ActivityIndicatorView/Source/Indicators/FlickeringDotsIndicatorView.swift

//
//  FlickeringDotsIndicatorView.swift
//  ActivityIndicatorView
//
//  Created by Daniil Manin on 10/7/20.
//  Copyright © 2020 Exyte. All rights reserved.
//

import SwiftUI

struct FlickeringDotsIndicatorView: View {

    private let count: Int = 8

    var body: some View {
        GeometryReader { geometry in
            ForEach(0..<self.count) { index in
                FlickeringDotsIndicatorItemView(index: index, count: self.count, size: geometry.size)
            }.frame(width: geometry.size.width, height: geometry.size.height)
        }
    }
}

struct FlickeringDotsIndicatorItemView: View {

    let index: Int
    let count: Int
    let size: CGSize

    @State private var scale: CGFloat = 0
    @State private var opacity: Double = 0

    var body: some View {
        let duration = 0.5
        let itemSize = size.height / 5
        let angle = 2 * CGFloat.pi / CGFloat(count) * CGFloat(index)
        let x = (size.width / 2 - itemSize / 2) * cos(angle)
        let y = (size.height / 2 - itemSize / 2) * sin(angle)

        let animation = Animation.linear(duration: duration)
            .repeatForever(autoreverses: true)
            .delay(duration * Double(index) / Double(count) * 2)

        return Circle()
            .frame(width: itemSize, height: itemSize)
            .scaleEffect(scale)
            .opacity(opacity)
            .onAppear {
                self.scale = 1
                self.opacity = 1
                withAnimation(animation) {
                    self.scale = 0.5
                    self.opacity = 0.3
                }
            }
            .offset(x: x, y: y)
    }
}