UIVisualEffectView subclass that allows to customise effect intensity

CustomIntensityVisualEffectView.swift

import UIKit

final class CustomIntensityVisualEffectView: UIVisualEffectView {
  /// Create visual effect view with given effect and its intensity
  ///
  /// - Parameters:
  ///   - effect: visual effect, eg UIBlurEffect(style: .dark)
  ///   - intensity: custom intensity from 0.0 (no effect) to 1.0 (full effect) using linear scale
  init(effect: UIVisualEffect, intensity: CGFloat) {
    theEffect = effect
    customIntensity = intensity
    super.init(effect: nil)
  }

  required init?(coder aDecoder: NSCoder) { nil }

  deinit {
    animator?.stopAnimation(true)
  }

  override func draw(_ rect: CGRect) {
    super.draw(rect)
    effect = nil
    animator?.stopAnimation(true)
    animator = UIViewPropertyAnimator(duration: 1, curve: .linear) { [unowned self] in
      self.effect = theEffect
    }
    animator?.fractionComplete = customIntensity
  }

  private let theEffect: UIVisualEffect
  private let customIntensity: CGFloat
  private var animator: UIViewPropertyAnimator?
}

ios 14.5 from background - effect reset

In order to solve the foreground/background issues try setting animator?.pausesOnCompletion = true inside func draw(_ rect: CGRect) after animator = UIViewPropertyAnimator(.....

Just as a heads up, this code has trouble with UITests.

Setting fractionComplete on an animator and not stopping the animation essentially indicates to the test runner that animations haven't settled, which results in tests hanging for long periods of time, showing the following message in the console:
Wait for <myAppIdentifier> to idle

From the docs on fractionComplete:

Changing the value of this property on an inactive animator moves it to the active state.

There's a bit more info on this here that I found helpful in solving this code's behavior with UITests:
https://diggingdeveloper.blog/2022/01/21/xcode-ui-tests-infinite-wait-for-app-to-idle/

As a workaround, you could choose to disable this code with a custom environment variable like isUITesting.

override func draw(_ rect: CGRect) {
    super.draw(rect)
    effect = nil
    animator?.stopAnimation(true)
    animator = UIViewPropertyAnimator(duration: 1, curve: .linear) { [unowned self] in
      self.effect = theEffect
    }
    animator?.fractionComplete = customIntensity
    DispatchQueue.main.async { [weak self] in
            self?.animator.stopAnimation(true)
            self?.animator.finishAnimation(at: .current)
        }
  }

for cells and view states, you can finish animation