What is the best way to create a shadow behind a UIImageView

There's a better and easier way to do this. UIImageView inherits from UIView so it has a layer property. You can access the layer's shadow properties and bam, you got a shadow.

If you have the UIImageView as an IBOutlet to a nib file, you can just implement the awakeFromNib e.g.

Objective-C

- (void)awakeFromNib {
    imageView.layer.shadowColor = [UIColor purpleColor].CGColor;
    imageView.layer.shadowOffset = CGSizeMake(0, 1);
    imageView.layer.shadowOpacity = 1;
    imageView.layer.shadowRadius = 1.0;
    imageView.clipsToBounds = NO;
}

Don't forget to #import "QuartzCore/CALayer.h"

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For Swift, you can go about it multiple ways. Create a class extension, subclass, or an imageView instance. Whichever the way, the process is the same in modifying the layers shadow property.

Swift 3

override func awakeFromNib() {
    super.awakeFromNib()

    imageView.layer.shadowColor = UIColor.purple.cgColor
    imageView.layer.shadowOffset = CGSize(width: 0, height: 1)
    imageView.layer.shadowOpacity = 1
    imageView.layer.shadowRadius = 1.0
    imageView.clipsToBounds = false
}

The simplest thing to do is add a shadow layer to your image view:

CALayer             *layer = [CALayer layer];
CGRect              bounds = self.bounds;

layer.bounds = bounds;
layer.position = CGPointMake(bounds.size.width / 2 + 3, bounds.size.height / 2 + 3);
layer.backgroundColor = [UIColor colorWithWhite: 0.25 alpha: 0.55].CGColor;
layer.zPosition = -5;

[self.layer addSublayer: layer];

Be sure "Clip Subviews" is turned off for the view

Swift solution with extension. Subclassing is not required. Call myImage.addShadow() from viewDidLoad(). This should work for UIView and UIImageView.

extension UIView {

    func addShadow() {
        layer.shadowColor = UIColor.black.cgColor
        layer.shadowOffset = CGSize(width: 0, height: 0)
        layer.shadowOpacity = 0.5
        layer.shadowRadius = 5
        clipsToBounds = false
    }
}