How to record video and play audio at the same time (swift tutorial)

So you want to record a video and play music from the user's library at the same time? Look no further. Below is the answer.

For the audio playback you will use AVAudioPlayer. All you have to do is to declare the AVAudioPlayer as a global variable (I named it audioPlayer) and implement the code below.

Use this in after the user chose the song he/she wants to play:

func mediaPicker(mediaPicker: MPMediaPickerController, didPickMediaItems mediaItemCollection: MPMediaItemCollection) {
    let pickerItem: MPMediaItem = mediaItemCollection.items[0]
    let songURL = pickerItem.valueForProperty(MPMediaItemPropertyAssetURL)
    if let sURL = songURL as? NSURL
    {
        songTitle = pickerItem.title!
        do
        {
            audioPlayer = try AVAudioPlayer(contentsOfURL: sURL)
        }
        catch
        {
            print("Can't Create Audio Player: \(error)")
        }
    }
    dismissViewControllerAnimated(true, completion: { () -> Void in
        audioPlayer.play()
    })
}

You will also need to set up the audio session(in viewDidLoad). It's crucial if you want audio to play while recording:

 // Audio Session Setup
    do
    {
        try audioSession.setCategory(AVAudioSessionCategoryPlayAndRecord)
    }
    catch
    {
        print("Can't Set Audio Session Category: \(error)")
    }
    AVAudioSessionCategoryOptions.MixWithOthers
    do
    {
        try audioSession.setMode(AVAudioSessionModeVideoRecording)
    }
    catch
    {
        print("Can't Set Audio Session Mode: \(error)")
    }
    // Start Session
    do
    {
        try audioSession.setActive(true)
    }
    catch
    {
        print("Can't Start Audio Session: \(error)")
    }

Now for the video recording. You will use AVCaptureSession. Declare the following as global variables:

let captureSession = AVCaptureSession()
var currentDevice: AVCaptureDevice?
var videoFileOutput: AVCaptureMovieFileOutput?
var cameraPreviewLayer: AVCaptureVideoPreviewLayer?

Then configure the session in viewDidLoad. Note: The video preview is in a container and the entire video related code is in a different view controller but just using a view instead of a container should work just as fine:

// Preset For 720p
captureSession.sessionPreset = AVCaptureSessionPreset1280x720

// Get Available Devices Capable Of Recording Video
let devices = AVCaptureDevice.devicesWithMediaType(AVMediaTypeVideo) as! [AVCaptureDevice]

// Get Back Camera
for device in devices
{
    if device.position == AVCaptureDevicePosition.Back
    {
        currentDevice = device
    }
}
let camera = AVCaptureDevice.defaultDeviceWithMediaType(AVMediaTypeVideo)

// Audio Input
let audioInputDevice = AVCaptureDevice.defaultDeviceWithMediaType(AVMediaTypeAudio)

do
{
    let audioInput = try AVCaptureDeviceInput(device: audioInputDevice)

    // Add Audio Input
    if captureSession.canAddInput(audioInput)
    {
        captureSession.addInput(audioInput)
    }
    else
    {
        NSLog("Can't Add Audio Input")
    }
}
catch let error
{
    NSLog("Error Getting Input Device: \(error)")
}

// Video Input
let videoInput: AVCaptureDeviceInput
do
{
    videoInput = try AVCaptureDeviceInput(device: camera)

    // Add Video Input
    if captureSession.canAddInput(videoInput)
    {
        captureSession.addInput(videoInput)
    }
    else
    {
        NSLog("ERROR: Can't add video input")
    }
}
catch let error
{
    NSLog("ERROR: Getting input device: \(error)")
}

// Video Output
videoFileOutput = AVCaptureMovieFileOutput()
captureSession.addOutput(videoFileOutput)

// Show Camera Preview
cameraPreviewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
view.layer.addSublayer(cameraPreviewLayer!)
cameraPreviewLayer?.videoGravity = AVLayerVideoGravityResizeAspectFill
let width = view.bounds.width
cameraPreviewLayer?.frame = CGRectMake(0, 0, width, width)

// Bring Record Button To Front & Start Session
view.bringSubviewToFront(recordButton)
captureSession.startRunning()
print(captureSession.inputs)

Then you create an @IBAction for handling when the user presses the record button (I just used a simple button which I made red and round):

@IBAction func capture(sender: AnyObject) {
    do
    {
        initialOutputURL = try NSFileManager.defaultManager().URLForDirectory(.DocumentDirectory, inDomain: .UserDomainMask, appropriateForURL: nil, create: true).URLByAppendingPathComponent("output").URLByAppendingPathExtension("mov")
    }
    catch
    {
        print(error)
    }
    if !isRecording
    {
        isRecording = true

        UIView.animateWithDuration(0.5, delay: 0.0, options: [.Repeat, .Autoreverse, .AllowUserInteraction], animations: { () -> Void in
            self.recordButton.transform = CGAffineTransformMakeScale(0.75, 0.75)
            }, completion: nil)

        videoFileOutput?.startRecordingToOutputFileURL(initialOutputURL, recordingDelegate: self)
    }
    else
    {
        isRecording = false

        UIView.animateWithDuration(0.5, delay: 0, options: [], animations: { () -> Void in
            self.recordButton.transform = CGAffineTransformMakeScale(1.0, 1.0)
            }, completion: nil)
        recordButton.layer.removeAllAnimations()
        videoFileOutput?.stopRecording()
    }
}

Then all there is left for you to do is to save the video to (presumably) the camera roll. But I won't include that. You must put in some effort yourselves. (hint: UISaveVideoAtPathToSavedPhotosAlbum)

So that's it folks. That's how you use AVFoundation to record a video and play music from the library at the same time.