UIScrollView 底部渐变透明

Question

我有一个UIScrollView，需要底部渐变为透明，以避免内容突然截断。 UIScrollView的背景是自定义颜色。到目前为止，我已经实现了以下代码，但是图层显示的是白色而不是自定义颜色。我想要的效果如下图所示，只在底部实现渐变。 以下是我的代码：

maskLayer = [CAGradientLayer layer];

            CGColorRef firstColor = [UIColor colorWithRed:141 green:211 blue:244 alpha:1.0].CGColor;
            CGColorRef secondColor = [UIColor colorWithRed:141 green:211 blue:244 alpha:0.8].CGColor;
            CGColorRef thirdColor = [UIColor colorWithRed:141 green:211 blue:244 alpha:0.2].CGColor;
            CGColorRef fourthColor = [UIColor colorWithRed:141 green:211 blue:244 alpha:0.0].CGColor;

            maskLayer.colors = [NSArray arrayWithObjects:(__bridge id)firstColor, (__bridge id)secondColor, (__bridge id)thirdColor, (__bridge id)fourthColor, nil];
            maskLayer.locations = [NSArray arrayWithObjects:[NSNumber numberWithFloat:0.0],
                                                        [NSNumber numberWithFloat:0.2],
                                                        [NSNumber numberWithFloat:0.8],
                                                        [NSNumber numberWithFloat:1.0], nil];
            maskLayer.frame = CGRectMake(0, 285, self.loginScrollView.frame.size.width, 40);
            maskLayer.anchorPoint = CGPointZero;
            [self.loginScrollView.layer addSublayer:maskLayer];

由于某种原因，该图层显示为白色，并且仅有scrollView宽度的一半。

- Chandler De Angelis

Answer 1

我采用的方法是子类化UIScrollView，并在layoutSubviews方法中创建遮罩层。

以下是我的代码，它将UIScrollView的顶部和底部从背景颜色渐变到透明：

#import "FadingScrollView.h"
#import <QuartzCore/QuartzCore.h>

static float const fadePercentage = 0.2;

@implementation FadingScrollView

// ...

- (void)layoutSubviews
{
    [super layoutSubviews];

    NSObject * transparent = (NSObject *) [[UIColor colorWithWhite:0 alpha:0] CGColor];
    NSObject * opaque = (NSObject *) [[UIColor colorWithWhite:0 alpha:1] CGColor];

    CALayer * maskLayer = [CALayer layer];
    maskLayer.frame = self.bounds;

    CAGradientLayer * gradientLayer = [CAGradientLayer layer];
    gradientLayer.frame = CGRectMake(self.bounds.origin.x, 0,
                                     self.bounds.size.width, self.bounds.size.height);

    gradientLayer.colors = [NSArray arrayWithObjects: transparent, opaque,
                            opaque, transparent, nil];

    // Set percentage of scrollview that fades at top & bottom
    gradientLayer.locations = [NSArray arrayWithObjects:
                               [NSNumber numberWithFloat:0],
                               [NSNumber numberWithFloat:fadePercentage],
                               [NSNumber numberWithFloat:1.0 - fadePercentage],
                               [NSNumber numberWithFloat:1], nil];

    [maskLayer addSublayer:gradientLayer];
    self.layer.mask = maskLayer;
}

@end

如果你只想淡化底部，请更改这一行：

// Fade bottom of scrollview only
gradientLayer.locations = [NSArray arrayWithObjects:
                           [NSNumber numberWithFloat:0],
                           [NSNumber numberWithFloat:0],
                           [NSNumber numberWithFloat:1.0 - fadePercentage],
                           [NSNumber numberWithFloat:1], nil];

当我自己实现时，我发现这个SO问题和Github上的这个代码片段对我很有帮助。 编辑：我已经将这个代码上传到Github上，请查看这里。

- Steph Sharp

---

我认为现在应该使用UIViewAnimationOptionCurveEaseIn。 - matrejek

Answer 2

我真的很喜欢Steph Sharp的实现方式，我将其转换为Swift并想与大家分享，以便其他人使用。

let fadePercentage = CGFloat(0.2)

override func layoutSubviews() {

    super.layoutSubviews()

    var transparent = UIColor.clearColor().CGColor
    var opaque = UIColor.blackColor().CGColor

    var maskLayer = CALayer()
    maskLayer.frame = self.bounds

    var gradientLayer = CAGradientLayer()
    gradientLayer.frame = CGRectMake(self.bounds.origin.x, 0, self.bounds.size.width, self.bounds.size.height)
    gradientLayer.colors = [transparent, opaque, opaque, transparent]
    gradientLayer.locations = [0, fadePercentage, 1 - fadePercentage, 1]

    maskLayer.addSublayer(gradientLayer)
    self.layer.mask = maskLayer

}

- Lorenzo

Answer 3

这是 Steph 回答的 Swift 3 版本：

let fadePercentage: Double = 0.2

override func layoutSubviews() {

    super.layoutSubviews()

    let transparent = UIColor.clear.cgColor
    let opaque = UIColor.black.cgColor

    let maskLayer = CALayer()
    maskLayer.frame = self.bounds

    let gradientLayer = CAGradientLayer()
    gradientLayer.frame = CGRect(x: self.bounds.origin.x, y: 0, width: self.bounds.size.width, height: self.bounds.size.height)
    gradientLayer.colors = [transparent, opaque, opaque, transparent]
    gradientLayer.locations = [0, NSNumber(floatLiteral: fadePercentage), NSNumber(floatLiteral: 1 - fadePercentage), 1]

    maskLayer.addSublayer(gradientLayer)
    self.layer.mask = maskLayer

}

- Joseph El Mallah

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谢谢你为我节省了一点时间。 - GoldenJoe

Answer 4

还有一个this的答案，它不包括任何子类。

- Aviel Gross