【iOS】図形（CGContext）にInnerShadowを加える

CGContext+Extension.swift

extension CGContext {

    func setInnerShadow(withPath path: CGPath, shadowColor: CGColor, offset: CGSize, blurRadius: CGFloat) {
        saveGState()
        addPath(path)
        clip()
        setAlpha(shadowColor.alpha)
        beginTransparencyLayer(auxiliaryInfo: nil)
        guard let opaqueShadowColor = shadowColor.copy(alpha: 1.0) else { return }
        setShadow(offset: offset, blur: blurRadius, color: opaqueShadowColor)
        setBlendMode(.sourceOut)
        setFillColor(opaqueShadowColor)
        addPath(path)
        fillPath()
        endTransparencyLayer()
        restoreGState()
    }

}