A simple fade-to function that employs a Timer for AVPlayer. Allows for specification of fade time rate / interval.

FadeTo.swift

/// Fades current player to specified `volume` with specified `duration`.
    public func fadeTo(volume: Float, duration: TimeInterval = 5.0, completionHandler: @escaping () -> Void) {
        
        // Current player
        let player = self.currentPlayer
        
        // Calculated increments for volume steps
        let volumeIncrementationRate = 0.15 // Volume adjustments pr. seconds
        let volumeDistance = player.volume - volume
        let volumeSteps = duration / volumeIncrementationRate
        let volumeDelta = abs(volumeDistance) / Float(volumeSteps)
        
        // Stop periodic observer for current player (to avoid cross-fading)
        self.removePeriodicTimeObserver(for: self.currentPlayer)
        
        // Start timer
        Timer.scheduledTimer(withTimeInterval: volumeIncrementationRate, repeats: true) { (timer) in
            
            // Fade complete
            func finish() {
                completionHandler()
                timer.invalidate()
            }
            
            // Negative volume adjustment
            if player.volume > min(volume, player.volume) {
                player.volume -= Float(volumeDelta)
                if player.volume < volume {
                   finish()
                }
            }
                
            // Positive volume adjustment
            else {
                player.volume += Float(volumeDelta)
                if player.volume > volume {
                    finish()
                }
            }
        }
    }