How to create a top fade effect using UIScrollView

问题:

I've got a UIScrollView in my app and I have seen in some other apps that when the user scrolls, the top section fades out on scroll rather than just dissapearing out.

I really love this effect and want to achieve it. Any ideas how it can be done?

回答1:

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

---

See my answer to a similar question.

My solution is to subclass UIScrollView, and create a mask layer in the layoutSubviews method.

#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

The code above fades the top and bottom of the UIScrollView from the background colour to transparent, but this can be easily changed to fade the top only (or fade to any colour you want).

Change this line to fade the top only:

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

---

EDIT 2:

Or fade the top only by changing these two lines:

// Fade top of scrollview only
    gradientLayer.colors = [NSArray arrayWithObjects: transparent, opaque, nil];

    gradientLayer.locations = [NSArray arrayWithObjects: [NSNumber numberWithFloat:0],
                                                         [NSNumber numberWithFloat:fadePercentage], nil];

Or, fade the bottom only:

// Fade bottom of scrollview only
    gradientLayer.colors = [NSArray arrayWithObjects: opaque, transparent, nil];

    gradientLayer.locations = [NSArray arrayWithObjects: [NSNumber numberWithFloat:1.0 - fadePercentage],
                                                         [NSNumber numberWithFloat:1], nil];

回答2:

Just based on Rob's great answer, here's a category on UIView,

@implementation UIView (Looks)

-(void)fadeTail
    {
    CAGradientLayer *gradient = [CAGradientLayer layer];
    gradient.frame = self.bounds;
    gradient.colors = @[
        (id)[[UIColor whiteColor] CGColor],
        (id)[[UIColor clearColor] CGColor]
        ];
    gradient.startPoint = CGPointMake(0.5, 0.93);
    gradient.endPoint = CGPointMake(0.5, 1);

    [self.layer setMask:gradient];
    }

@end

which does this .. seen here on a small UIWebView ...

(To be clear, the web view itself is white; the web view happens to be sitting on a greyish background. So, the fadeTail function basically fades out the "whole" web view, to whatever happens to be underneath.)

Hope it saves someone a little typing, cheers

回答3:

You can use a CAGradientLayer by

1. Adding the QuartzCore.framework to your project (see Linking to Library or Framework).

2. Add #import of the QuartzCore headers:

   #import <QuartzCore/QuartzCore.h>
   

3. And then use CAGradientLayer:

   - (void)addGradientMaskToView:(UIView *)view
   {
       CAGradientLayer *gradient = [CAGradientLayer layer];
       gradient.frame = view.bounds;
       gradient.colors = @[(id)[[UIColor clearColor] CGColor], (id)[[UIColor whiteColor] CGColor]];
       gradient.startPoint = CGPointMake(0.5, 0.0); // this is the default value, so this line is not needed
       gradient.endPoint = CGPointMake(0.5, 0.20);
   
       [view.layer setMask:gradient];
   }
   

Note, this CAGradientLayer is a gradient from a color with alpha of 0.0 (e.g. clearColor) to a color to a color with alpha of 1.0 (e.g. whiteColor), not just from black to white. You can adjust the startPoint (the default value is probably fine) and the endPoint to adjust where you want the gradient to be applied.

And generally, when doing this with a UIScrollView, unless you want the gradient to scroll with you, you make the UIScrollView a subview of some other UIView and apply this gradient to that container view, not the scroll view itself.

回答4:

You add an alpha mask layer to a view containing your scroll view like this:

CALayer *mask = [CALayer layer];
CGImageRef maskRef = [UIImage imageNamed:@"scrollMask"].CGImage;
CGImageRef maskImage = CGImageMaskCreate(CGImageGetWidth(maskRef),
                                             CGImageGetHeight(maskRef),
                                             CGImageGetBitsPerComponent(maskRef),
                                             CGImageGetBitsPerPixel(maskRef),
                                             CGImageGetBytesPerRow(maskRef),
                                             CGImageGetDataProvider(maskRef), NULL, false);
mask.contents = (__bridge id)maskImage;
mask.frame = CGRectMake(0, 0, view.bounds.size.width, view.bounds.size.height);
view.layer.mask = mask;
view.layer.masksToBounds = YES;
CGImageRelease(maskImage);

where "scrollMask" is a grayscale image defining mask region: white == fully masked, black == not masked at all and gray == partially masked.

To create the effect you're looking for, the mask image would be black with a white gradient at the top like this:

For more details, take a look at the documentation for CGImageMaskCreate.

