added lots of comments to explain the code

nathangitter · Sep 4, 2017 · 6c06e71 · 6c06e71
1 parent ea61739
commit 6c06e71
Showing 1 changed file with 66 additions and 0 deletions.
diff --git a/InteractiveAnimations/InteractiveAnimations/ViewController.swift b/InteractiveAnimations/InteractiveAnimations/ViewController.swift
@@ -9,6 +9,8 @@
 import UIKit
 import UIKit.UIGestureRecognizerSubclass
 
+// MARK: - State
+
 private enum State {
     case closed
     case open
@@ -23,10 +25,16 @@ extension State {
     }
 }
 
+// MARK: - View Controller
+
 class ViewController: UIViewController {
 
+    // MARK: - Constants
+
     private let popupOffset: CGFloat = 440
 
+    // MARK: - Views
+
     private lazy var contentImageView: UIImageView = {
         let imageView = UIImageView()
         imageView.image = #imageLiteral(resourceName: "content")
@@ -76,6 +84,8 @@ class ViewController: UIViewController {
         return imageView
     }()
 
+    // MARK: - View Controller Lifecycle
+
     override func viewDidLoad() {
         super.viewDidLoad()
         layout()
@@ -86,6 +96,8 @@ class ViewController: UIViewController {
         return true
     }
 
+    // MARK: - Layout
+
     private var bottomConstraint = NSLayoutConstraint()
 
     private func layout() {
@@ -133,10 +145,15 @@ class ViewController: UIViewController {
 
     }
 
+    // MARK: - Animation
+
+    /// The current state of the animation. This variable is changed only when an animation completes.
     private var currentState: State = .closed
 
+    /// All of the currently running animators.
     private var runningAnimators = [UIViewPropertyAnimator]()
 
+    /// The progress of each animator. This array is parallel to the `runningAnimators` array.
     private var animationProgress = [CGFloat]()
 
     private lazy var panRecognizer: InstantPanGestureRecognizer = {
@@ -145,8 +162,13 @@ class ViewController: UIViewController {
         return recognizer
     }()
 
+    /// Animates the transition, if the animation is not already running.
     private func animateTransitionIfNeeded(to state: State, duration: TimeInterval) {
+
+        // ensure that the animators array is empty (which implies new animations need to be created)
         guard runningAnimators.isEmpty else { return }
+
+        // an animator for the transition
         let transitionAnimator = UIViewPropertyAnimator(duration: duration, dampingRatio: 1, animations: {
             switch state {
             case .open:
@@ -164,7 +186,11 @@ class ViewController: UIViewController {
             }
             self.view.layoutIfNeeded()
         })
+
+        // the transition completion block
         transitionAnimator.addCompletion { position in
+
+            // update the state
             switch position {
             case .start:
                 self.currentState = state.opposite
@@ -173,14 +199,21 @@ class ViewController: UIViewController {
             case .current:
                 ()
             }
+
+            // manually reset the constraint positions
             switch self.currentState {
             case .open:
                 self.bottomConstraint.constant = 0
             case .closed:
                 self.bottomConstraint.constant = self.popupOffset
             }
+
+            // remove all running animators
             self.runningAnimators.removeAll()
+
         }
+
+        // an animator for the title that is transitioning into view
         let inTitleAnimator = UIViewPropertyAnimator(duration: duration, curve: .easeIn, animations: {
             switch state {
             case .open:
@@ -190,6 +223,8 @@ class ViewController: UIViewController {
             }
         })
         inTitleAnimator.scrubsLinearly = false
+
+        // an animator for the title that is transitioning out of view
         let outTitleAnimator = UIViewPropertyAnimator(duration: duration, curve: .easeOut, animations: {
             switch state {
             case .open:
@@ -199,35 +234,60 @@ class ViewController: UIViewController {
             }
         })
         outTitleAnimator.scrubsLinearly = false
+
+        // start all animators
         transitionAnimator.startAnimation()
         inTitleAnimator.startAnimation()
         outTitleAnimator.startAnimation()
+
+        // keep track of all running animators
         runningAnimators.append(transitionAnimator)
         runningAnimators.append(inTitleAnimator)
         runningAnimators.append(outTitleAnimator)
+
     }
 
     @objc private func popupViewPanned(recognizer: UIPanGestureRecognizer) {
         switch recognizer.state {
         case .began:
+
+            // start the animations
             animateTransitionIfNeeded(to: currentState.opposite, duration: 1)
+
+            // pause all animations, since the next event may be a pan changed
             runningAnimators.forEach { $0.pauseAnimation() }
+
+            // keep track of each animator's progress
             animationProgress = runningAnimators.map { $0.fractionComplete }
+
         case .changed:
+
+            // variable setup
             let translation = recognizer.translation(in: popupView)
             var fraction = -translation.y / popupOffset
+
+            // adjust the fraction for the current state and reversed state
             if currentState == .open { fraction *= -1 }
             if runningAnimators[0].isReversed { fraction *= -1 }
+
+            // apply the new fraction
             for (index, animator) in runningAnimators.enumerated() {
                 animator.fractionComplete = fraction + animationProgress[index]
             }
+
         case .ended:
+
+            // variable setup
             let yVelocity = recognizer.velocity(in: popupView).y
             let shouldClose = yVelocity > 0
+
+            // if there is no motion, continue all animations and exit early
             if yVelocity == 0 {
                 runningAnimators.forEach { $0.continueAnimation(withTimingParameters: nil, durationFactor: 0) }
                 break
             }
+
+            // reverse the animations based on their current state and pan motion
             switch currentState {
             case .open:
                 if !shouldClose && !runningAnimators[0].isReversed { runningAnimators.forEach { $0.isReversed = !$0.isReversed } }
@@ -236,14 +296,20 @@ class ViewController: UIViewController {
                 if shouldClose && !runningAnimators[0].isReversed { runningAnimators.forEach { $0.isReversed = !$0.isReversed } }
                 if !shouldClose && runningAnimators[0].isReversed { runningAnimators.forEach { $0.isReversed = !$0.isReversed } }
             }
+
+            // continue all animations
             runningAnimators.forEach { $0.continueAnimation(withTimingParameters: nil, durationFactor: 0) }
+
         default:
             ()
         }
     }
 
 }
 
+// MARK: - InstantPanGestureRecognizer
+
+/// A pan gesture that enters into the `began` state on touch down instead of waiting for a touches moved event.
 class InstantPanGestureRecognizer: UIPanGestureRecognizer {
 
     override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent) {