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A powerful, elegant, and modular animation library for Swift.
.package(url: "https://github.com/poetmountain/MotionMachine.git", from: "2.0.1")

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MotionMachine provides a modular, powerful, and generic platform for manipulating values, whether that be animating UI elements or interpolating property values in your own classes. It offers sensible default functionality that abstracts most of the hard work away, allowing you to focus on your work. While it is type-agnostic, MotionMachine does support most major UIKit types out of the box and provides syntactic sugar to easily manipulate them. But it's also easy to dive in and modify for your own needs, whether that be custom motion classes, supporting custom value types, or new easing equations.

  • Animation engine built from the ground up (not tied to Core Animation).
  • Motions can be grouped, sequenced, and nested in any arrangement and have reversing and repeating actions applied at any level.
  • Animate properties of UIKit or any generic classes.
  • Includes both static and physics-based motion classes, and both support additive animation.
  • Powerfully modular – most aspects can be customized or outright replaced to fit your specific needs.
  • Provides status callback closures for many types of motion events.
  • Fully tested
  • Fully documented

Getting Started

Get started with the Motion Classes guide for detailed explanations and examples.

Also check out the Examples project to see all the MotionMachine classes in action, or dive deep into the source Documentation.


MotionGroup animation

This complex animation was created with the code sample below. These Motion classes animate the NSLayoutConstraints of the circle views (the constraints object in the target parameter is a dictionary of NSLayoutConstraint references) as well as one of their backgroundColor properties. A MotionGroup object is used to synchronize the four Motion objects and reverse their movements.

let group = MotionGroup()

.add(Motion(target: constraints["circleX"]!,
        properties: [PropertyData("constant", 200.0)],
          duration: 1.0,
            easing: EasingQuartic.easeInOut()))

.add(Motion(target: constraints["circleY"]!,
        properties: [PropertyData("constant", 250.0)],
          duration: 1.4,
            easing: EasingElastic.easeInOut()))

.add(Motion(target: circle,
        properties: [PropertyData("backgroundColor.blue", 0.9)],
          duration: 1.2,
            easing: EasingQuartic.easeInOut()))

.add(Motion(target: constraints["circle2X"]!,
        properties: [PropertyData("constant", 300.0)],
          duration: 1.2,
            easing: EasingQuadratic.easeInOut())
            .reverses(withEasing: EasingQuartic.easeInOut()))


How does this work?

All of the included motion classes in MotionMachine adopt the Moveable protocol, which enables them to work seamlessly together. By using the MotionGroup and MotionSequence collection classes to control multiple motion objects – even nesting multiple layers – you can create complex animations with little effort.


Motion uses a KVC keyPath (i.e. "frame.origin.x") to target specific properties of an object and transform their values over a period of time via an easing equation. In this example, we pass in PropertyStates structs to the statesForProperties convenience initializer to provide ending value states for the transform and backgroundColor properties of the target object.

let new_color = UIColor.init(red: 91.0/255.0, green:189.0/255.0, blue:231.0/255.0, alpha:1.0)
let circle = circles[0]

motion = Motion(target: circle,
   statesForProperties: [
    PropertyStates(path: "transform", end: circle.transform.scaledBy(x: 1.5, y: 1.5)),
    PropertyStates(path: "backgroundColor", end: new_color)
              duration: 2.0,
                easing: EasingBack.easeInOut(overshoot: 0.5))

Motion animation


MotionGroup is a MoveableCollection class that manages a group of Moveable objects, controlling their movements in parallel. It's handy for controlling and synchronizing multiple Moveable objects. MotionGroup can even control other MoveableCollection objects. In the below example, we told the MotionGroup to reverse and synchronize its child motions while doing so. What this means is that it will pause all motions after the forward movement is done, and only then will it reverse them. In this case, the horizontal movements pause while waiting for the Motion which modifies the second circle's backgroundColor to finish its 3 second duration.

