ball.layer.cornerRadius = 25
  }

  // MARK: Update
  private func update(old: VM?) {
    self.setNeedsLayout()
  }

  // MARK: Layout
  override func layoutSubviews() {
    super.layoutSubviews()

    self.ball.center = self.center + model?.offset
    self.ball.bounds.size = CGSize(width: 50, height: 50)
  }
}

// MARK: - VC
class VC: UIViewController {

  // Helper to retrieve a correctly typed view
  var rootView: View {
    return self.view as! View
  }

  // This override of `loadView` creates the right type of view and sets the initial vm
  override func loadView() {
    super.loadView()
    let view = View()
    view.model = VM(offset: .zero)
    self.view = view
  }

  // The `viewDidLoad` method is used to connect the interactions
  override func viewDidLoad() {
    super.viewDidLoad()
    self.setupInteraction()
  }

  // It connects all the interactions from the View to the VC.
  // Specifically, it receives the drag interaction in the view and updates the VM accordingly.
  func setupInteraction() {
    self.rootView.userDragged = { [unowned self] point in
      self.rootView.model = VM(offset: point)
    }
  }
}

// MARK: - Helpers
extension CGPoint {
  static func +(lhs: Self, rhs: Self?) -> CGPoint {
    guard let right = rhs else {
      return lhs
    }
    return .init(x: lhs.x + right.x, y: lhs.y + right.y)
  }
}