Slider + create thumb

Slider.swift

// Step 5
private func createThumbImageView() {
    let thumbSize = (3 * frame.height) / 4
    let thumbView = ThumbView(frame: .init(x: 0, 
                                           y: 0, 
                                           width: thumbSize, 
                                           height: thumbSize))
    thumbView.layer.cornerRadius = thumbSize / 2
    let thumbSnapshot = thumbView.snapshot
    setThumbImage(thumbSnapshot, for: .normal)
}