启用了UserInteraction的FadeIn＆FadeOut UISlider

I created a video duration line by using UISlider. It works nicely; and I can get slider value change. Then, I tried to add a visual (fade-in & fade-out) to it, so when user taps on the screen, the slider fades in, and fades-out if no other interaction is made. I created a video duration line by using UISlid