回答5:

Thanks to Fattie's answer I created the following UIView extension, in swift, that takes care of the gradient fading and provides more styles (bottom, top, left right, vertical and horizontal)

The Extension:

extension UIView {

    enum UIViewFadeStyle {
        case bottom
        case top
        case left
        case right

        case vertical
        case horizontal
    }

    func fadeView(style: UIViewFadeStyle = .bottom, percentage: Double = 0.07) {
        let gradient = CAGradientLayer()
        gradient.frame = bounds
        gradient.colors = [UIColor.white.cgColor, UIColor.clear.cgColor]

        let startLocation = percentage
        let endLocation = 1 - percentage

        switch style {
        case .bottom:
            gradient.startPoint = CGPoint(x: 0.5, y: endLocation)
            gradient.endPoint = CGPoint(x: 0.5, y: 1)
        case .top:
            gradient.startPoint = CGPoint(x: 0.5, y: startLocation)
            gradient.endPoint = CGPoint(x: 0.5, y: 0.0)
        case .vertical:
            gradient.startPoint = CGPoint(x: 0.5, y: 0.0)
            gradient.endPoint = CGPoint(x: 0.5, y: 1.0)
            gradient.colors = [UIColor.clear.cgColor, UIColor.white.cgColor, UIColor.white.cgColor, UIColor.clear.cgColor]
            gradient.locations = [0.0, startLocation, endLocation, 1.0] as [NSNumber]

        case .left:
            gradient.startPoint = CGPoint(x: startLocation, y: 0.5)
            gradient.endPoint = CGPoint(x: 0.0, y: 0.5)
        case .right:
            gradient.startPoint = CGPoint(x: endLocation, y: 0.5)
            gradient.endPoint = CGPoint(x: 1, y: 0.5)
        case .horizontal:
            gradient.startPoint = CGPoint(x: 0.0, y: 0.5)
            gradient.endPoint = CGPoint(x: 1.0, y: 0.5)
            gradient.colors = [UIColor.clear.cgColor, UIColor.white.cgColor, UIColor.white.cgColor, UIColor.clear.cgColor]
            gradient.locations = [0.0, startLocation, endLocation, 1.0] as [NSNumber]
        }

        layer.mask = gradient
    }

}

For any comments/recommendations, please let me know at the gist I've created.

回答6:

We built onto Steph Sharp's code to only fade when necessary. Our code is setup as a static utility method instead of a subclass so that we could reuse the method in our subclasses of UIScrollView and UITableView. Here is our static utility method:

#define kScrollViewFadeColorLight [UIColor colorWithRed:0.56 green:0.56 blue:0.56 alpha:0.0]
#define kScrollViewFadeColorDark [UIColor colorWithRed:0.56 green:0.56 blue:0.56 alpha:1.0]

#define kScrollViewScrollBarWidth 7.0
#define kScrollViewFadingEdgeLength 40.0

+ (void) applyFadeToScrollView:(UIScrollView*) scrollView {
  CGFloat topOffset = -scrollView.contentInset.top;
  CGFloat bottomOffset = scrollView.contentSize.height + scrollView.contentInset.bottom - scrollView.bounds.size.height;
  CGFloat distanceFromTop = scrollView.contentOffset.y - topOffset;
  CGFloat distanceFromBottom = bottomOffset - scrollView.contentOffset.y;
  BOOL isAtTop = distanceFromTop < 1.0;
  BOOL isAtBottom = distanceFromBottom < 1.0;
  if (isAtTop && isAtBottom) {
    // There is no scrolling to be done here, so don't fade anything!
    scrollView.layer.mask = nil;
    return;
  }
  NSObject* transparent = (NSObject*)[kScrollViewFadeColorLight CGColor];
  NSObject* opaque = (NSObject*)[kScrollViewFadeColorDark CGColor];

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

  CALayer* scrollGutterLayer = [CALayer layer];
  scrollGutterLayer.frame = CGRectMake(scrollView.bounds.size.width - kScrollViewScrollBarWidth, 0.0,
                                       kScrollViewScrollBarWidth, scrollView.bounds.size.height);
  scrollGutterLayer.backgroundColor = (__bridge CGColorRef)(opaque);
  [maskLayer addSublayer:scrollGutterLayer];

  CAGradientLayer* gradientLayer = [CAGradientLayer layer];
  gradientLayer.frame = CGRectMake(0.0, 0.0, scrollView.bounds.size.width, scrollView.bounds.size.height);
  CGFloat fadePercentage = kScrollViewFadingEdgeLength / scrollView.bounds.size.height;
  NSMutableArray* colors = [[NSMutableArray alloc] init];
  NSMutableArray* locations = [[NSMutableArray alloc] init];
  if (!isAtTop) {
    [colors addObjectsFromArray:@[transparent, opaque]];
    [locations addObjectsFromArray:@[@0.0, [NSNumber numberWithFloat:fadePercentage]]];
  }
  if (!isAtBottom) {
    [colors addObjectsFromArray:@[opaque, transparent]];
    [locations addObjectsFromArray:@[[NSNumber numberWithFloat:1.0 - fadePercentage], @1.0]];
  }
  gradientLayer.colors = colors;
  gradientLayer.locations = locations;
  [maskLayer addSublayer:gradientLayer];
  scrollView.layer.mask = maskLayer;
}

Here is our usage of that method.

FadingScrollView.h

@interface FadingScrollView : UIScrollView
@end

FadingScrollView.m

@implementation FadingScrollView
- (void) layoutSubviews {
  [super layoutSubviews];
  [Utils applyFadeToScrollView:self];
}
@end

FadingTableView.h

@interface FadingTableView : UITableView    
@end

FadingTableView.m

@implementation FadingTableView
- (void) layoutSubviews {
  [super layoutSubviews];
  [Utils applyFadeToScrollView:self];
}
@end