INTUAnimationEngine makes it easy to build advanced custom animations on iOS.

INTUAnimationEngine provides a friendly interface to drive custom animations using a CADisplayLink, inspired by the UIView block-based animation API. It enables interactive animations (normally driven by user input, such as a pan or pinch gesture) to run automatically over a given duration. It can also be used to get a callback every frame of an animation.

INTUAnimationEngine includes an extensive library of easing functions that can be used to customize animation timing, as well as a complete library of interpolation functions to animate any type of value or property including those that are not animatable by Core Animation.

The project also includes a standalone spring physics library to simulate damped harmonic motion. This is used under the hood to power a spring animation API on INTUAnimationEngine that allows full control over the damping, stiffness, and mass parameters. Since the spring solver is a completely independent and generic library implemented in pure C, it can be used on its own for many other applications apart from animation.

Installation

INTUAnimationEngine requires iOS 5.0 or later.

Installing the Spring Solver Library Only

The Spring Solver used by INTUAnimationEngine is available as a standalone C library, where it can be used for other applications (including ones that are not related to animation). The Spring Solver has its own CocoaPods subspec so that it can be installed separately from the rest of the INTUAnimationEngine project. To install the Spring Solver only, add the following line to your Podfile:

pod 'INTUAnimationEngine/SpringSolver'

Note that installing INTUAnimationEngine using pod 'INTUAnimationEngine' automatically includes the Spring Solver library as a dependency.

Manually from GitHub

1. Download the contents of the INTUAnimationEngine directory.
2. Add all the files to your Xcode project (drag and drop is easiest).
3. Import the INTUAnimationEngine.h header.

Usage

The primary difference between INTUAnimationEngine and the UIView animation methods is how the animations block works. With the UIView methods, the animations block is only executed once, and the changes made to views within this block represent the new state at the end of the animation.

With INTUAnimationEngine, the animations block is executed many times during the animation (once per frame), and each time it is executed, your code inside the block should update the state of views based upon the current value of the percentage or progress passed into the block. Typically, you'll want to use one of the interpolation functions included in this library to help generate all the intermediate values between the start and end states for a given property.

Animation Engine

There are a few different API methods on INTUAnimationEngine that can be used to start an animation.

Without Easing (Linear)

+ (INTUAnimationID)animateWithDuration:(NSTimeInterval)duration
                                 delay:(NSTimeInterval)delay
                            animations:(void (^)(CGFloat percentage))animations
                            completion:(void (^)(BOOL finished))completion;

This method will start an animation that calls the animations block each frame of the animation, passing in a percentage value that represents the animation's current percentage complete. The completion block will be executed when the animation completes, with the finished parameter indicating whether the animation was canceled.

With Easing

+ (INTUAnimationID)animateWithDuration:(NSTimeInterval)duration
                                 delay:(NSTimeInterval)delay
                                easing:(INTUEasingFunction)easingFunction
                            animations:(void (^)(CGFloat progress))animations
                            completion:(void (^)(BOOL finished))completion;

This method will start an animation that calls the animations block each frame of the animation, passing in a progress value that represents the current progress of the animation (taking into account the easing function). The easingFunction can be any of the easing functions in INTUEasingFunctions.h, or a block that defines a custom easing curve. The completion block will be executed when the animation completes, with the finished parameter indicating whether the animation was canceled.

There is also another variant of the above method that takes an options: parameter, which is a mask of INTUAnimationOptions. This can be used to repeat or autoreverse animations.

Using a Spring

+ (INTUAnimationID)animateWithDamping:(CGFloat)damping
                            stiffness:(CGFloat)stiffness
                                 mass:(CGFloat)mass
                                delay:(NSTimeInterval)delay
                           animations:(void (^)(CGFloat progress))animations
                           completion:(void (^)(BOOL finished))completion;

This method will start a spring animation that calls the animations block each frame of the animation, passing in a progress value that represents the current progress of the animation. The animation will simulate the physics of a spring-mass system with the specified properties:

• damping – The amount of friction. Must be greater than or equal to zero. If exactly zero, the harmonic motion will continue indefinitely. Typical range: 1.0 to 30.0
• stiffness – The stiffness of the spring. Must be greater than zero. Typical range: 1.0 to 500.0
• mass – The amount of mass being moved by the spring. Must be greater than zero. Typical range: 0.1 to 10.0

Note that the total duration of the animation is determined by simulating a spring-mass system with the above parameters until it reaches a resting state. The completion block will be executed when the animation completes, with the finished parameter indicating whether the animation was canceled.

