How to merge two UIImages?

I am trying to merge two different images and create a new one. This is the way I would like to do: I have this image (A):

It's a PNG image and I would like to merge this one with another image (B) which I took from the phone to create something like this:

I need a function who merge A with B creating C. The size must remain from the A image and the image B should auto adapt the size to fit into the polaroid (A). Is it possible to do that? Thank for your help!

UPDATE Just one thing, the image (A) is a square and the image i took is a 16:9, how can i fix that?? If i use your function the image (B) that i took become stretched!

Hope this may help you,

var bottomImage = UIImage(named: "bottom.png")
var topImage = UIImage(named: "top.png")
    
var size = CGSize(width: 300, height: 300)
UIGraphicsBeginImageContext(size)
    
let areaSize = CGRect(x: 0, y: 0, width: size.width, height: size.height)
bottomImage!.draw(in: areaSize)
    
topImage!.draw(in: areaSize, blendMode: .normal, alpha: 0.8)
    
var newImage:UIImage = UIGraphicsGetImageFromCurrentImageContext()!
UIGraphicsEndImageContext()

All the Best :)

Swift 5: Extension for UIImage

extension UIImage {
  func mergeWith(topImage: UIImage) -> UIImage {
    let bottomImage = self

    UIGraphicsBeginImageContext(size)


    let areaSize = CGRect(x: 0, y: 0, width: bottomImage.size.width, height: bottomImage.size.height)
    bottomImage.draw(in: areaSize)

    topImage.draw(in: areaSize, blendMode: .normal, alpha: 1.0)

    let mergedImage = UIGraphicsGetImageFromCurrentImageContext()!
    UIGraphicsEndImageContext()
    return mergedImage
  }
}

Swift 4 UIImage extension that enables easy image merging / overlaying.

extension UIImage { 

  func overlayWith(image: UIImage, posX: CGFloat, posY: CGFloat) -> UIImage {
    let newWidth = size.width < posX + image.size.width ? posX + image.size.width : size.width
    let newHeight = size.height < posY + image.size.height ? posY + image.size.height : size.height
    let newSize = CGSize(width: newWidth, height: newHeight)
    
    UIGraphicsBeginImageContextWithOptions(newSize, false, 0.0)
    draw(in: CGRect(origin: CGPoint.zero, size: size))
    image.draw(in: CGRect(origin: CGPoint(x: posX, y: posY), size: image.size))
    let newImage = UIGraphicsGetImageFromCurrentImageContext()!
    UIGraphicsEndImageContext()
    
    return newImage
  }

}

This way the overlay picture will be much cleaner:

class func mergeImages(imageView: UIImageView) -> UIImage {
    UIGraphicsBeginImageContextWithOptions(imageView.frame.size, false, 0.0)
    imageView.superview!.layer.renderInContext(UIGraphicsGetCurrentContext()!)
    let image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    
    return image 
}

Objective C version of this solution with top image re-centered logic :

-(UIImage *)getImageInclosedWithinAnotherImage
{
    float innerImageSize = 20;
    UIImage *finalImage;

    UIImage *outerImage = [UIImage imageNamed:@"OuterImage.png"];
    UIImage *innerImage = [UIImage imageNamed:@"InnerImage.png"];

    CGSize outerImageSize = CGSizeMake(40, 40); // Provide custom size or size of your actual image
    UIGraphicsBeginImageContext(outerImageSize);

    //calculate areaSize for re-centered inner image
    CGRect areSize = CGRectMake(((outerImageSize.width/2) - (innerImageSize/2)), ((outerImageSize.width/2) - (innerImageSize/2)), innerImageSize, innerImageSize);
    [outerImage drawInRect:CGRectMake(0, 0, outerImageSize.width, outerImageSize.height)];
    [innerImage drawInRect:areSize blendMode:kCGBlendModeNormal alpha:1.0];

    finalImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return finalImage;
}

The upvoted answer stretches the background image changing its ratio. The solution below fixes that by rendering the image from a UIView that contains the two image views as subviews.

ANSWER YOU ARE LOOKING FOR (Swift 4):

func blendImages(_ img: UIImage,_ imgTwo: UIImage) -> Data? {
    let bottomImage = img
    let topImage = imgTwo
    
    let imgView = UIImageView(frame: CGRect(x: 0, y: 0, width: 306, height: 306))
    let imgView2 = UIImageView(frame: CGRect(x: 0, y: 0, width: 306, height: 306))

    // - Set Content mode to what you desire
    imgView.contentMode = .scaleAspectFill
    imgView2.contentMode = .scaleAspectFit

    // - Set Images
    imgView.image = bottomImage
    imgView2.image = topImage
    
    // - Create UIView
    let contentView = UIView(frame: CGRect(x: 0, y: 0, width: 306, height: 306))
    contentView.addSubview(imgView)
    contentView.addSubview(imgView2)
    
    // - Set Size
    let size = CGSize(width: 306, height: 306)

    // - Where the magic happens
    UIGraphicsBeginImageContextWithOptions(size, true, 0)

    contentView.drawHierarchy(in: contentView.bounds, afterScreenUpdates: true)

    guard let i = UIGraphicsGetImageFromCurrentImageContext(),
        let data = UIImageJPEGRepresentation(i, 1.0)
        else {return nil}
    
    UIGraphicsEndImageContext()
    
    return data
}

The returned image data doubles the size of the image, so set the size of the views at half the desired size.

EXAMPLE: I wanted the width and height of the image to be 612, so I set the view frames width and height to 306) // Enjoy :)

Slightly modified version of answer by budidino. This implementation also handles negative posX and posY correctly.

extension UIImage {
    func overlayWith(image: UIImage, posX: CGFloat, posY: CGFloat) -> UIImage {
        let newWidth = posX < 0 ? abs(posX) + max(self.size.width, image.size.width) :
            size.width < posX + image.size.width ? posX + image.size.width : size.width
        let newHeight = posY < 0 ? abs(posY) + max(size.height, image.size.height) :
            size.height < posY + image.size.height ? posY + image.size.height : size.height
        let newSize = CGSize(width: newWidth, height: newHeight)

        UIGraphicsBeginImageContextWithOptions(newSize, false, 0.0)
        let originalPoint = CGPoint(x: posX < 0 ? abs(posX) : 0, y: posY < 0 ? abs(posY) : 0)
        self.draw(in: CGRect(origin: originalPoint, size: self.size))
        let overLayPoint = CGPoint(x: posX < 0 ? 0 : posX, y: posY < 0 ? 0 : posY)
        image.draw(in: CGRect(origin: overLayPoint, size: image.size))
        let newImage = UIGraphicsGetImageFromCurrentImageContext()!
        UIGraphicsEndImageContext()

        return newImage
    }
}