CALayer: add a border only at one side

Question

I can add a border to a CALayer in this way:

[webView.layer setBorderColor: [[UIColor colorWithRed:0.6 green:0.7 blue:0.2 alpha:1] CGColor]];
[webView.layer          


        

Answer 1

My Swift Solution. It involves four different functions, all extensions to UIView. Each function adds a different border.

extension UIView {
@discardableResult func addRightBorder(color: UIColor, width: CGFloat) -> UIView {
    let layer = CALayer()
    layer.borderColor = color.cgColor
    layer.borderWidth = width
    layer.frame = CGRect(x: self.frame.size.width-width, y: 0, width: width, height: self.frame.size.height)
    self.layer.addSublayer(layer)
    return self
    }
@discardableResult func addLeftBorder(color: UIColor, width: CGFloat) -> UIView {
    let layer = CALayer()
    layer.borderColor = color.cgColor
    layer.borderWidth = width
    layer.frame = CGRect(x: 0, y: 0, width: width, height: self.frame.size.height)
    self.layer.addSublayer(layer)
    return self
    }
@discardableResult func addTopBorder(color: UIColor, width: CGFloat) -> UIView {
    let layer = CALayer()
    layer.borderColor = color.cgColor
    layer.borderWidth = width
    layer.frame = CGRect(x: 0, y: 0, width: self.frame.size.width, height: width)
    self.layer.addSublayer(layer)
    return self
    }
@discardableResult func addBottomBorder(color: UIColor, width: CGFloat) -> UIView {
    let layer = CALayer()
    layer.borderColor = color.cgColor
    layer.borderWidth = width
    layer.frame = CGRect(x: 0, y: self.frame.size.height-width, width: self.frame.size.width, height: width)
    self.layer.addSublayer(layer)
    return self
    }
}

Answer 2

The easiest way is to add a subLayer that will draw the selective borders, but there are some things to consider when choosing this solution, the biggest are making sure that the borders subLayer is always on top, and that the borders change when you change the frame of your layer.

I implemented a drop in open source solution that takes care of those issues, and let you declare selective borders like this:

myView.borderDirection = AUIFlexibleBordersDirectionRight | AUIFlexibleBordersDirectionTop;

You can get the code, and read about it some more here

Answer 3

There's another way of doing this. The CAShapeLayer has properties called "strokeStart" and "strokeEnd". Considering the stroking starts from 0 and ends at 1 when it reaches the origin back, thus completing the path, you can set values to start and end to draw the border on one side.

The relative location at which to begin stroking the path. Animatable. The value of this property must be in the range 0.0 to 1.0. The default value of this property is 0.0. Combined with the strokeEnd property, this property defines the subregion of the path to stroke. The value in this property indicates the relative point along the path at which to begin stroking while the strokeEnd property defines the end point. A value of 0.0 represents the beginning of the path while a value of 1.0 represents the end of the path. Values in between are interpreted linearly along the path length.

Example:

let path = UIBezierPath(roundedRect: bounds, byRoundingCorners: corners, cornerRadii: CGSize(width: radius, height: radius))
let mask = CAShapeLayer()
mask.path = path.CGPath
mask.strokeColor = UIColor.redColor().CGColor
mask.strokeStart = 0.45
mask.strokeEnd = 0.92
self.layer.mask = mask

Answer 4

The following works with Swift 4.2 for applying a border along an existing UIView.

extension UIView {
    enum Side {
        case top
        case bottom
        case left
        case right
    }

    func addBorder(to side: Side, color: UIColor, borderWidth: CGFloat) {
        let subLayer = CALayer()
        subLayer.borderColor = color.cgColor
        subLayer.borderWidth = borderWidth
        let origin = findOrigin(side: side, borderWidth: borderWidth)
        let size = findSize(side: side, borderWidth: borderWidth)
        subLayer.frame = CGRect(origin: origin, size: size)
        layer.addSublayer(subLayer)
    }

    private func findOrigin(side: Side, borderWidth: CGFloat) -> CGPoint {
        switch side {
        case .right:
            return CGPoint(x: frame.maxX - borderWidth, y: 0)
        case .bottom:
            return CGPoint(x: 0, y: frame.maxY - borderWidth)
        default:
            return .zero
        }
    }

    private func findSize(side: Side, borderWidth: CGFloat) -> CGSize {
        switch side {
        case .left, .right:
            return CGSize(width: borderWidth, height: frame.size.height)
        case .top, .bottom:
            return CGSize(width: frame.size.width, height: borderWidth)
        }
    }
}

And you can call it with this:

myView.addBorder(to: .left, color: .black, borderWidth: 2.0)

What is going on:

• The user provides a Side as defined in the enum, along with a color and borderWidth
• A new subLayer is created and given a borderColor and borderWidth
• The origin is calculated, as determined by the side
• The size is calculated, also as determined by the side
• Finally, the layer is given a frame and then added as a sublayer

Answer 5

I made a right border using this:

leftScrollView.clipsToBounds = YES;

CALayer *rightBorder = [CALayer layer];
rightBorder.borderColor = [UIColor darkGrayColor].CGColor;
rightBorder.borderWidth = 1;
rightBorder.frame = CGRectMake(-1, -1, CGRectGetWidth(leftScrollView.frame), CGRectGetHeight(leftScrollView.frame)+2);

[leftScrollView.layer addSublayer:rightBorder];

Answer 6

Here's the swift equivalent

leftScrollView.clipsToBounds = true
let rightBorder: CALayer = CALayer()
rightBorder.borderColor = UIColor.darkGrayColor().CGColor
rightBorder.borderWidth = 1
rightBorder.frame = CGRectMake(-1, -1, CGRectGetWidth(leftScrollView.frame), CGRectGetHeight(leftScrollView.frame)+2)
leftScrollView.layer.addSublayer(rightBorder)