A simple button where the left inset is adjusted to a fixed value, pinning the content to the right edge. Used in Stack Exchange.app to have a variable width empty button with an icon on the right side. Uses IBDesignable and IBInspectable so we never have to reference the button in code. :)

SEUILeftInsetFromRightButton.swift

import UIKit

@IBDesignable
class SEUILeftInsetFromRightButton: UIButton {

    @IBInspectable var leftInsetFromRight:CGFloat = 0 {
        didSet {
            updateContentEdgeInsetsWithWidth(CGRectGetWidth(bounds))
        }
    }
    
    override var bounds:CGRect {
        willSet(newValue) {
            if CGRectGetWidth(newValue) != CGRectGetWidth(bounds) {
                updateContentEdgeInsetsWithWidth(CGRectGetWidth(newValue))
            }
        }
    }
    
    override var frame:CGRect {
        willSet(newValue) {
            if CGRectGetWidth(newValue) != CGRectGetWidth(frame) {
                updateContentEdgeInsetsWithWidth(CGRectGetWidth(newValue))
            }
        }
    }
    
    private func updateContentEdgeInsetsWithWidth(width:CGFloat) {
        let leftInset = leftInsetFromRight > 0 ? width - leftInsetFromRight : 0
        contentEdgeInsets = UIEdgeInsets(top: 0, left: leftInset, bottom: 0, right: 0)
    }
    
    override func intrinsicContentSize() -> CGSize {
        var size = super.intrinsicContentSize()
        size.width -= contentEdgeInsets.left
        return size
    }
}