How to launch camera on viewDidLoad with UIImagePickerControllerDelegate?

I have a new iOS app in Swift where I'm trying to show the camera as soon as the app is launched. In my view controller I have a single UIView object which I try to fill with the camera.

The code in my view controller calls UIImagePickerController in the viewDidLoad method but nothing happens.

Here is the code, any idea why nothing happens? Isn't the camera supposed to open on app launch with this code?

import UIKit

class ViewController: UIViewController, UINavigationControllerDelegate, UIImagePickerControllerDelegate {
    let imagePicker = UIImagePickerController()
    @IBOutlet weak var imageViewer: UIImageView!

    override func viewDidLoad() {
        super.viewDidLoad()
        if UIImagePickerController.isCameraDeviceAvailable(   UIImagePickerControllerCameraDevice.Front) {
            imagePicker.delegate = self
            imagePicker.sourceType = UIImagePickerControllerSourceType.Camera
            presentViewController(imagePicker, animated: true, completion: nil)
        } else {
            println("no front camera available")
        }

        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    func imagePickerController(picker: UIImagePickerController, didFinishPickingImage image: UIImage!, editingInfo: [NSObject : AnyObject]!) {
        dismissViewControllerAnimated(true, completion: nil)
        imageViewer.image = image
    }
}

UPDATE: if I add a button to the storyboard, connect it to the code and put the code from viewDidLoad in it, the camera works when the button is clicked:

@IBAction func presentImagePicker(sender: AnyObject) {

    if UIImagePickerController.isCameraDeviceAvailable( UIImagePickerControllerCameraDevice.Front) {
        imagePicker.delegate = self
        imagePicker.sourceType = UIImagePickerControllerSourceType.Camera
        presentViewController(imagePicker, animated: true, completion: nil)
    }
}

I know that the camera works, but how to make it work without having to press a button?