animate a UIView using CADisplayLink 'combined' with CAMediaTimingFunction. (to get a arbitrary curve)

Question

I am using a CADisplayLink to do a view animation that just interpolates a value and redraws the view itself.

e.g. I have a view MyView and it has a property value, whenever value is set I call setNeedsDisplay and the view knows what to draw.

to animate this I use CADisplayLink and I want the view to 'morph' between values. I do this by just interpolating the value from the animation's start and stop Value:

- (CGFloat)interpolatedValue:(CGFloat)sourceValue withValue:(CGFloat)targetValue forProgress:(CGFloat)progress;

now getting a linear progress is easy and getting a 'specific curve' (ok) but I want to be able to take advantage of CAMediaTimingFunction to do this (or some other preexisting logic - I don't want to reinvent the wheel again' :)

Answer 1

There's this awesome gist RSTiming which might be handy in your case. You can define timing functions either using standard CAMediaTimingFunction or even define custom ones using 2 control points to define a bezier curve.

If I got your setup, you may have something like this:

ViewController

#import "ViewController.h"
#import "AnimatedView.h"
#include <stdlib.h>

@interface ViewController ()

@property (nonatomic, strong) CADisplayLink *displayLink;
@property (weak, nonatomic) IBOutlet AnimatedView *viewToAnimate;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(updateFrame)];
    [self.displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
}

- (void)updateFrame {
    [self.viewToAnimate updateAnimation];
}

- (IBAction)updateAnimationTapped:(id)sender {
    self.viewToAnimate.value = arc4random_uniform(101) / 100.0;
    NSLog(@"Next animation value: %f", self.viewToAnimate.value);
}

@end

AnimatedView

#import "AnimatedView.h"
#import "RSTimingFunction.h"

@interface AnimatedView()
{
    CGFloat _lastValue;
    CGFloat _progressStep;
    CGFloat _currentProgress;
}

@property (nonatomic, strong) RSTimingFunction *animationProgress;

@end

@implementation AnimatedView

- (instancetype)initWithCoder:(NSCoder *)aDecoder
{
    if ((self = [super initWithCoder:aDecoder]))
    {
        _progressStep = 0.01; // defines animation speed
        _currentProgress = 1.0;
        self.animationProgress = [RSTimingFunction timingFunctionWithName:kRSTimingFunctionEaseInEaseOut];
    }

    return self;
}

- (void)setValue:(CGFloat)value {
    if (_value != value)
    {
        _lastValue = _value;
        _value = value;
        _currentProgress = 0.0;
    }
}

- (void)updateAnimation
{
    if (_currentProgress > 1.0)
        return;

    _currentProgress += _progressStep;
    CGFloat currentAnimationValue = _lastValue + (_value - _lastValue) * [self.animationProgress valueForX:_currentProgress];

    self.alpha = currentAnimationValue; // as an example animate alpha
}

@end

As said you can even set 2 control points to create a timing function modelled on a cubic Bezier curve.

self.animationProgress = [RSTimingFunction timingFunctionWithControlPoint1:CGPointMake(0.6, 0.6) controlPoint2:CGPointMake(0.1, 0.8)];

Which will produce the following timing animation (produced using CAMediaTimingFunction playground)