CircularCountdownView.swift

import Foundation
import UIKit
import RxSwift
import RxCocoa

final class CircularCountdownView: UIView {
    let disposeBag = DisposeBag()

    private var progressView = UIView()
    private var progressLayer: CAShapeLayer = {
        let layer = CAShapeLayer()
        layer.lineCap = kCALineCapRound
        layer.fillRule = kCALineJoinRound
        layer.fillColor = UIColor.clear.cgColor
        return layer
    }()

    public var progressStrokeColor: UIColor = .white {
        didSet {
            progressLayer.strokeColor = progressStrokeColor.cgColor
            self.layoutIfNeeded()
        }
    }

    public var progressLineWidth: CGFloat = 3 {
        didSet {
            progressLayer.lineWidth = progressLineWidth
            self.layoutIfNeeded()
        }
    }

    public let totalTime = BehaviorRelay<Int>(value: 0)

    public var finished: Observable<Bool> {
        return finishedSubject.asObservable()
    }

    private var finishedSubject = PublishSubject<Bool>()

    private var secondsElapsed: TimeInterval = 0
    private var totalTimeValue: TimeInterval {
        return TimeInterval(self.totalTime.value)
    }

    override init(frame: CGRect) {
        super.init(frame: frame)

        self.setupUI()

    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func layoutSubviews() {
        super.layoutSubviews()

        let lineWidthExpaded = progressLineWidth * 2
        let expandedWidth = self.boundsWidth + lineWidthExpaded
        let expandedHeight = self.boundsHeight + lineWidthExpaded
        let size = CGSize(width: expandedWidth, height: expandedHeight)
        self.progressView.frameSize = size
        self.progressView.center = self.ownCenter
        self.progressLayer.frame.size = size
    }

    func setupUI() {

        self.addSubview(progressView)
        self.progressView.layer.addSublayer(progressLayer)
        self.progressLayer.lineWidth = progressLineWidth
        self.progressLayer.strokeColor = progressStrokeColor.cgColor

    }

    private func updateProgressLayer(progress: CGFloat) {
        let sizeCenter = CGPoint(x: self.progressView.frameWidth * 0.5,
                                 y: self.progressView.frameHeight * 0.5)
        let radius = (self.progressView.frameWidth - progressLineWidth) * 0.5

        let startAngle = -(CGFloat.pi / 2)
        let endAngle = (progress * CGFloat.pi * 2) - CGFloat.pi / 2

        let path = UIBezierPath(arcCenter: sizeCenter,
                                radius: radius,
                                startAngle: startAngle,
                                endAngle: endAngle,
                                clockwise: true)
        progressLayer.path = path.cgPath
    }

    fileprivate var displayLink: CADisplayLink?

    func start() {
        secondsElapsed = 0

        displayLink?.invalidate()
        displayLink = UIScreen.main.displayLink(withTarget: self, selector: #selector(CircularCountdownView.tick))
        displayLink!.add(to: RunLoop.current, forMode: RunLoopMode.defaultRunLoopMode)

    }

    func stop() {
        displayLink?.invalidate()
        displayLink = nil
    }

    @objc func tick() {
        secondsElapsed += displayLink!.duration
        let progress: CGFloat = CGFloat(secondsElapsed) / CGFloat(totalTimeValue)
        self.updateProgressLayer(progress: progress)
        if progress >= 1.0 {
            stop()
            finishedSubject.onNext(true)
        }
    }

}