InnerShadowView.swift

// Adapted from http://blog.helftone.com/demystifying-inner-shadows-in-quartz/

import UIKit

class InnerShadowView: UIView {
    var blurRadius = CGFloat(3) {
        didSet { setNeedsDisplay() }
    }
    var offset = CGSize(width: 0, height: 2) {
        didSet { setNeedsDisplay() }
    }
    var path = UIBezierPath() {
        didSet { setNeedsDisplay() }
    }
    var shadowColor = UIColor.blackColor().colorWithAlphaComponent(0.5) {
        didSet { setNeedsDisplay() }
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        
        commonInit()
    }
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        commonInit()
    }
    
    func commonInit() {
        backgroundColor = UIColor.clearColor()
    }
    
    override func drawRect(rect: CGRect) {
        super.drawRect(rect)
        
        let context = UIGraphicsGetCurrentContext()
        
        CGContextSaveGState(context)
        
        CGContextAddPath(context, path.CGPath)
        CGContextClip(context)
        
        let opaqueShadowColor = CGColorCreateCopyWithAlpha(shadowColor.CGColor, 1.0)
        
        CGContextSetAlpha(context, CGColorGetAlpha(shadowColor.CGColor))
        CGContextBeginTransparencyLayer(context, nil)
        CGContextSetShadowWithColor(context, offset, blurRadius, opaqueShadowColor)
        CGContextSetBlendMode(context, CGBlendMode.SourceOut)
        CGContextSetFillColorWithColor(context, opaqueShadowColor)
        CGContextAddPath(context, path.CGPath)
        CGContextFillPath(context)
        CGContextEndTransparencyLayer(context)
        
        CGContextRestoreGState(context)
    }
}