将 UIScrollView 的底部淡化为透明

Question

我有一个 UIScrollView，我需要底部淡入透明，这样它就不会突然切断内容。 UIScrollView 的背景是自定义颜色。这是我到目前为止所拥有的，但是该图层显示为白色，而不是自定义颜色。这就是我想要的，但只是在底部。

:

这是我目前所拥有的：

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，图层显示为白色，只有滚动视图宽度的一半。

Answer 1

我使用的方法是继承UIScrollView，并在layoutSubviews 方法中创建遮罩层。

这是我的代码，它将UIScrollView 的顶部和底部从背景颜色淡化为透明：

#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，改变这一行：

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

当我自己实现这个时，我发现this SO question 很有帮助，this gist 在 github 上。

编辑：我已将此代码放在 github 上，请参阅 here。

Answer 2

真的很喜欢 Steph Sharp 的实现，我将它转换为 swift 并认为我会分享以防其他人需要它。

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

}

Answer 3

Steph 回答的 swift 3 版本如下：

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

}

Answer 4

还有 this 答案不包含任何子类化。