使用UILabel子图层切掉覆盖图像的角

Question

I've encountered a problem with code I'd written to cut off the corners of a UILabel (or, indeed, any UIView-derived object to which you can add sublayers) -- I do have to thank Kurt Revis for his answer to Use a CALayer to add a diagonal banner/badge to the corner of a UITableViewCell that pointed me in this direction.

I don't have a problem if the corner overlays a solid color -- it's simple enough to make the cut-off corner match that color. But if the corner overlays an image, how would you let the image show through?

I've searched SO for anything similar to this problem, but most of those answers have to do with cells in tables and all I'm doing here is putting a label on a screen's view.

Here's the code I use:

-(void)returnChoppedCorners:(UIView *)viewObject
{ 
    NSLog(@"Object Width = %f", viewObject.layer.frame.size.width);
    NSLog(@"Object Height = %f", viewObject.layer.frame.size.height);

    CALayer* bannerLeftTop = [CALayer layer];

    bannerLeftTop.backgroundColor = [UIColor blackColor].CGColor;
    // or whatever color the background is

    bannerLeftTop.bounds = CGRectMake(0, 0, 25, 25);
    bannerLeftTop.anchorPoint = CGPointMake(0.5, 1.0);
    bannerLeftTop.position = CGPointMake(10, 10);
    bannerLeftTop.affineTransform = CGAffineTransformMakeRotation(-45.0 / 180.0 * M_PI);

    [viewObject.layer addSublayer:bannerLeftTop];

    CALayer* bannerRightTop = [CALayer layer];

    bannerRightTop.backgroundColor = [UIColor blackColor].CGColor;

    bannerRightTop.bounds = CGRectMake(0, 0, 25, 25);
    bannerRightTop.anchorPoint = CGPointMake(0.5, 1.0);
    bannerRightTop.position = CGPointMake(viewObject.layer.frame.size.width - 10.0, 10.0);
    bannerRightTop.affineTransform = CGAffineTransformMakeRotation(45.0 / 180.0 * M_PI);

    [viewObject.layer addSublayer:bannerRightTop];
}

I'll be adding similar code to do the BottomLeft and BottomRight corners, but, right now, those are corners that overlay an image. The bannerLeftTop and bannerRightTop are actually squares that are rotated over the corner against a black background. Making them clear only lets the underlying UILabel background color appear, not the image. Same for using the z property. Is masking the answer? Oo should I be working with the underlying image instead?

I'm also encountering a problem with the Height and Width being passed to this method -- they don't match the constrained Height and Width of the object. But we'll save that for another question.

Answer 1

What you need to do, instead of drawing an opaque corner triangle over the label, is mask the label so its corners aren't drawn onto the screen.

Since iOS 8.0, UIView has a maskView property, so we don't actually need to drop to the Core Animation level to do this. We can draw an image to use as a mask, with the appropriate corners clipped. Then we'll create an image view to hold the mask image, and set it as the maskView of the label (or whatever).

The only problem is that (in my testing) UIKit won't resize the mask view automatically, either with constraints or autoresizing. We have to update the mask view's frame “manually” if the masked view is resized.

I realize your question is tagged objective-c , but I developed my answer in a Swift playground for convenience. It shouldn't be hard to translate this to Objective-C. I didn't do anything particularly “Swifty”.

So... here's a function that takes an array of corners (specified as UIViewContentMode cases, because that enum includes cases for the corners), a view, and a “depth”, which is how many points each corner triangle should measure along its square sides:

func maskCorners(corners: [UIViewContentMode], ofView view: UIView, toDepth depth: CGFloat) {

In Objective-C, for the corners argument, you could use a bitmask (eg (1 << UIViewContentModeTopLeft) | (1 << UIViewContentModeBottomRight) ), or you could use an NSArray of NSNumber s (eg @[ @(UIViewContentModeTopLeft), @(UIViewContentModeBottomRight) ] ).

Anyway, I'm going to create a square, 9-slice resizable image. The image will need one point in the middle for stretching, and since each corner might need to be clipped, the corners need to be depth by depth points. Thus the image will have sides of length 1 + 2 * depth points:

    let s = 1 + 2 * depth

Now I'm going to create a path that outlines the mask, with the corners clipped.

    let path = UIBezierPath()

So, if the top left corner is clipped, I need the path to avoid the top left point of the square (which is at 0, 0). Otherwise, the path includes the top left point of the square.

    if corners.contains(.TopLeft) {
        path.moveToPoint(CGPoint(x: 0, y: 0 + depth))
        path.addLineToPoint(CGPoint(x: 0 + depth, y: 0))
    } else {
        path.moveToPoint(CGPoint(x: 0, y: 0))
    }