Canceling Animations

+ (void)cancelAnimationWithID:(INTUAnimationID)animationID;

When starting an animation, you can store the returned animation ID, and pass it to the above method to cancel the animation before it completes. If the animation is canceled, the completion block will execute with finished parameter equal to NO.

Easing Functions

INTUEasingFunctions.h is a library of standard easing functions. Here's a handy cheat sheet that includes visualizations and animation demos for these functions.

Interpolation Functions

INTUInterpolationFunctions.h is a library of interpolation functions.

Proximal Interpolation

For discrete values (where linear interpolation does not make sense), there are two proxmial interpolation functions. For example:

INTUInterpolateDiscrete(NSTextAlignmentLeft, NSTextAlignmentRight, progress)
// Returns NSTextAlignmentLeft when progress is < 0.5, NSTextAlignmentRight otherwise

[INTUInterpolateDiscreteValues(@[@(NSTextAlignmentLeft), @(NSTextAlignmentCenter), @(NSTextAlignmentRight)], progress) integerValue]
// Returns NSTextAlignmentLeft, then NSTextAlignmentCenter, and finally NSTextAlignmentRight as progress increases from 0.0 to 1.0

Linear Interpolation

For continuous values, there are a variety of linear interpolation functions. The following types are supported:

• CGFloat
• CGPoint
• CGSize
• CGRect
• CGVector
• UIOffset
• UIEdgeInsets
• UIColor / CGColor

There is also an untyped function INTUInterpolate() that takes values of type id and returns an interpolated value by automatically determining the type of the values. Proximal interpolation is used if the value types do not match, or if linear interpolation isn't supported for their type.

CGAffineTransform & CATransform3D

There are no functions that directly interpolate transforms. This is by design: linear interpolation of raw matrices often yields unexpected or invalid results. To interpolate between two transforms, decompose them into their translation, rotation, and scale components:

CGFloat rotation = INTUInterpolateCGFloat(0.0, M_PI, progress);
view.transform = CGAffineTransformMakeRotation(rotation);
// view will rotate from upright (progress = 0.0) to upside down (progress = 1.0)

You can concatenate transforms to combine them:

CGFloat rotation = INTUInterpolateCGFloat(0.0, M_PI, progress);
CGFloat scale = INTUInterpolateCGFloat(1.0, 0.5, progress);
view.transform = CGAffineTransformConcat(CGAffineTransformMakeScale(scale, scale), CGAffineTransformMakeRotation(rotation));
// view will rotate from upright and full size (progress = 0.0), to upside down and half size (progress = 1.0)

UIColor / CGColor

When interpolating between two colors, both colors must be in the same color space (grayscale, RGB, or HSB). Interpolating between colors in the HSB color space will generally yield better visual results than the RGB color space.

[UIColor colorWithWhite:1.0 alpha:1.0] // Grayscale color space; white
[UIColor colorWithRed:1.0 green:1.0 blue:1.0 alpha:1.0] // RGB color space; white
[UIColor colorWithHue:0.0 saturation:0.0 brightness:1.0 alpha:1.0] // HSB color space; white

Spring Solver

The SpringSolver directory in the project contains a spring physics library to simulate damped harmonic motion, based on the spring solver that powers Facebook's Pop. The INTUAnimationEngine spring solver has been extensively refactored for simplicity and performance, and as a fully independent pure C library is highly portable to any platform and can be leveraged for other use cases beyond animation.

Example Project

An example project is provided. It requires Xcode 6 and iOS 6.0 or later.

Issues & Contributions

Please open an issue here on GitHub if you have a problem, suggestion, or other comment.

Pull requests are welcome and encouraged! There are no official guidelines, but please try to be consistent with the existing code style.

License

INTUAnimationEngine is provided under the MIT license. The spring solver library within the project is provided under a BSD license.

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