Wave.swift

struct Wave: Shape {

    var offset: Angle
    var percent: Double

    var animatableData: Double {
        get { offset.degrees }
        set { offset = Angle(degrees: newValue) }
    }

    func path(in rect: CGRect) -> Path {
        
        var p = Path()
        let waveHeight = 0.015 * rect.height
        let yOffset = CGFloat(1 - percent) * (rect.height - 4 * waveHeight) + 2 * waveHeight
        let startAngle = offset
        let endAngle = offset + Angle(degrees: 360)
        p.move(to: CGPoint(x: 0, y: yOffset + waveHeight * CGFloat(sin(offset.radians))))

        for angle in stride(from: startAngle.degrees, through: endAngle.degrees, by: 5) {
            let x = CGFloat((angle - startAngle.degrees) / 360) * rect.width
            p.addLine(to: CGPoint(x: x, y: yOffset + waveHeight * CGFloat(sin(Angle(degrees: angle).radians))))
        }

        p.addLine(to: CGPoint(x: rect.width, y: rect.height))
        p.addLine(to: CGPoint(x: 0, y: rect.height))
        p.closeSubpath()

        return p
    }
}