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A tweening library to animate arbitrary numerical values.
// JTSSwiftTweener.swift
// JTSSwiftTweener
// Created by Joshua Sullivan on 12/10/16.
// Copyright © 2016 Josh Sullivan. All rights reserved.
import UIKit
/// The `Tweener` class allows for the easy animation of arbitrary numeric values,
/// including those that are not associated with views.
public final class Tweener: Equatable {
/// A closure invoked when the tween updates.
/// - Parameter Double: This parameter is the current value of the tween.
/// - Parameter Tweener: A reference to the tweener invoking the closure.
/// - Note: The `TweenProgress` closure is gauranteed to be called with the `from` value first and the `to` value of the tween immediately prior to the `TweenComplete` closure being called.
public typealias TweenProgress = (Double, Tweener) -> Void
/// A closure invoked when the tween completes or is canceled.
/// - Parameter Bool: Will be `true` if the the tween completed and `false` if it was canceled.
/// - Parameter Tweener: A reference to the tweener invoking the closure.
public typealias TweenComplete = (Bool, Tweener) -> Void
/// The currently active tweens.
fileprivate static var tweens: [Tweener] = [] {
didSet {
if tweens.isEmpty {
} else {
/// The display link that drives the animation engine.
fileprivate static let displayLink: CADisplayLink = {
let dl = CADisplayLink(target: Tweener.self, selector: #selector(tick(link:)))
dl.add(to: .main, forMode: .commonModes)
dl.isPaused = true
return dl
/// Used to generate unique ids for the individual `Tweener` instances.
fileprivate static var idCounter: Int = 0
/// The last update of the tween, used to determine the elapsed time between ticks.
fileprivate static var previousTimestamp: CFTimeInterval = 0.0
/// Handle the display link tick.
@objc fileprivate static func tick(link: CADisplayLink) {
let dt = link.timestamp - previousTimestamp
// Send a tick to all unpaused tweens. If any of them returns a true (indicating completion),
// we will do a pruning pass afterwards.
let tweensDidFinish = tweens.filter({ !$0.isPaused })
.reduce(false, { return $0 || $1.tick(elapsedTime: dt) })
if tweensDidFinish {
// Remove all tweens flagged as complete.
tweens = tweens.filter({ !$0.isComplete })
previousTimestamp = link.timestamp
/// Start sending updates to the Tweener instances.
fileprivate static func resume() {
guard displayLink.isPaused else { return }
previousTimestamp = CACurrentMediaTime()
displayLink.isPaused = false
/// Stop sending updates to Tweener instances.
fileprivate static func pause() {
displayLink.isPaused = true
/// Create a Tweener instance with the specified parameters.
/// - Warning: If you keep a strong reference to the returned tweener object you could create
/// a retain cycle unless you're careful to only use `weak` or `unowned` self in the
/// progress and completion closures.
/// - Parameters:
/// - duration: The duration of the tween, in seconds.
/// - from: The starting value of the tween.
/// - to: The ending value of the tween.
/// - easing: The easing function to apply to the Tween. By default, `Quadratic.easeInOut` is used.
/// - progress: The closure to be invoked when the tween updates.
/// - completion: The optional closure to be invoked when the tween completes.
/// - Returns: The tweener object which is useful for pausing or canceling the tween.
@discardableResult public static func tween(duration: CFTimeInterval, delay: CFTimeInterval = 0.0, from: Double = 0.0, to: Double = 1.0, easing: @escaping TweenerEasing.EasingTransform = TweenerEasing.Quadratic.easeInOut, progress: @escaping TweenProgress, completion: TweenComplete? = nil) -> Tweener {
let tweener = Tweener(id: idCounter, duration: duration, delay: delay, from: from, to: to, easing: easing, progress: progress, completion: completion)
idCounter += 1
return tweener
// MARK: Instance Variables
/// An internal means for uniquely identifying Tweener instance.
fileprivate let id: Int
/// The duration of the tween.
fileprivate let duration: CFTimeInterval
/// The initial delay before the tween starts.
fileprivate let delay: CFTimeInterval
/// The starting value of the tween.
fileprivate let fromValue: Double
/// The ending value of the tween.
fileprivate let toValue: Double
/// The easing transform to use.
fileprivate let easing: TweenerEasing.EasingTransform
/// The progress closure for the tween.
fileprivate let progress: TweenProgress
/// The completion closure for the tween.
fileprivate let completion: TweenComplete?