// the MotionGroup will wait for all child motions to finish moving forward before starting their reverse motions
group = MotionGroup().reverses(syncsChildMotions: true)

// move first circle horizontally
let horizontal1 = Motion(target: constraints["x1"]!,
                         properties: [PropertyData("constant", 250.0)],
                         duration: 1.5,
                         easing: EasingSine.easeOut())

// reverse and repeat horizontal movement of second circle once, with a subtle overshoot easing
let horizontal2 = Motion(target: constraints["x2"]!,
                  properties: [PropertyData("constant", 250.0)],
                  duration: 1.0,
                  easing: EasingBack.easeOut(overshoot: 0.12))

// change backgroundColor of second circle
let color = Motion(target: circles[1],
                   statesForProperties: [PropertyStates(path: "backgroundColor", end: UIColor.init(red: 91.0/255.0, green:189.0/255.0, blue:231.0/255.0, alpha:1.0))],
                   duration: 3.0,
                   easing: EasingQuadratic.easeInOut())


MotionGroup animation


MotionSequence is a MoveableCollection class which moves a collection of Moveable objects in sequential order, even other MoveableCollection objects. MotionSequence provides a powerful and easy way of chaining together value transformations of object properties to do keyframing or to create complex and fluid compound animations of many objects.

// create a reversing MotionSequence with its reversingMode set to contiguous to create a fluid animation from its child motions
sequence = MotionSequence().reverses(.contiguous)

// set up motions for each circle and add them to the MotionSequence
for x in 0..<4 {
    // motion to animate a topAnchor constraint down
    let down = Motion(target: constraints["y\(x)"]!,
                      properties: [PropertyData("constant", 60.0)],
                      duration: 0.4,
                      easing: EasingQuartic.easeInOut())

    // motion to change background color of circle
    let color = Motion(target: circles[x],
                       statesForProperties: [PropertyStates(path: "backgroundColor", end: UIColor.init(red: 91.0/255.0, green:189.0/255.0, blue:231.0/255.0, alpha:1.0))],
                       duration: 0.3,
                       easing: EasingQuadratic.easeInOut())

    // wrap the Motions in a MotionGroup and set it to reverse
    let group = MotionGroup(motions: [down, color]).reverses(syncsChildMotions: true)

    // add group to the MotionSequence

MotionSequence animation


MotionMachine supports Swift Packages in Xcode 11 and above.

If you use CocoaPods, add this pod to your Podfile:

pod 'MotionMachine', '~> 2.0'

Or add the Sources directory to your project.


MotionMachine currently requires:

  • Swift 5.0
  • Xcode 10.0 or later
  • iOS 8.0 or later, tvOS 9.0 or later


  • MotionMachine uses Key-Value Coding (KVC) to introspect objects and retrieve and set their property values using keypaths. Because Swift currently offers no native ability in this regard, objects whose properties should be modified by MotionMachine must inherit from NSObject. If and when more dynamism is added to Swift (and the author of this library hopes that is the case), MotionMachine will hopefully be able to do away with this restriction. Note that as of Swift 4.0, any properties of a custom class you wish to manipulate must be prefixed with @objc, or add @objcMembers above the class if all properties should be exposed.

  • Because native Swift structs cannot inherit from NSObject, Swift structs unfortunately cannot be used directly with MotionMachine at this time, though you can use them in a keyPath if you're not targeting one of their properties directly.

  • The KVC provided by NSObject is not able to evaluate Optional values. Properties you wish to modify with MotionMachine must not be Optionals.

  • Swift on Linux is not currently supported due to the lack of Foundation and Core Graphics frameworks on that platform.


MotionMachine was created by Brett Walker. It is loosely based on the author's Objective-C library PMTween.


MotionMachine is licensed under the MIT License. See LICENSE for details.

I'd love to know if you use MotionMachine in your projects!


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iOS Software Engineer @ Perry Street Software
Perry Street Software is Jack’d and SCRUFF. We are two of the world’s largest gay, bi, trans and queer social dating apps on iOS and Android. Our brands reach more than 20 million members worldwide so members can connect, meet and express themselves on a platform that prioritizes privacy and security. We invest heavily into SwiftUI and using Swift Packages to modularize the codebase.

Release Notes

2 weeks ago
  • Fixed some retain cycles that were holding on to target objects
  • Updated examples project

Swiftpack is being maintained by Petr Pavlik | @ptrpavlik | @swiftpackco | API