透過性のあるUIViewサブクラスの影の描画と実装方法について ref: http://qiita.com/shoheiyokoyama/items/d7d02f16b4f632ba6584

Objective-C

    shadowButton.layer.cornerRadius = 15;
    shadowButton.layer.shadowOffset = CGSizeMake(0.0f, 0.0f);
    shadowButton.backgroundColor = [UIColor clearColor];

    CGFloat w = 6.0f;
    CGFloat wh = w / 2;
    CGFloat sw = shadowButton.frame.size.width;
    CGFloat sh = shadowButton.frame.size.height;
    CGFloat c = shadowButton.layer.cornerRadius;

    CGMutablePathRef pathRef = CGPathCreateMutable();
    //Start Point
    CGPathMoveToPoint(pathRef   , nil, -wh, -wh + c);
    CGPathAddArcToPoint(pathRef , nil, -wh, -wh, -wh + c, -wh, c);
    //Top Outside
    CGPathAddLineToPoint(pathRef, nil, sw + wh - c, -wh);
    CGPathAddArcToPoint(pathRef , nil, sw+wh, -wh, sw + wh, -wh + c, c);
    CGPathAddLineToPoint(pathRef, nil, sw - wh, wh + c);
    CGPathAddArcToPoint(pathRef , nil, sw - wh, wh, sw - wh - c, wh, c);
    //Top Inside
    CGPathAddLineToPoint(pathRef, nil, wh + c, wh);
    CGPathAddArcToPoint(pathRef , nil, wh, wh, wh, wh + c, c);
    //Left Inside
    CGPathAddLineToPoint(pathRef, nil, wh, sh - wh - c);
    CGPathAddArcToPoint(pathRef , nil, wh, sh - wh, wh + c, sh - wh, c);
    //Bottom Inside
    CGPathAddLineToPoint(pathRef, nil, sw - wh - c, sh - wh);
    CGPathAddArcToPoint(pathRef , nil, sw - wh, sh - wh, sw - wh, sh - wh - c, c);
    //Right Inside
    CGPathAddLineToPoint(pathRef, nil, sw - wh, wh + c);
    CGPathAddLineToPoint(pathRef, nil, sw + wh, -wh + c);
    //Right Outside
    CGPathAddLineToPoint(pathRef, nil, sw + wh, sh + wh - c);
    CGPathAddArcToPoint(pathRef , nil, sw + wh, sh + wh, sw + wh - c, sh + wh, c);
    //Bottom Outside
    CGPathAddLineToPoint(pathRef, nil, -wh + c, sh + wh);
    CGPathAddArcToPoint(pathRef , nil, -wh, sh + wh, -wh, sh + wh - c, c);
    //Left Outside
    CGPathAddLineToPoint(pathRef, nil, -wh, -wh + c);

    CGPathCloseSubpath(pathRef);
    shadowButton.layer.shadowPath = pathRef;

Swift

        shadowButton.layer.cornerRadius = 15
        shadowButton.layer.shadowOffset = CGSizeMake(0.0, 0.0)
        shadowButton.backgroundColor = UIColor.clearColor()

        let w: CGFloat = 6.0
        let wh: CGFloat = w / 2
        let sw = shadowButton.frame.width
        let sh = shadowButton.frame.height
        let c = shadowButton.layer.cornerRadius

        let pathRef: CGMutablePathRef = CGPathCreateMutable()
        //Start Point
        CGPathMoveToPoint(pathRef   , nil, -wh, -wh + c)
        CGPathAddArcToPoint(pathRef , nil, -wh, -wh, -wh + c, -wh, c)
        //Top Outside
        CGPathAddLineToPoint(pathRef, nil, sw + wh - c, -wh)
        CGPathAddArcToPoint(pathRef , nil, sw + wh, -wh, sw + wh, -wh+c, c)
        CGPathAddLineToPoint(pathRef, nil, sw - wh, wh + c)
        CGPathAddArcToPoint(pathRef , nil, sw - wh, wh, sw - wh - c, wh, c)
        //Top Inside
        CGPathAddLineToPoint(pathRef, nil, wh + c, wh)
        CGPathAddArcToPoint(pathRef , nil, wh, wh, wh, wh + c, c)
        //Left Inside
        CGPathAddLineToPoint(pathRef, nil, wh, sh - wh - c)
        CGPathAddArcToPoint(pathRef , nil, wh, sh - wh, wh + c, sh - wh, c)
        //Bottom Inside
        CGPathAddLineToPoint(pathRef, nil, sw - wh - c, sh - wh)
        CGPathAddArcToPoint(pathRef , nil, sw - wh, sh - wh, sw - wh, sh - wh - c, c)
        //Right Inside
        CGPathAddLineToPoint(pathRef, nil, sw - wh, wh + c)
        CGPathAddLineToPoint(pathRef, nil, sw + wh, -wh + c)
        //Right Outside
        CGPathAddLineToPoint(pathRef, nil, sw + wh, sh + wh - c)
        CGPathAddArcToPoint(pathRef , nil, sw + wh, sh + wh, sw + wh - c, sh + wh, c)
        //Bottom Outside
        CGPathAddLineToPoint(pathRef, nil, -wh + c, sh + wh)
        CGPathAddArcToPoint(pathRef , nil, -wh, sh + wh, -wh, sh + wh - c, c)
        //Left Outside
        CGPathAddLineToPoint(pathRef, nil, -wh, -wh + c)

        CGPathCloseSubpath(pathRef)
        shadowButton.layer.shadowPath = pathRef