How to set the system background color below presented view controllers in ios 13

Question

See the pic below, i presented a viewcontroller in my app, here comes 3 layers on screen:

1. Black, provided by system
2. My app, the presenting view controller, white
3. My app, the presented view controller, white

My question is, shouldn't the first layer be a blurred screen shot of a previously running app, why it's black? can i set the color?

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Edit: it turns out the first layer is not provided by system, but simply app's key window background! see my answer below

Answer 1

I managed to solve it by setting the key window background color, eg:

// inside appDelegate's didFinishLaunchingWithOptions
// The first layer will be white now
window?.backgroundColor = UIColor.white

Answer 2

In the viewWillAppear of the root view controller, you need to reset background color of it by put the code bellow:

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    self.view.backgroundColor = .white
}

In put this at where you push the new view controller:

self.navigationController?.pushViewController(anotherVC, completion: {
            self.view.backgroundColor = .black
        })