Blur effect with given intensity

UIView+Blur.swift

import UIKit
import SnapKit

extension UIView {
    
    // MARK: Blur
    
    func addBlur() {
        backgroundColor = .clear
        let blurView = makeBlurView()
        insertSubview(blurView, at: 0)
        blurView.snp.makeConstraints { make in
            make.edges.equalToSuperview()
        }
        
        func makeBlurView(with intensity: CGFloat = 0.4) -> UIVisualEffectView {
            let blurEffect = UIBlurEffect(style: .dark)
            let blurView = UIVisualEffectView(effect: blurEffect)
            blurView.setIntensity(intensity)
            return blurView
        }
    }
}

UIVisualEffectView+Blur.swift

import UIKit

extension UIVisualEffectView {
    
    private var key: UnsafeRawPointer? { UnsafeRawPointer(bitPattern: 16) }

    private var interface: VisualEffectViewInterface {
        if let key = key, let visualEffectViewInterface = objc_getAssociatedObject(self, key) as? VisualEffectViewInterface {
            return visualEffectViewInterface
        }
        let visualEffectViewInterface = VisualEffectViewInterface()
        
        if let key = key {
            objc_setAssociatedObject(self, key, visualEffectViewInterface, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
        }
        return visualEffectViewInterface
    }

    func setIntensity(_ intensity: CGFloat) {
        interface.setIntensity(effectView: self, intensity: intensity)
    }
    
    final private class VisualEffectViewInterface {
        
        private var animator: UIViewPropertyAnimator!
        
        func setIntensity(effectView: UIVisualEffectView, intensity: CGFloat) {
            let effect = effectView.effect
            effectView.effect = nil
            animator = UIViewPropertyAnimator(duration: 1, curve: .linear) { [weak effectView] in effectView?.effect = effect }
            animator.fractionComplete = intensity
        }
    }
}