Popsicle is an easy-as-pie approach to creating and managing interpolations of different value types, as well as implementing your own custom value interpolations.

Note: this framework comes out as an abstraction of former DRDynamicSlideShow's keyframing logic. You can combine this framework with components such as DRPageScrollView or vanilla UIScrollView and still get the same functionality provided by DRDynamicSlideShow.

Installation

Carthage

github "Dromaguirre/Popsicle"

CocoaPods

pod 'Popsicle'

Manual

Drag and copy all files in the Popsicle folder into your project.

At a glance

Interpolating UIView values

Popsicle is basically divided in two classes: PSInterpolator and PSInterpolation.

First, you need a retained instance of PSInterpolator, which is as simple as declaring it as a property or ivar (commonly in your view controller) and initializing it like this:

interpolator = [PSInterpolator new];

Note: interpolator has a time property. Every PSInterpolation instance you add to that PSInterpolator instance will depend on that property to determine its current value. time is an abstract concept here, and has nothing to do with literal time. You may define time however you want. For example, time may be the position of a finger on the screen from 0 to 240, determined by a UIPanGestureRecognizer, or the amount of brightness of the screen from 0 to 1, determined by UIScreen.

Next, you need to add some PSInterpolation instances to your interpolator. In the example below, we are going to interpolate the alpha value of a UIView through a PSFloatInterpolation, with a time range of 0 to 100:

[self.interpolator addInterpolations:PS(PSFloatInterpolation, 0, 100, 1, 0) forObjects:view keyPath:@"alpha"];

Note: for the sake of simplicity, Popsicle provides a great macro to create instances of PSInterpolation: PS(INTERPOLATION_CLASS, START_TIME, END_TIME, FROM_VALUE, TO_VALUE). Also note you can specify multiple interpolations for multiple objects at once.

Finally, just make your interpolator vary its time depending on whatever you want. For example, in case you're using a UIScrollView:

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
	interpolator.time = scrollView.contentOffset.x
	// view.alpha = 1 when scrollView.contentOffset.x = 0
	// view.alpha = 0.75 when scrollView.contentOffset.x = 25
	// view.alpha = 0.5 when scrollView.contentOffset.x = 50
	// ... and so on.
}

Creating custom value interpolations

You can create your own value interpolations subclassing PSInterpolation.

In the example below, you'll see how PSFloatInterpolation has been implemented.

// PSFloatInterpolation.h

#import "PSInterpolation.h"

@interface PSFloatInterpolation : PSInterpolation

// This allows PS() macro to work correctly.
+ (instancetype)interpolationWithStartTime:(float)startTime endTime:(float)endTime fromValue:(float)fromValue toValue:(float)toValue;

@end

// PSFloatInterpolation.h

#import "PSFloatInterpolation.h"
#import "PSInterpolation+Subclass.h" // Notice you need this file for Interpolation() to work.

@implementation PSFloatInterpolation

// Remember fromValue and toValue are objects, so we need to pass in a NSNumber. You may make any necessary value conversions as shown below.
// If this library was written in Swift, this "problem" wouldn't exist, since values like float are already objects in Swift. Give me some time :)
+ (instancetype)interpolationWithStartTime:(float)startTime endTime:(float)endTime fromValue:(float)fromValue toValue:(float)toValue {
	return [super interpolationWithStartTime:startTime endTime:endTime fromValue:@(fromValue) toValue:@(toValue)];
}

// This method is called whenever PSInterpolator performs a new interpolation, so you must return a value here depending on timeFraction (from 0 to 1).
// The easiest way possible is using Interpolation(), already included in PSInterpolation+Subclass.h.
- (id)valueForTimeFraction:(float)timeFraction {
	// Here we retrieve our fromValue and toValue and convert them to float in order to manage them with Interpolation().
	float fromValue = [self.fromValue floatValue];
	float toValue = [self.toValue floatValue];
	
	// Here we interpolate fromValue and toValue depending on the parameter timeFraction in order to get the intermediate value we need to return.
	float value = Interpolation(fromValue, toValue, timeFraction);
	
	// Here we retrieve our fromValue and toValue and convert them to float in order to manage them with Interpolation().
	return @(value);
}

@end

License

Popsicle is available under the MIT license.