PannableViewControllerTests

PannableViewController.swift

import UIKit

class PannableViewController: ViewController {
  override func viewDidLoad() {
    super.viewDidLoad()
    
    let panGesture = UIPanGestureRecognizer(target: self, action: #selector(panGesture(_:)))
    view.addGestureRecognizer(panGesture)
  }
}

// MARK: - Private Methods
private extension PannableViewController {
  @objc func panGesture(_ gesture: UIPanGestureRecognizer) {
    let translation = gesture.translation(in: view)
    guard translation.y >= 0 else { return }
    
    switch gesture.state {
    case .began, .changed:
      let offset = max(safeAreaTopInset, translation.y + safeAreaTopInset)
      view.frame.origin.y = offset
    case .ended:
      let velocity = gesture.velocity(in: view)
      let shouldDismiss = velocity.y > 0 && (velocity.y > minimumVelocityToHide || translation.y > view.frame.height * minimumScreenRatioToHide)
      if shouldDismiss {
        self.view.frame.origin.y = self.view.frame.height
        self.dismiss(animated: false, completion: nil)
      } else {
        self.view.frame.origin.y = 0
      }
    case .possible, .cancelled, .failed:
      self.view.frame.origin.y = 0
    }
  }
}

PannableViewControllerTests.swift

import XCTest
@testable import Project

class PannableViewControllerTests: XCTestCase {
  private let vc = PannableViewControllerInjector()
  
  override func setUp() {
    super.setUp()
    _ = vc.view
  }
  
  func testSetup() {
    XCTAssertNotNil(vc.view.gestureRecognizers?.first(where: { $0 is UIPanGestureRecognizerMock }))
  }
  
  func testPanBegan() {
    vc.gestureRecognizer?.pan(location: nil, translation: .zero, state: .began)
    XCTAssertEqual(vc.view.frame.minY, 0)
  }
  
  func testPanDownwards() {
    vc.gestureRecognizer?.pan(location: nil, translation: CGPoint(x: 0, y: 200), state: .changed)
    XCTAssertEqual(vc.view.frame.minY, 200)
  }
  
  func testPanUpwardsFromStart() {
    vc.gestureRecognizer?.pan(location: nil, translation: CGPoint(x: 0, y: -200), state: .changed)
    XCTAssertEqual(vc.view.frame.minY, 0)
  }
  
  func testPanUpwardsFromMiddle() {
    vc.gestureRecognizer?.pan(location: nil, translation: CGPoint(x: 0, y: 200), state: .changed)
    vc.gestureRecognizer?.pan(location: nil, translation: CGPoint(x: 0, y: 100), state: .changed)
    XCTAssertEqual(vc.view.frame.minY, 100)
  }
  
  func testPanEndedShouldDismiss() {
    vc.gestureRecognizer?.gestureVelocity = .init(x: 0, y: 1)
    let offset = vc.view.frame.height * vc.minimumScreenRatioToHide + 1
    vc.gestureRecognizer?.pan(location: nil, translation: CGPoint(x: 0, y: offset), state: .ended)
    XCTAssertEqual(vc.view.frame.minY, vc.view.frame.height)
  }
  
  func testPanEndedShouldResetDueToLowVelocity() {
    vc.gestureRecognizer?.gestureVelocity = .init(x: 0, y: 0)
    let offset = vc.view.frame.height * vc.minimumScreenRatioToHide + 1
    vc.gestureRecognizer?.pan(location: nil, translation: CGPoint(x: 0, y: offset), state: .ended)
    XCTAssertEqual(vc.view.frame.minY, 0)
  }
  
  func testPanEndedShouldResetDueMinimumScreenRatioNotMet() {
    vc.gestureRecognizer?.gestureVelocity = .init(x: 0, y: 1)
    let offset = vc.view.frame.height * vc.minimumScreenRatioToHide - 1
    vc.gestureRecognizer?.pan(location: nil, translation: CGPoint(x: 0, y: offset), state: .ended)
    XCTAssertEqual(vc.view.frame.minY, 0)
  }
  
  func testPanCancelled() {
    vc.gestureRecognizer?.pan(location: nil, translation: CGPoint(x: 0, y: 1), state: .cancelled)
    XCTAssertEqual(vc.view.frame.minY, 0)
  }
}

private class PannableViewControllerInjector: PannableViewController {
  var gestureRecognizer: UIPanGestureRecognizerMock?
  
  override func viewDidLoad() {
    super.viewDidLoad()
    guard let existingGestureRecognizer = view.gestureRecognizers?.first(where: { $0 is UIPanGestureRecognizer }) as? UIPanGestureRecognizer else { return }
    view.removeGestureRecognizer(existingGestureRecognizer)
    
    let action = Selector("panGesture:") // we need to create Objective-C selector instead because a caller metod is private
    let newGestureRecognizer = UIPanGestureRecognizerMock(target: self, action: action)
    view.addGestureRecognizer(newGestureRecognizer)
    gestureRecognizer = newGestureRecognizer
  }
}

UIPanGestureRecognizerMock.swift

import UIKit

class UIPanGestureRecognizerMock: UIPanGestureRecognizer {
  private let target: Any?
  private let action: Selector?
  var gestureState: UIGestureRecognizerState?
  var gestureLocation: CGPoint?
  var gestureTranslation: CGPoint?
  var gestureVelocity: CGPoint?
  
  override init(target: Any?, action: Selector?) {
    self.target = target
    self.action = action
    super.init(target: target, action: action)
  }
  
  override func location(in view: UIView?) -> CGPoint {
    if let gestureLocation = gestureLocation {
      return gestureLocation
    }
    return super.location(in: view)
  }
  
  override func translation(in view: UIView?) -> CGPoint {
    if let gestureTranslation = gestureTranslation {
      return gestureTranslation
    }
    return super.translation(in: view)
  }
  
  override func velocity(in view: UIView?) -> CGPoint {
    if let gestureVelocity = gestureVelocity {
      return gestureVelocity
    }
    return super.velocity(in: view)
  }
  
  override var state: UIGestureRecognizerState {
    get {
      if let gestureState = gestureState {
        return gestureState
      }
      return super.state
    }
    set {
      self.state = newValue
    }
  }
}

extension UIPanGestureRecognizerMock {
  func pan(location: CGPoint?, translation: CGPoint?, state: UIGestureRecognizerState) {
    guard let action = action else { return }
    gestureState = state
    gestureLocation = location
    gestureTranslation = translation
    (target as? NSObject)?.perform(action, on: Thread.current, with: self, waitUntilDone: true)
  }
}