StopWatch.swift

public class StopWatch {

    private var elapsed: Double = 0
    private var isRunning: Bool = false
    private var startTime: Double = 0

    public init() {}

    public var elapsedTimeSeconds: Double {
        if isRunning {
            return elapsed + max(0, Date.timeIntervalSinceReferenceDate - startTime)
        } else {
            return elapsed
        }
    }

    public func start() {
        guard !isRunning else { return }
        startTime = Date.timeIntervalSinceReferenceDate
        isRunning = true
    }

    public func stop() {
        guard isRunning else { return }
        elapsed += max(0, Date.timeIntervalSinceReferenceDate - startTime)
        isRunning = false
    }

    public func reset() {
        isRunning = false
        startTime = 0
        elapsed = 0
    }
}