Do the same for each corner in turn, going around the square:

    if corners.contains(.TopRight) {
        path.addLineToPoint(CGPoint(x: s - depth, y: 0))
        path.addLineToPoint(CGPoint(x: s, y: 0 + depth))
    } else {
        path.addLineToPoint(CGPoint(x: s, y: 0))
    }

    if corners.contains(.BottomRight) {
        path.addLineToPoint(CGPoint(x: s, y: s - depth))
        path.addLineToPoint(CGPoint(x: s - depth, y: s))
    } else {
        path.addLineToPoint(CGPoint(x: s, y: s))
    }

    if corners.contains(.BottomLeft) {
        path.addLineToPoint(CGPoint(x: 0 + depth, y: s))
        path.addLineToPoint(CGPoint(x: 0, y: s - depth))
    } else {
        path.addLineToPoint(CGPoint(x: 0, y: s))
    }

Finally, close the path so I can fill it:

    path.closePath()

Now I need to create the mask image. I'll do this using an alpha-only bitmap:

    let colorSpace = CGColorSpaceCreateDeviceGray()
    let scale = UIScreen.mainScreen().scale
    let gc = CGBitmapContextCreate(nil, Int(s * scale), Int(s * scale), 8, 0, colorSpace, CGImageAlphaInfo.Only.rawValue)!

I need to adjust the coordinate system of the context to match UIKit:

    CGContextScaleCTM(gc, scale, -scale)
    CGContextTranslateCTM(gc, 0, -s)

Now I can fill the path in the context. The use of white here is arbitrary; any color with an alpha of 1.0 would work:

    CGContextSetFillColorWithColor(gc, UIColor.whiteColor().CGColor)
    CGContextAddPath(gc, path.CGPath)
    CGContextFillPath(gc)

Next I create a UIImage from the bitmap:

    let image = UIImage(CGImage: CGBitmapContextCreateImage(gc)!, scale: scale, orientation: .Up)

If this were in Objective-C, you'd want to release the bitmap context at this point, with CGContextRelease(gc) , but Swift takes care of it for me.

Anyway, I convert the non-resizable image to a 9-slice resizable image:

    let maskImage = image.resizableImageWithCapInsets(UIEdgeInsets(top: depth, left: depth, bottom: depth, right: depth))

Finally, I set up the mask view. I might already have a mask view, because you might have clipped the view with different settings already, so I'll reuse an existing mask view if it is an image view:

    let maskView = view.maskView as? UIImageView ?? UIImageView()
    maskView.image = maskImage

Finally, if I had to create the mask view, I need to set it as view.maskView and set its frame:

    if view.maskView != maskView {
        view.maskView = maskView
        maskView.frame = view.bounds
    }
}

OK, how do I use this function? To demonstrate, I'll make a purple background view, and put an image on top of it:

let view = UIImageView(image: UIImage(named: "Kaz-256.jpg"))
view.autoresizingMask = [ .FlexibleWidth, .FlexibleHeight ]
let backgroundView = UIView(frame: view.frame)
backgroundView.backgroundColor = UIColor.purpleColor()
backgroundView.addSubview(view)
XCPlaygroundPage.currentPage.liveView = backgroundView

Then I'll mask some corners of the image view. Presumably you would do this in, say, viewDidLoad :

maskCorners([.TopLeft, .BottomRight], ofView: view, toDepth: 50)

Here's the result:

You can see the purple background showing through the clipped corners.

If I were to resize the view, I'd need to update the mask view's frame. For example, I might do this in my view controller:

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        self.cornerClippedView.maskView?.frame = self.cornerClippedView.bounds
    }

Here's a gist of all the code , so you can copy and paste it into a playground to try out. You'll have to supply your own adorable test image.

UPDATE

Here's code to create a label with a white background, and overlay it (inset by 20 points on each side) on the background image:

let backgroundView = UIImageView(image: UIImage(named: "Kaz-256.jpg"))
let label = UILabel(frame: backgroundView.bounds.insetBy(dx: 20, dy: 20))
label.backgroundColor = UIColor.whiteColor()
label.font = UIFont.systemFontOfSize(50)
label.text = "This is the label"
label.lineBreakMode = .ByWordWrapping
label.numberOfLines = 0
label.textAlignment = .Center
label.autoresizingMask = [ .FlexibleWidth, .FlexibleHeight ]
backgroundView.addSubview(label)
XCPlaygroundPage.currentPage.liveView = backgroundView

maskCorners([.TopLeft, .BottomRight], ofView: label, toDepth: 50)

Result: