Fade bottom of UIScrollView to transparent

问题:

I have a UIScrollView, and I need the bottom to fade to transparent, so that it does not abruptly cut off the content. The background of the UIScrollView is a custom color. This is what I have so far, but the layer is showing up white, instead of the custom color. This is what I am going for, but only on the bottom.

:

Here is what I have so far:

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];

For some reason, the layer is showing up as white, and only half the width of the scrollView.

回答1:

The approach I used was to subclass UIScrollView, and create the mask layer in the layoutSubviews method.

Here's my code, which fades the top and bottom of the UIScrollView from the background colour to transparent:

#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

If you just want to fade the bottom, change this line:

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

When I was implementing this myself, I found this SO question helpful, and this gist on github.

EDIT: I've put this code up on github, see here.

回答2:

Really like Steph Sharp's implementation, I converted it to swift and thought I'd share in case anyone else needs it.

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

}

回答3:

Here goes the swift 3 version of Steph's answer:

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

}

回答4:

Theres also this answer which does not include any subclassing.