SwiftUI监听scenePhase时onChange报错：无法接受双参数闭包

问题重现 #

代码示例：

struct MyView: View {
    @Environment(\.scenePhase) var scenePhase
    @State private var timerStatus: TimerStatus

    var body: some View {
        VStack {
            Text(...)
            Button(...)
        }
        .onChange(of: scenePhase) { oldState, newState in // 报错行
            if newState == .active {
                // some code
            } else if oldState == .active {
                // some code
            }
        }
        .onChange(of: timerStatus) { oldState, newState in
            // some code
        }
    }
}

enum TimerStatus {
    case paused
    case runningActive
    case runningInactive
}

报错信息：

"Contextual closure type '(ScenePhase) -> Void' expects 1 argument, but 2 were used in closure body"

开发环境：iOS 18.2 + Xcode，跳转至scenePhase对应的onChange定义显示旧版本：

@available(iOS 14.0, macOS 11.0, tvOS 14.0, watchOS 7.0, *)
extension View {
    @inlinable nonisolated public func onChange<V>(of value: V, perform action: @escaping (_ newValue: V) -> Void) -> some View where V : Equatable
}

而监听自定义TimerStatus枚举的onChange显示新版本：

@available(iOS 17.0, macOS 14.0, tvOS 17.0, watchOS 10.0, *)
extension View {
    nonisolated public func onChange<V>(of value: V, initial: Bool = false, _ action: @escaping (_ oldValue: V, _ newValue: V) -> Void) -> some View where V : Equatable
}

错误原因 #

核心原因是**@Environment(.scenePhase)的调用没有匹配到iOS 17+推出的双参数onChange重载**。虽然ScenePhase本身遵循Equatable协议，但环境值的传递机制导致编译器优先匹配了iOS 14就存在的单参数版本（仅接收newValue），而自定义@State变量因为是本地可绑定的Equatable类型，成功匹配了新版本的双参数重载。

解决方法 #

方案1：使用旧版单参数onChange，手动保存旧状态 #

自己维护一个@State变量记录scenePhase的旧值，实现新旧状态的判断：

struct MyView: View {
    @Environment(\.scenePhase) var scenePhase
    @State private var lastScenePhase: ScenePhase = .active
    @State private var timerStatus: TimerStatus

    var body: some View {
        VStack {
            Text(...)
            Button(...)
        }
        .onChange(of: scenePhase) { newState in
            if newState == .active {
                // some code
            } else if lastScenePhase == .active {
                // some code
            }
            // 更新旧状态记录
            lastScenePhase = newState
        }
        .onChange(of: timerStatus) { oldState, newState in
            // some code
        }
    }
}

方案2：将scenePhase转为本地@State变量，适配新版双参数onChange #

先把环境值同步到本地@State变量，再监听这个变量的变化，就能匹配新版双参数重载：

struct MyView: View {
    @Environment(\.scenePhase) var scenePhase
    @State private var localScenePhase: ScenePhase = .active
    @State private var timerStatus: TimerStatus

    var body: some View {
        VStack {
            Text(...)
            Button(...)
        }
        .onAppear {
            localScenePhase = scenePhase
        }
        .onChange(of: scenePhase) { newState in
            localScenePhase = newState
        }
        .onChange(of: localScenePhase) { oldState, newState in
            if newState == .active {
                // some code
            } else if oldState == .active {
                // some code
            }
        }
        .onChange(of: timerStatus) { oldState, newState in
            // some code
        }
    }
}

方案3：显式调用新版onChange重载（iOS 17+专属） #

如果项目只需要兼容iOS 17及以上版本，可以通过显式指定initial参数来强制匹配新版重载：

.onChange(of: scenePhase, initial: false) { oldState, newState in
    if newState == .active {
        // some code
    } else if oldState == .active {
        // some code
    }
}

内容的提问来源于stack exchange，提问作者Kbb Bio