updated AudioPlayer.swift for medium post about Audio Unit V3 Extensions

AudioPlayer.swift

public func selectAudioUnitWithComponentDescription(_ componentDescription: AudioComponentDescription?, completionHandler: @escaping (() -> Void)) {
        // Internal function to resume playing and call the completion handler.
        func done() {
            if isPlaying {
                playerNode.play()
            }
            completionHandler()
        }
        
        let hardwareFormat = self.engine.outputNode.outputFormat(forBus: 0)
        
        self.engine.connect(self.engine.mainMixerNode, to: self.engine.outputNode, format: hardwareFormat)
        
        /*
         Pause the player before re-wiring it. (It is not simple to keep it
         playing across an insertion or deletion.)
         */
        if isPlaying {
            playerNode.pause()
        }
        
        // Destroy any pre-existing unit.
        
        if audioUnitNode != nil {
            // Break player -> effect connection.
            engine.disconnectNodeInput(audioUnitNode!)
            
            
            // Break audioUnitNode -> mixer connection
            engine.disconnectNodeInput(engine.mainMixerNode)
            
            // Connect player -> mixer.
            engine.connect(playerNode, to: engine.mainMixerNode, format: file!.processingFormat)
            
            // We're done with the unit; release all references.
            engine.detach(audioUnitNode!)
            
            audioUnitNode = nil
            audioUnit = nil
        }
        
        // Insert the audio unit, if any.
        if let componentDescription = componentDescription {
            AVAudioUnit.instantiate(with: componentDescription, options: []) { avAudioUnit, error in
                guard let avAudioUnit = avAudioUnit else {
                    NSLog("avAudioUnit nil")
                    return
                }
                
                self.audioUnitNode = avAudioUnit
                self.engine.attach(avAudioUnit)
                
                // Disconnect player -> mixer.
                self.engine.disconnectNodeInput(self.engine.mainMixerNode)
                
                // Connect player -> effect -> mixer.
                self.engine.connect(self.playerNode, to: avAudioUnit, format: self.file!.processingFormat)
                self.engine.connect(avAudioUnit, to: self.engine.mainMixerNode, format: self.file!.processingFormat)
                
                self.audioUnit = avAudioUnit.auAudioUnit
                avAudioUnit.auAudioUnit.contextName = "running in AUv3Host"
                
                done()
            }
        } else {
            done()
        }
    }