Gradually change background color based on scroll

Question

I have a scrollview that when it is scrolled down the background changes color.

I know I could use UIView animations to make this automatic. But I want to set a color based on the percentage of the scroll.

I want set the 0% and the 100% colors and the current color would be calculated and set on scrollViewDidScroll

 0%               50%              100%
yellow           green             blue

EDIT

How can I calculate the new color based on the scroll position?

Answer 1

Here's a solution for Swift 3.

// This function calculates a new color by blending the two colors.
// A percent of 0.0 gives the "from" color
// A percent of 1.0 gives the "to" color
// Any other percent gives an appropriate color in between the two
func blend(from: UIColor, to: UIColor, percent: Double) -> UIColor {
    var fR : CGFloat = 0.0
    var fG : CGFloat = 0.0
    var fB : CGFloat = 0.0
    var tR : CGFloat = 0.0
    var tG : CGFloat = 0.0
    var tB : CGFloat = 0.0

    from.getRed(&fR, green: &fG, blue: &fB, alpha: nil)
    to.getRed(&tR, green: &tG, blue: &tB, alpha: nil)

    let dR = tR - fR
    let dG = tG - fG
    let dB = tB - fB

    let rR = fR + dR * CGFloat(percent)
    let rG = fG + dG * CGFloat(percent)
    let rB = fB + dB * CGFloat(percent)

    return UIColor(red: rR, green: rG, blue: rB, alpha: 1.0)
}

// Pass in the scroll percentage to get the appropriate color    
func scrollColor(percent: Double) -> UIColor {
    var start : UIColor
    var end : UIColor
    var perc = percent
    if percent < 0.5 {
        // If the scroll percentage is 0.0..<0.5 blend between yellow and green
        start = UIColor.yellow
        end = UIColor.green
    } else {
        // If the scroll percentage is 0.5..1.0 blend between green and blue
        start = UIColor.green
        end = UIColor.blue
        perc -= 0.5
    }

    return blend(from: start, to: end, percent: perc * 2.0)
}

// In your "scrollViewDidScroll" delegate, calculate the scroll 
// percentage as a value from 0.0 to 1.0
// Then call "scrollColor"
let scrollPercentage = ... // your calculation
let scrollColor = scrollColor(percent: scrollPercentage)