DownloadProgress.swift

//
//  DownloadProgress.swift
//  

import Foundation

class DownloadProgress {
    
    // MARK: - Properties
    
    var estimatedTimeRemaining: TimeInterval?
    var onEstimatedTimeRemainingUpdate: ((_ time: TimeInterval?) -> Void)?
    
    private(set) var progress: Progress?
    
    private var remainingTimeTimer: Timer?
    
    private let smoothingFactor: Double = 0.05
    private var averageSpeed: Double = 0
    private var lastSpeed: Int64 = 0
    private var lastCompletedUnitCount: Int64 = 0
    
    // MARK: - Lifecycle
    
    init() {
        startRemainingTimeTimer()
    }
    
    deinit {
        stopRemainingTimeTimer()
    }
    
    // MARK: - Actions
    
    func update(with progress: Progress) {
        self.progress = progress
    }
    
    private func calculateTimeRemaining() {
        
        guard let progress = self.progress else {
            estimatedTimeRemaining = nil
            return
        }
        
        lastSpeed = progress.completedUnitCount - lastCompletedUnitCount
        lastCompletedUnitCount = progress.completedUnitCount
        
        averageSpeed = smoothingFactor * Double(lastSpeed) + (1 - smoothingFactor) * averageSpeed
        
        estimatedTimeRemaining = Double(progress.totalUnitCount - progress.completedUnitCount) / averageSpeed
        
        onEstimatedTimeRemainingUpdate?(estimatedTimeRemaining)
    }
    
    // MARK: - Timer
    
    private func startRemainingTimeTimer() {
        
        remainingTimeTimer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { [weak self] _ in
            
            self?.calculateTimeRemaining()
        }
    }
    
    private func stopRemainingTimeTimer() {
        
        remainingTimeTimer?.invalidate()
        remainingTimeTimer = nil
    }
}

Download progress class for calculating download time remaining.
Usage:

// store an instance somewhere
private let downloadProgress = DownloadProgress()

// when a `Progress` swift object arrives (from Alamofire for example)
downloadProgress.update(with: progress)

// call manually at any time
downloadProgress.estimatedTimeRemaining

// or use callback 
downloadProgress.onEstimatedTimeRemainingUpdate = { [weak self] _ in
    self?.updateDownloadEstimatedTimeRemaining()
}