keyframes.swift

//
//  ContentView.swift
//  KeyFrameAnimation
//
//  Created by Chris Eidhof on 21.10.19.
//  Copyright © 2019 objc.io. All rights reserved.
//

import SwiftUI

extension CGSize {
    static func *(lhs: CGSize, rhs: CGFloat) -> CGSize {
        return CGSize(width: lhs.width * rhs, height: lhs.height * rhs)
    }
}

extension VectorArithmetic {
    func scaled(by amount: Double) -> Self {
        var result = self
        result.scale(by: amount)
        return result
    }
}

extension CGSize: VectorArithmetic {
    public static func -= (lhs: inout CGSize, rhs: CGSize) {
        lhs = lhs - rhs
    }
    
    public static func - (lhs: CGSize, rhs: CGSize) -> CGSize {
        CGSize(width: lhs.width - rhs.width, height: lhs.height - rhs.height)
    }
    
    public static func += (lhs: inout CGSize, rhs: CGSize) {
        lhs = lhs + rhs
    }
    
    public mutating func scale(by rhs: Double) {
        width *= CGFloat(rhs)
        height *= CGFloat(rhs)
    }
    
    public var magnitudeSquared: Double {
        Double((width*width) + (height*height))
    }
    
    public static func + (lhs: CGSize, rhs: CGSize) -> CGSize {
        return CGSize(width: lhs.width + rhs.width, height: lhs.height + rhs.height)
    }
    
    
}


struct Keyframed<D: VectorArithmetic> {
    var start: D
    var keyframes: [(position: Double, value: D)] = []
    var end: D
    
    func value(position: Double) -> D {
        if position >= 1 { return end }
        var lastValue = start
        var lastPosition: Double = 0
        var frames = keyframes
        frames.append((1, end))
        for frame in frames {
            if position < frame.position {
                let amount = (position - lastPosition) / (frame.position - lastPosition)
                return lastValue + (frame.value - lastValue).scaled(by: amount)
            } else {
                lastValue = frame.value
                lastPosition = frame.position
            }
        }
        return end
    }
}

struct KeyframeAnimation: AnimatableModifier {
    var progress: Double = 0
    var values = Keyframed<CGSize>(start: .zero, keyframes: [(position: 0.5, value: CGSize(width: 100, height: 0))], end: CGSize(width: 100, height: 100))
    
    var animatableData: Double {
        get { progress }
        set { progress = newValue }
    }
    
    func body(content: Content) -> some View {
        let offset = values.value(position: progress)
        print(progress, offset)
        return content.offset(offset)
    }
}

struct ContentView: View {
    @State var progress: Double = 0
    var body: some View {
        Text("Hello World")
        	.modifier(KeyframeAnimation(progress: progress))
            .animation(Animation.default.speed(0.2))
            .onAppear { self.progress = 1 }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}