Animates changing the brightness of a `UIScreen`

ExampleViewController.swift

import UIKit

final class ExampleViewController: UViewController {

    private var usersBrightness = UIScreen.main.brightness
    
    private var willEnterForegroundWasCalled = false
    private var viewWillDisappearWasCalled = false

    override func viewDidLoad() {
        super.viewDidLoad()
        
        usersBrightness = UIScreen.main.brightness

        NotificationCenter.default.addObserver(self, selector: #selector(applicationWillEnterForeground), name: .UIApplicationWillEnterForeground, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(applicationDidBecomeActive), name: .UIApplicationDidBecomeActive, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(applicationWillResignActive), name: .UIApplicationWillResignActive, object: nil)
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        usersBrightness = UIScreen.main.brightness
        UIScreen.main.animateBrightness(to: 1, duration: 0.6)
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        usersBrightness = UIScreen.main.brightness
        UIScreen.main.animateBrightness(to: 1, duration: 0.6)
    }

    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)

        viewWillDisappearWasCalled = true
        UIScreen.main.animateBrightness(to: usersBrightness, duration: 0.3)
    }

    @objc private func applicationWillResignActive() {
        UIScreen.main.animateBrightness(to: usersBrightness, duration: 0.3)
    }

    @objc private func applicationWillEnterForeground() {
        willEnterForegroundWasCalled = true
        usersBrightness = UIScreen.main.brightness
        UIScreen.main.animateBrightness(to: 1, duration: 0.6)
    }

    @objc private func applicationDidBecomeActive() {
        // When the app enters the foreground again because the user closed notification
        // or control center then `UIApplicationWillEnterForeground` is not called, necessitating
        // also listening to `UIApplicationDidBecomeActive`.

        // This guard ensures the brightness is not increased when the app is opened from the home
        // screen, the multitasker, etc., and also when the view controller is dismissing, which can
        // occur if the user closes the view control quickly after closing notification or control center
        guard !willEnterForegroundWasCalled, !viewWillDisappearWasCalled else {
            willEnterForegroundWasCalled = false
            return
        }
        
        usersBrightness = UIScreen.main.brightness
        UIScreen.main.animateBrightness(to: 1, duration: 0.6)
    }
}

UIScreen+AnimatedBrightness.swift

import UIKit

extension UIScreen {

    public func animateBrightness(to newValue: CGFloat, duration: TimeInterval, ticksPerSecond: Double = 60) {
        let startingBrightness = UIScreen.main.brightness
        let delta = newValue - startingBrightness
        let totalTicks = Int(ticksPerSecond * duration)
        let changePerTick = delta / CGFloat(totalTicks)
        let delayBetweenTicks = 1 / ticksPerSecond

        brightness += changePerTick

        DispatchQueue.global(qos: .userInteractive).async {
            var previousValue = self.brightness

            for _ in 2...totalTicks {
                Thread.sleep(forTimeInterval: delayBetweenTicks)

                guard previousValue == self.brightness else {
                    // Value has changed since thread went to sleep 😴
                    return
                }
                let nextValue = min(1, self.brightness + changePerTick)

                self.brightness = nextValue

                // Don't use `nextValue` here as iOS appears to do some rounding, so
                // the actual value of `self.brightness` may differ from `nextValue`
                previousValue = self.brightness
            }
        }
    }

}

@JosephDuffy thanks so much for this!

Thanks, but I don't know why it's not working on my project. But the logic for the app in background is perfect.

The method view viewWillAppear is repeated and it's missing the deinit method to remove the NotificationCenterObserver:

deinit {
        NotificationCenter.default.removeObserver(self, name: .UIApplicationWillEnterForeground, object: nil)
        NotificationCenter.default.removeObserver(self, name: .UIApplicationDidBecomeActive, object: nil)
        NotificationCenter.default.removeObserver(self, name: .UIApplicationWillResignActive, object: nil)
    }