CAGradientLayer send to back

The simplest solution is not to insert the gradient layer as a sublayer of self.view.layer. Instead, it should be self.view.layer. A UIView subclass has a class method layerClass that lets you say what class its layer should be.

For example, in your UIView subclass you would add this:

override class var layerClass: AnyClass {
    return CAGradientLayer.self
}

And when referencing self.layer within the view, wrap it a guard let or if let to convert the type like such:

guard let layer = self.layer as? CAGradientLayer else { ... }

Or, it could be the layer of some other subview of self.view, again easily arranged by using the same class method layerClass.

Otherwise, you will have to use layer commands, not view commands, to order the sublayers of self.view. That includes its subviews. In other words, if v is a subview of self.view, then v.layer is a sublayer of self.view.layer and you can order it among its sublayers by rearranging the sublayers array.

.layer has a sublayers property which is a simple array. So you can insert at any index you want. If you want your new layer to be at the back, just insert it at 0, like this:

let newLayer = CALayer() // your layer that you want at the back
view.layer.insertSublayer(newLayer, at: 0)

Create a view that contains only the gradient