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How to create keyframe animations using animateKeyframes()

Swift version: 5.1

Paul Hudson    @twostraws   

If you need to combine a selection of animations over time the easiest thing to do is create a keyframe animation. This starts with a call to animateKeyframes(), then you go ahead and call addKeyframe() as many times as you need.

Each keyframe you add has a relative start time and relative duration, so they work independently of the overall timing. You also provide each keyframe with the animation it should perform, again not worrying about the rest of the animation. When it runs, iOS combines them all together, blending one animation seamlessly into the next.

To give you an example, here’s some code that moves and scales an image view around the screen:

let start = self.imageView.center

UIView.animateKeyframes(withDuration: 5, delay: 0, options: .calculationModeCubic, animations: {
    UIView.addKeyframe(withRelativeStartTime: 0.0, relativeDuration: 0.25) {
        self.imageView.transform = CGAffineTransform(scaleX: 2, y: 2)
    }

    UIView.addKeyframe(withRelativeStartTime: 0.25, relativeDuration: 0.25) {
        self.imageView.center = CGPoint(x: self.view.bounds.midX, y: self.view.bounds.maxY)
    }

    UIView.addKeyframe(withRelativeStartTime: 0.5, relativeDuration: 0.25) {
        self.imageView.center = CGPoint(x: self.view.bounds.width, y: start.y)
    }

    UIView.addKeyframe(withRelativeStartTime: 0.75, relativeDuration: 0.25) {
        self.imageView.center = start
    }
})

The calculationModeCubic option tells iOS to blend the animations together, so you’ll see the image view overshoot one animation as it curves into the next. You should also try using calculationModeCubicPaced instead – in the above code it makes the scaling part run over the entire length of the animation.

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