根据UIScrollView的contentOffset在颜色之间切换

Question

我有一个水平的UIScrollView，宽度为960，足以容纳3个UIViewController视图。

每个视图都有一个背景颜色。第一个是粉红色，第二个是蓝色，第三个是绿色。

我希望在用户滚动时将可见颜色混合/混合/淡化。

因此，如果您从第1页（粉红色）滚动到第2页（蓝色），当用户完全刷到第2页时，最终会落在最终的蓝色上之前会有某种类型的粉色和蓝色混合/混合/淡入淡出。

我发现了一个关于我正在尝试做什么的问题，我已经实现了可以在这里找到的答案：https://stackoverflow.com/a/26159561/3344977

这个答案确实有效，我唯一的问题是这个答案只是为了使用2个屏幕/颜色，我有3个屏幕/颜色。

我理解这个答案的基础知识，它取决于UIScrollView的contentOffset.x来计算当前的颜色，但除此之外，我在数学方面很糟糕，这让我不知道如何修改它以使用第三种颜色。

Answer 1

是的，你肯定可以使用它三种颜色（或更多）。

对于三种颜色（比如红色，绿色，蓝色），红色为0.0，绿色为0.5，蓝色为1.0。所以你只需要在两个渐变之间拆分方法（红色 - 绿色和绿色 - 蓝色将分开计算）。

在获得方法之前使用我的代码...

// this just gets the percentage offset.
// 0,0 = no scroll
// 1,1 = maximum scroll
- (void)scrollView:(UIScrollView *)scrollView didScrollToPercentageOffset:(CGPoint)percentageOffset
{
    // get your colours to fade between
    NSArray *colours = @[[UIColor redColor], [UIColor yellowColor], [UIColor purpleColor]];

    // choose the colours to fade between based on the percentage.
    if (percentageOffset.x < 0.5) {
        // multiply the offset by 2 because we want 0.5 to be 100%
        self.backgroundColor = [self fadeFromColor:colours[0] toColor:colours[1] withPercentage:percentageOffset.x*2];
    } else {
        // minus 0.5 because we want 0.5 to be 0%
        self.backgroundColor = [self fadeFromColor:colours[1] toColor:colours[2] withPercentage:(percentageOffset.x-0.5)*2];
    }
}

// this is a more generic method to fade between two colours
// it allows the colours to be passed in as parameters
- (UIColor *)fadeFromColor:(UIColor *)fromColor toColor:(UIColor *)toColor withPercentage:(CGFloat)percentage
{
    // get the RGBA values from the colours
    CGFloat fromRed, fromGreen, fromBlue, fromAlpha;
    [fromColor getRed:&fromRed green:&fromGreen blue:&fromBlue alpha:&fromAlpha];

    CGFloat toRed, toGreen, toBlue, toAlpha;
    [toColor getRed:&toRed green:&toGreen blue:&toBlue alpha:&toAlpha];

    //calculate the actual RGBA values of the fade colour
    CGFloat red = (toRed - fromRed) * percentage + fromRed;
    CGFloat green = (toGreen - fromGreen) * percentage + fromGreen;
    CGFloat blue = (toBlue - fromBlue) * percentage + fromBlue;
    CGFloat alpha = (toAlpha - fromAlpha) * percentage + fromAlpha;

    // return the fade colour
    return [UIColor colorWithRed:red green:green blue:blue alpha:alpha];
}

这将在所有三种颜色之间很好地消失。

您可以为此添加更多颜色，并更改它分割百分比的方式。

Answer 2

Fogmeister发布的答案完美无缺！

以下是转换为Swift的相同答案：

// this just gets the percentage offset.
// 0,0 = no scroll
// 1,1 = maximum scroll

func scrollViewdidScrollToPercentageOffset(scrollView: UIScrollView, percentageOffset: CGPoint) {
    // get your colours to fade between
    var colors = [UIColor.redColor(), UIColor.blueColor(), UIColor.greenColor()]

    // choose the colours to fade between based on the percentage.
    if (percentageOffset.x < 0.5) {
        // multiply the offset by 2 because we want 0.5 to be 100%
        scrollView.backgroundColor = fadeFromColor(colors[0], colors[1], percentageOffset.x*2)
    } else {
        // minus 0.5 because we want 0.5 to be 0%
        scrollView.backgroundColor = fadeFromColor(colors[1], colors[2], (percentageOffset.x - 0.5)*2)
    }
}

func fadeFromColor(fromColor: UIColor, toColor: UIColor, withPercentage: CGFloat) -> UIColor {
    var fromRed: CGFloat = 0.0
    var fromGreen: CGFloat = 0.0
    var fromBlue: CGFloat = 0.0
    var fromAlpha: CGFloat = 0.0

    fromColor.getRed(&fromRed, green: &fromGreen, blue: &fromBlue, alpha: &fromAlpha)

    var toRed: CGFloat = 0.0
    var toGreen: CGFloat = 0.0
    var toBlue: CGFloat = 0.0
    var toAlpha: CGFloat = 0.0

    toColor.getRed(&toRed, green: &toGreen, blue: &toBlue, alpha: &toAlpha)

    //calculate the actual RGBA values of the fade colour
    var red = (toRed - fromRed) * withPercentage + fromRed
    var green = (toGreen - fromGreen) * withPercentage + fromGreen
    var blue = (toBlue - fromBlue) * withPercentage + fromBlue
    var alpha = (toAlpha - fromAlpha) * withPercentage + fromAlpha

    // return the fade colour
    return UIColor(red: red, green: green, blue: blue, alpha: alpha)
}

Answer 3

这是使用Swift 4.2的任何数量的颜色（或我的UICollectionView中的页面）的答案：

func scrollViewDidScroll(_ scrollView: UIScrollView) {
    let maximumOffset = scrollView.contentSize.width - scrollView.frame.width
    let currentOffset = scrollView.contentOffset.x
    let offsetPercentage = currentOffset / maximumOffset
    let sharesCount = CGFloat(colorsArray.count - 1)

    var currentShareIndex: CGFloat = 0
    switch offsetPercentage {
    case  ..<0:
        currentShareIndex = 0
    case 0..<1:
        currentShareIndex = floor(offsetPercentage * sharesCount)
    case 1... :
        currentShareIndex = sharesCount - 1
    default: break
    }

    let firstColorIndex = Int(currentShareIndex)
    let secondColorIndex = Int(currentShareIndex) + 1
    let colorPercentage = (offsetPercentage - currentShareIndex / sharesCount) * sharesCount

    scrollView.backgroundColor = UIColor.fade(from: colorsArray[firstColorIndex], to: colorsArray[secondColorIndex], with: colorPercentage)
}