iOS应用退至后台或关闭时，如何恢复屏幕亮度？

问题描述 #

我的应用内有一个展示条码的页面（作为TabView中的标签页），当用户进入该页面时，将屏幕亮度设置为最大值；用户离开页面时恢复为原始亮度。但当用户将应用退至后台或完全关闭时，亮度无法恢复。

尝试了以下Swift代码，通过.onDisappear和监听scenePhase切换至.background时恢复亮度：

struct BarcodeView: View {
    @Environment(\.scenePhase) var scenePhase

    @State private var originalBrightness: CGFloat = 0.0

    var body: some View {
        Barcode()
            .onAppear {
                originalBrightness = UIScreen.main.brightness
                UIScreen.main.brightness = 1.0
            }
            .onDisappear {
                UIScreen.main.brightness = originalBrightness
            }
            .onChange(of: scenePhase) { _, newPhase in
                if newPhase == .background {
                    UIScreen.main.brightness = originalBrightness
                }
            }
    }
}

虽然这些修饰符会触发，但屏幕亮度并未改变。经了解这是苹果的预期行为——应用在非活跃状态下无法修改亮度设置。请问是否有恢复原始屏幕亮度的可行方法？

可行解决方案 #

方案1：通过AppDelegate监听应用生命周期 #

苹果允许应用在即将进入后台或终止的瞬间（此时应用仍处于短暂活跃状态）修改系统设置，我们可以利用AppDelegate的生命周期方法实现恢复逻辑，同时需要将原始亮度值存储到全局可访问的位置：

1. 创建全局亮度管理单例

class BrightnessManager {
    static let shared = BrightnessManager()
    var originalBrightness: CGFloat = 0.0
}

2. 修改BarcodeView，用单例存储原始亮度

struct BarcodeView: View {
    let brightnessManager = BrightnessManager.shared

    var body: some View {
        Barcode()
            .onAppear {
                brightnessManager.originalBrightness = UIScreen.main.brightness
                UIScreen.main.brightness = 1.0
            }
            .onDisappear {
                UIScreen.main.brightness = brightnessManager.originalBrightness
            }
    }
}

3. 实现AppDelegate的生命周期回调

class AppDelegate: NSObject, UIApplicationDelegate {
    let brightnessManager = BrightnessManager.shared

    func applicationDidEnterBackground(_ application: UIApplication) {
        UIScreen.main.brightness = brightnessManager.originalBrightness
    }

    func applicationWillTerminate(_ application: UIApplication) {
        UIScreen.main.brightness = brightnessManager.originalBrightness
    }
}

4. 在App结构体中注册AppDelegate

@main
struct YourApp: App {
    @UIApplicationDelegateAdaptor(AppDelegate.self) var appDelegate

    var body: some Scene {
        WindowGroup {
            TabView {
                BarcodeView()
                    .tabItem {
                        Label("条码", systemImage: "barcode.viewfinder")
                    }
                // 其他标签页
            }
        }
    }
}

方案2：基于SceneDelegate的场景生命周期处理（iOS 13+） #

如果应用采用Scene-Based架构，可在SceneDelegate中监听场景状态变化：

1. 复用上述BrightnessManager单例

2. 实现SceneDelegate的生命周期方法

class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    let brightnessManager = BrightnessManager.shared

    func sceneDidEnterBackground(_ scene: UIScene) {
        UIScreen.main.brightness = brightnessManager.originalBrightness
    }

    func sceneWillResignActive(_ scene: UIScene) {
        UIScreen.main.brightness = brightnessManager.originalBrightness
    }

    func sceneWillTerminate(_ scene: UIScene) {
        UIScreen.main.brightness = brightnessManager.originalBrightness
    }
}

核心说明 #

• 应用进入后台后确实无法修改亮度，但在applicationDidEnterBackground、sceneDidEnterBackground等回调触发时，应用还未完全进入非活跃状态，此时可以执行亮度修改操作。
• 使用全局单例存储原始亮度，避免View的@State变量在应用退后台时无法被正确访问的问题。

内容的提问来源于stack exchange，提问作者Pieter