#import <QuartzCore/CALayer.h>


UIImageView *imageView = [UIImageView alloc]init];
imageView.layer.masksToBounds = YES;
imageView.layer.borderColor = [UIColor blackColor].CGColor;
imageView.layer.borderWidth = 1;    

This code can be used for adding UIImageView view border.

all these answers work fine BUT add a rect to an image. Suppose You have a shape (in my case a butterfly) and You want to add a border (a red border):

we need two steps: 1) take the image, convert to CGImage, pass to a function to draw offscreen in a context using CoreGraphics, and give back a new CGImage

2) convert to uiimage back and draw:

// remember to release object!
+ (CGImageRef)createResizedCGImage:(CGImageRef)image toWidth:(int)width
andHeight:(int)height
{
// create context, keeping original image properties
CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(NULL, width,
                                             height,
                                             8
                                             4 * width,
                                             colorspace,
                                             kCGImageAlphaPremultipliedFirst
                                             );

 CGColorSpaceRelease(colorspace);

if(context == NULL)
    return nil;

// draw image to context (resizing it)
CGContextSetInterpolationQuality(context, kCGInterpolationDefault);

CGSize offset = CGSizeMake(2,2);
CGFloat blur = 4;   
CGColorRef color = [UIColor redColor].CGColor;
CGContextSetShadowWithColor ( context, offset, blur, color);

CGContextDrawImage(context, CGRectMake(0, 0, width, height), image);
// extract resulting image from context
CGImageRef imgRef = CGBitmapContextCreateImage(context);
CGContextRelease(context);
return imgRef;

}

- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.

CGRect frame = CGRectMake(0,0,160, 122);
UIImage * img = [UIImage imageNamed:@"butterfly"]; // take low res OR high res, but frame should be the low-res one.
imgV = [[UIImageView alloc]initWithFrame:frame];
[imgV setImage: img];
imgV.center = self.view.center;
[self.view addSubview: imgV];

frame.size.width = frame.size.width * 1.3;
frame.size.height = frame.size.height* 1.3;
CGImageRef cgImage =[ViewController createResizedCGImage:[img CGImage] toWidth:frame.size.width andHeight: frame.size.height ];

imgV2 = [[UIImageView alloc]initWithFrame:frame];
[imgV2 setImage: [UIImage imageWithCGImage:cgImage] ];

// release:
if (cgImage) CGImageRelease(cgImage);

[self.view addSubview: imgV2];

}

I added a normal butterfly and a red-bordered bigger butterfly.

I have created a class which adds a border to imageView h. Use this class instead of UIImageView.I have given a padding of 4. You can give as per your wish.

class UIBorderImageView: UIView {

private lazy var imageView: UIImageView = {
    let imageView = UIImageView()
    imageView.contentMode = .scaleAspectFit
    imageView.translatesAutoresizingMaskIntoConstraints = false
    return imageView
}()

override init(frame: CGRect) {
    super.init(frame: frame)
    self.backgroundColor = UIColor.White()
    self.layer.borderColor = UIColor.GreyMedium().cgColor
    self.layer.borderWidth = 1.0
    self.layer.cornerRadius = 4.0
    self.layer.masksToBounds = true
    self.setUpViews()
}

required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
}

private func setUpViews(){
    self.addSubview(imageView)
    self.addConstraintsWithFormat(format: "H:|-4-[v0]-4-|", views: imageView)
    self.addConstraintsWithFormat(format: "V:|-4-[v0]-4-|", views: imageView)
}

func configureImageViewWith(image:UIImage){
    self.imageview.image = image 
}}

I use this method to add a border outside the image. You can customise the border width in boderWidth constant.

Swift 3

func addBorderToImage(image : UIImage) -> UIImage {
    let bgImage = image.cgImage
    let initialWidth = (bgImage?.width)!
    let initialHeight = (bgImage?.height)!
    let borderWidth = Int(Double(initialWidth) * 0.10);
    let width = initialWidth + borderWidth * 2
    let height = initialHeight + borderWidth * 2
    let data = malloc(width * height * 4)

    let context = CGContext(data: data,
                        width: width,
                        height: height,
                        bitsPerComponent: 8,
                        bytesPerRow: width * 4,
                        space: (bgImage?.colorSpace)!,
                        bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue);

    context?.draw(bgImage!, in: CGRect(x: CGFloat(borderWidth), y: CGFloat(borderWidth), width: CGFloat(initialWidth), height: CGFloat(initialHeight)))
    context?.setStrokeColor(UIColor.white.cgColor)
    context?.setLineWidth(CGFloat(borderWidth))
    context?.move(to: CGPoint(x: 0, y: 0))
    context?.addLine(to: CGPoint(x: 0, y: height))
    context?.addLine(to: CGPoint(x: width, y: height))
    context?.addLine(to: CGPoint(x: width, y: 0))
    context?.addLine(to: CGPoint(x: 0, y: 0))
    context?.strokePath()

    let cgImage = context?.makeImage()
    let uiImage = UIImage(cgImage: cgImage!)

    free(data)

    return uiImage;
}