Fading out any content which approaches edges of U

问题:

As the title says, I am trying to give to some UIImageViews a fading out effect as they get closer and closer to any of the four edges of my UIScrollView. Since the user can drag the UIImages, if he drags them towards the edges, they start fading out, instead of giving that cut out effect as if there were borders to the UIScrollView. I tried this tutorial:

http://www.cocoanetics.com/2011/08/adding-fading-gradients-to-uitableview/

suggested in another stack overflow question, but it can only be applied to UITableViews. I would like the image to start fading as it gets one centimeter away from the border.

回答1:

Similar to what was done in the Cocoanetics post you link to, you can create a CAGradientLayer to cover your scroll view. Make it fade out to the left, right, top and bottom edges, using the background color of your scroll view (in my example, white):

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

   // first, define a horizontal gradient (left/right edges)
   CAGradientLayer* hMaskLayer = [CAGradientLayer layer];
   hMaskLayer.opacity = .7;
   hMaskLayer.colors = [NSArray arrayWithObjects:(id)outerColor,
                        (id)innerColor, (id)innerColor, (id)outerColor, nil];
   hMaskLayer.locations = [NSArray arrayWithObjects:[NSNumber numberWithFloat:0.0],
                           [NSNumber numberWithFloat:0.15],
                           [NSNumber numberWithFloat:0.85],
                           [NSNumber numberWithFloat:1.0], nil];
   hMaskLayer.startPoint = CGPointMake(0, 0.5);
   hMaskLayer.endPoint = CGPointMake(1.0, 0.5);
   hMaskLayer.bounds = self.scrollView.bounds;
   hMaskLayer.anchorPoint = CGPointZero;

   CAGradientLayer* vMaskLayer = [CAGradientLayer layer];
   // without specifying startPoint and endPoint, we get a vertical gradient
   vMaskLayer.opacity = hMaskLayer.opacity;
   vMaskLayer.colors = hMaskLayer.colors;
   vMaskLayer.locations = hMaskLayer.locations;
   vMaskLayer.bounds = self.scrollView.bounds;
   vMaskLayer.anchorPoint = CGPointZero;

   // you must add the masks to the root view, not the scrollView, otherwise
   //  the masks will move as the user scrolls!
   [self.view.layer addSublayer: hMaskLayer];
   [self.view.layer addSublayer: vMaskLayer];

Disclaimer: this does sort of double-up the gradient/fade at the four corners. You can take a look at the results and decide whether they're good enough for you. If not, you could also try drawing a transparent image in something like Photoshop, and adding a UIImageView subview on top as the mask, using the image you drew.

Youtube screen capture

回答2:

Swift version of Nate answer, but only for top and bottom:

        let innerColor = UIColor(white: 1.0, alpha: 0.0).CGColor
        let outerColor = UIColor(white: 1.0, alpha: 1.0).CGColor;

        // define a vertical gradient (up/bottom edges)
        let colors = [outerColor, innerColor,innerColor,outerColor]
        let locations = [0.0, 0.15,0.85,1.0]

        var vMaskLayer : CAGradientLayer = CAGradientLayer()// layer];
        // without specifying startPoint and endPoint, we get a vertical gradient
        vMaskLayer.opacity = 0.7
        vMaskLayer.colors = colors;
        vMaskLayer.locations = locations;
        vMaskLayer.bounds = self.scrollView.bounds;
        vMaskLayer.anchorPoint = CGPointZero;

        // you must add the mask to the root view, not the scrollView, otherwise
        //  the masks will move as the user scrolls!
        self.view.layer.addSublayer(vMaskLayer)

回答3:

Couple of postings on this issue here:

Changing the alpha inside UIScrollView

setting Alpha for UIView that is a subview of UIScrollVIew very slow

and some sample code here:

https://gist.github.com/MaximKeegan/2478842

---

If you want to go your own way then first get the visible part of the UIScrollView:

Getting the visible rect of an UIScrollView's content

and then fade the UIView:

UIView border with fade or blur effect