/// The paused state of the tween. Setting this to true will prevent the progress closure from being called.
/// When unpaused, the tween resumes where it left off, regardless of the elapsed time.
public var isPaused: Bool = false
/// Set once the tween is completed.
fileprivate var isComplete: Bool = false
/// The amount of time that has elapsed since the tween was created. This value does not increase while the tween is paused, but does increase during the delay interval.
fileprivate(set) var elapsedTime: CFTimeInterval = 0.0
/// The time elapsed as the tween progresses. Will be 0.0 if the tween is in its `delay` period and will not exceed `duration`.
var progressTime: CFTimeInterval {
return max(0.0, min(duration, elapsedTime - delay))
/// Keeps track of whether the initial progress callback invocation has happened.
fileprivate var initialProgressCalled: Bool = false
// MARK: Lifecycle
/// The initializer is fileprivate. Please use `Tweener.tween(...)` to instantiate tweeners.
fileprivate init(id: Int, duration: CFTimeInterval, delay: CFTimeInterval, from: Double, to: Double, easing: @escaping TweenerEasing.EasingTransform, progress: @escaping TweenProgress, completion: TweenComplete?) { = id
self.duration = max(0.0, duration)
self.delay = max(0.0, delay)
self.fromValue = from
self.toValue = to
self.progress = progress
self.easing = easing
self.completion = completion
if delay <= 0.0 {
self.progress(from, self)
/// Cancel a tween. Calling this method will cause the completion closure to be invoked with a value of false.
public func cancel() {
isComplete = true
completion?(false, self)
/// Invoked by the class to advance the tween.
/// - Returns: `true` if the tween finished, otherwise `false`.
fileprivate func tick(elapsedTime dt: CFTimeInterval) -> Bool {
// Do nothing if this tween is paused (this shouldn't happen).
guard !isPaused else { return false }
// Add to the elapsed time.
elapsedTime += dt
// If the elapsed time has not exceeded the delay, nothing will happen yet.
guard elapsedTime >= delay else { return false }
// If the initial progress call has not been made, make it now.
guard initialProgressCalled else {
initialProgressCalled = true
self.progress(0.0, self)
return false
let progressTime = elapsedTime - delay
// Check whether this tween has completed. If so, take appropriate action.
guard progressTime < duration else {
// Call the progress handler with final values.
self.progress(toValue, self)
// Call the completion handler, if present.
completion?(true, self)
// Mark self complete.
isComplete = true
// Report that we finished so the static tick method performs a cleanup sweep.
return true
// Calcualte the tween's progress toward its duration.
let progress = progressTime / duration
// Run that value through the tween's easing function.
let eased = easing(progress)
// Calculate how the eased progress translates into the value range of the tween.
let value = (toValue - fromValue) * eased + fromValue
// Call the progress closure.
self.progress(value, self)
// This tween did not finish.
return false
// Implementing equality operator to conform to the Equatible protocol.
public func == (t0: Tweener, t1: Tweener) -> Bool { return == }
public enum TweenerEasing {
/// A closure which takes progress scalar (0.0 - 1.0) and transforms it to a different progress scalar.
public typealias EasingTransform = (Double) -> Double
/// The `Linear` easing does not ease the progress.
public enum Linear {
public static let easeNone: EasingTransform = {
(progress: Double) -> Double in return progress
/// The `Quadratic` easing is an average easing family, similar to what is provided by CAAnimation.
public enum Quadratic {
/// The animation starts slowly and ends more quickly.
public static let easeIn: EasingTransform = {
(progress: Double) -> Double in
return progress * progress
/// The animation starts quickly and ends more slowly.
public static let easeOut: EasingTransform = {
(progress: Double) -> Double in
return -(progress * (progress - 2.0))
/// The animation is slow at both ends and quicker in the middle.
public static let easeInOut: EasingTransform = {
(progress: Double) -> Double in
if (progress < 0.5) {
return 2.0 * progress * progress;
else {
return (-2.0 * progress * progress) + (4.0 * progress) - 1.0;
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tfe commented Jun 22, 2021

Hello, just wanted to let you know that I found this useful and am using it in a project. I think I found and fixed a bug though; on this line the initial progress call should use the fromValue rather than 0:

Does that seem right?

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