Objective C - 在UICollectionView的左/右侧实现渐变效果

Question

我在视图控制器的主视图内有一个水平滚动的UICollectionView，如下所示（灰色是UIView，木头是UICollectionView）： 我想在这个UICollectionView的最左边和最右边添加固定的渐变效果，使得滚动时看起来像是消失了。我该如何实现？是否需要使用CAGradientLayer？如果您能给我任何帮助，我将不胜感激！

- Nate Kimball

Answer 1

我实际上通过使用一个遮罩层，借助于cocoanetics的这篇教程，成功地解决了这个问题。下面是我的操作步骤：

@interface ScalesViewController : UIViewController
{
    CAGradientLayer *maskLayer;
}
@end

接下来，在 .m 文件中，我添加了以下内容：

- (void)viewDidAppear:(BOOL)animated
{

    [super viewWillAppear: animated];

    if (!maskLayer)
    {
        maskLayer = [CAGradientLayer layer];

        CGColorRef outerColor = [[UIColor colorWithWhite:0.0 alpha:1.0] CGColor];
        CGColorRef innerColor = [[UIColor colorWithWhite:0.0 alpha:0.0] CGColor];

        maskLayer.colors = [NSArray arrayWithObjects:
                           (__bridge id)outerColor,
                           (__bridge id)innerColor,
                           (__bridge id)innerColor,
                           (__bridge id)outerColor, nil];

        maskLayer.locations = [NSArray arrayWithObjects:
                              [NSNumber numberWithFloat:0.0],
                              [NSNumber numberWithFloat:0.125],
                              [NSNumber numberWithFloat:0.875],
                              [NSNumber numberWithFloat:1.0], nil];

        [maskLayer setStartPoint:CGPointMake(0, 0.5)];
        [maskLayer setEndPoint:CGPointMake(1, 0.5)];

        maskLayer.bounds = self.mainCollectionView.bounds;
        maskLayer.anchorPoint = CGPointZero;

        [self.mainCollectionView.layer insertSublayer: maskLayer atIndex: 0];
    }
}

这会在我的集合视图两侧创建一个漂亮的“淡出至黑色”的效果。可以添加更多颜色到位置和颜色属性中以精细调整渐变混合。起始/终点确定了渐变的方向和位置。

- Nate Kimball

Answer 2

尝试在集合视图子层上添加两个CAGradientLayer图层：

#import <QuartzCore/QuartzCore.h>

CAGradientLayer *leftShadow = [CAGradientLayer layer];
leftShadow.frame = CGRectMake(0, 0, 100, self.collectionView.frame.size.height);
leftShadow.startPoint = CGPointMake(0, 0.5);
leftShadow.endPoint = CGPointMake(1.0, 0.5);
leftShadow.colors = [NSArray arrayWithObjects:(id)[[UIColor colorWithWhite:0.0 alpha:0.4f] CGColor], (id)[[UIColor clearColor] CGColor], nil];

[self.collectionView.layer addSublayer:leftShadow];

CAGradientLayer *rightShadow = [CAGradientLayer layer];
rightShadow.frame = CGRectMake(CGRectGetWidth(self.collectionView.frame)-100.0, 0, 100, self.collectionView.frame.size.height);
rightShadow.startPoint = CGPointMake(1.0, 0.5);
rightShadow.endPoint = CGPointMake(0, 0.5);
rightShadow.colors = [NSArray arrayWithObjects:(id)[[UIColor colorWithWhite:0.0 alpha:0.4f] CGColor], (id)[[UIColor clearColor] CGColor], nil];

[self.collectionView.layer addSublayer:rightShadow];

- Valent Richie