I’m an iOS Engineer at where I develop for game devs. Not so long ago I got a task to completely reimplement UI and animations. One of the most interesting parts was implementing of balance view with rolling animation of the number in it: Triumph Labs TriumphSDK https://vimeo.com/813889559 At the first sight, it looks pretty simple but we needed a well-configured custom solution. Of course, I checked existing libraries that I could reuse and adjust this kind of animation for our needs. After the first try, I figured out that the libraries are not so adjustable and we cannot control the animation of each number. I won’t go into details too much, however, I decided to make my own solution from scratch. Now it is an open-source project that is available as Cocoapod and Swift package: https://github.com/maxkalik/RollingNumbers CALayers I had a list of requirements — how should the animation look, but also I added another one — the component should be implemented only using . Why? Most of the libraries that I checked were implemented using only . The typical approach is making or inside of a where arranged views are UILabels. The rolling animation is implemented by using changing constraints. That’s absolutely ok but I wanted as much as possible to reduce the workload of performance. CALayers UIViews UIScrollViews UIStackVeiws UIView UIView.animate represent the visual content of . Layers provide low-level API of an efficient and more detailed configuration of rendering and animation using a low-cost framework. Another important thing about Layers is that they are rendered using only GPU resources. This means the CPU will be free for another calculation task while the animation is running — that’s what I wanted to mention about performance. CALayers UIView Core Animation Solution If to check open-source solutions, the approach of how to construct the view is everywhere almost the same: a stack of numbers but visible ones only based on the state. My wireframe looks the same but different :) As you can see in the picture the stacks of numbers are doubled, this means the number of elements in one stack is 20. This particular number of elements needs a smooth transition from an initial state to the next one and in the meantime the direction of rolling I wanted to be fully configurable. Let’s say, if a stack would have only 10 elements (and it would be pretty reasonable from the first sight) then the rolling direction from one digit to another would be always different. I needed to obey the direction rolling, for example, all digits from up to down. Each stack in the view is where sublayers are numbers. The numbers are arranged by the same height, which means it’s easy to calculate the next position of the future number. CALayer Characters All characters are and they have their own width. If to use it’s not needed to care about width, all will be arranged by view configuration, but in my case — don’t have such default implementation so it means it should be calculated. CATextLayers UIStackView CALayers To get the width of a character is a pretty straight-forward task using and font: NSAttributedString } private func prepareWidthOfChar ( _ char : Character ) -> Double { let fontAttributes = [ NSAttributedString . Key .font: font] let size = String (char).size(withAttributes: fontAttributes as [ NSAttributedString .Key : Any ]) return size.width * characterSpacing This function also helps to calculate an actual with of all columns in the Rolling Numbers view. To get this information a developer can use the public property . width Animation Configuration To achieve the spring animation effect I used changing position of the — a column where sublayers are numbers . CASpringAnimation y CALayer CATextLayers } } private func animate ( config : RollingNumbersView . AnimationConfiguration , completion : (() -> Void ) ? = nil ) { CATransaction .begin() let animation: CASpringAnimation = CASpringAnimation (keyPath: "position.y" ) let fromValue: CGFloat = position.y let toValue: CGFloat = moveToDigit() animation.fromValue = fromValue animation.toValue = toValue animation.duration = config.duration animation.speed = config.speed animation.damping = config.damping animation.initialVelocity = config.initialVelocity animation.isRemovedOnCompletion = false animation.fillMode = .forwards CATransaction .setCompletionBlock { completion ? () add(animation, forKey: nil ) CATransaction .commit() Additionally, I used to catch the animation completion moment which reference I exposed in the Rolling Numbers view API as trailing completion. It happens only once after setting a new number with animation. CATransaction } rollingNumbersView.setNumberWithAnimation( 245699 ) { // completion For the default animation config, I prepared a separate called: . There are 4 initial configurations of the spring animation that are publically accessible. struct AnimationConfiguration duration: CFTimeInterval = 1 , speed: Float = 0.3 , damping: CGFloat = 17 , initialVelocity: CGFloat = 1 Animation Type There are 4 animation types prepared for usage: } public enum AnimationType { case allNumbers case onlyChangedNumbers case allAfterFirstChangedNumber case noAnimation is accessible using public property . AnimationType animationType Let’s consider an example formatted as US currency. The initial value is $4.588.77. The future value is $4.5 7 76.6 (the changed numbers in the price I highlighted in bold). By default, the animation is set up with . This means if a future number is in the middle of the horizontal string then all others digits after this number will be animated. If to use — this means literally: if a future number is different then only this number column will be scrolled. But somebody will need to roll all digits so for this you can use just . .allAfterFirstChangedNumber .onlyChangedNumbers .allNumbers Rolling Direction The numbers rolling direction can be configured using specific public properties : obviously only two directions: and . For example, if to set up this property as then the numbers in the view will always move in the up direction. However, initially, this property is nil which means by default the direction depends on a future number. If the future number will be less than the old one then the numbers column will move down and vice versa. rollingDirection RollingDirectionwhere .up .down .up Formatting There are several options to format the Rolling Numbers: alignment, character spacing, font, , and text color. NumberFormatterconfigurator }() var rollingNumbersView = { // Initialize Rolling Numbers view with initial number value let view = RollingNumbersView (number: 1234.56 ) // Spacing between numbers view.characterSpacing = 1 // Text color view.textColor = .black // Alignment within UIView view.alignment = .left // UIFont view.font = .systemFont(ofSize: 48 , weight: .medium) let formatter = NumberFormatter () formatter.numberStyle = .currency view.formatter = formatter return view Changing the size of the font keep in mind that the actual height of the view can be smaller, this means the digits can be invisible. Besides general public property , there is another interesting public method: textColor rollingNumbersView.setTextColor(.blue, withAnimationDuration: 3 ) Under the hood, the animation implemented using of .The public method helps to change the color of the text (numbers) while moving animation is happening! As an option changing color can be also an animation with duration. CABasicAnimation forgroundColor setTextColor Wrapping up I enjoy building UI for mobile apps and solving performance problems. If it’s needed to use more than just then Apple Documentation and tons of examples can help to make a more efficient solution. UIView When I finished building the Rolling Numbers component, I decided to share my solution with everyone — making a public library. Full documentation of usage of Rolling Number you can get from the GitHub repository: . Welcome to everyone who can contribute and suggest improvements to this project via PR! https://github.com/maxkalik/RollingNumbers Links Core Animation Programming Guide CALayer Rolling Numbers

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