How to make UIViewcontroller transparent in swift ? clear() doesn't work

Question

I have a question : How to make UIviewcontroller be transparent. I want to make background image of iPhone as the background image as my application

I want to make a interface like the this sujet:Creating a blurring overlay view

the code works very well:

if !UIAccessibilityIsReduceTransparencyEnabled() {
        self.view.backgroundColor = UIColor.clearColor()

        let blurEffect = UIBlurEffect(style: UIBlurEffectStyle.Light)
        let blurEffectView = UIVisualEffectView(effect: blurEffect)
        //always fill the view
        blurEffectView.frame = self.view.bounds
        blurEffectView.autoresizingMask = [.FlexibleWidth, .FlexibleHeight]

        //self.view.addSubview(blurEffectView) //if you have more UIViews, use an insertSubview API to place it where needed
        self.view.insertSubview(blurEffectView, atIndex: 0)
    }
    else {
        self.view.backgroundColor = UIColor.clearColor()
    }

But after self.view.backgroundColor = UIColor.clearColor(), the background of my application est always white...the background is not transparent...

I want to make the background of application transparent to show the image background of iPhone, how to do it ?

PS: I have second solution to show background image of iPhone as the background image as my application, it is get the background image of iPhone...but How can I get the background image of iPhone before starting application?

Thank you for all of you.