Slider + Track positioning

Slider.swift

private func setup() {
    clear()
    createBaseLayer() // Step 3
    createThumbImageView() // Step 5
    configureTrackLayer() // Step 7
    addTarget(self, action: #selector(valueChanged(_:)), for: .valueChanged) // Step 8
}

// Step 8
@objc private func valueChanged(_ sender: Slider) {
    let thumbRectA = thumbRect(forBounds: bounds, 
                               trackRect: trackRect(forBounds: bounds), 
                               value: value)
    trackLayer.frame = .init(x: 0, 
                             y: frame.height / 4, 
                             width: thumbRectA.midX, 
                             height: frame.height / 2)
}