SwiftUI中无法从@objc函数修改类属性的问题求助

尝试在SwiftUI中捕获距离传感器（ProximitySensor）的活动，创建了ProximityObserver类，试图在通知回调中更新state属性，但执行state = device.proximityState后，state从未改变。相关代码如下：

import SwiftUI
import UIKit

class ProximityObserver {
    @State var state = false;
    
    @objc func didChange(notification: NSNotification) {
        print("MyView::ProximityObserver.didChange")
        if let device = notification.object as? UIDevice {
            print(device.proximityState)
            state = device.proximityState
            print(state)
        }
    }
}

struct ContentView: View {
    @State var proximityObserver = ProximityObserver()
        
    func activateProximitySensor() {
        print("MyView::activateProximitySensor")
        if !UIDevice.current.isProximityMonitoringEnabled {
            UIDevice.current.isProximityMonitoringEnabled = true
            
            if UIDevice.current.isProximityMonitoringEnabled {
                NotificationCenter.default.addObserver(proximityObserver, selector: #selector(proximityObserver.didChange), name: UIDevice.proximityStateDidChangeNotification, object: UIDevice.current)
            }
        }
    }
    
    func deactivateProximitySensor() {
        print("MyView::deactivateProximitySensor")
        UIDevice.current.isProximityMonitoringEnabled = false
        NotificationCenter.default.removeObserver(proximityObserver, name: UIDevice.proximityStateDidChangeNotification, object: UIDevice.current)
    }

    var body: some View {
        Text(proximityObserver.state ? "true" : "false" )
                .animation(.linear(duration: 20).delay(20), value: proximityObserver.state)
        .onAppear() {
            self.activateProximitySensor()
        }
        .onDisappear() {
            self.deactivateProximitySensor()
        }
  }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

日志输出显示：

MyView::ProximityObserver.didChange
true
false

问题原因 #

• @State属性包装器仅适用于SwiftUI的View结构体内部，不能用于普通类。@State的核心作用是让SwiftUI管理视图的状态，类实例中的@State不会被SwiftUI正确跟踪，赋值操作无法生效。
• ContentView中用@State存储ProximityObserver实例也是错误用法：@State设计用于值类型，引用类型需要用@ObservedObject或@StateObject来管理，这样SwiftUI才能监听实例内部的状态变化。

修复方案 #

需要调整状态管理的方式，具体改动如下：

1. 让ProximityObserver遵循ObservableObject协议，用@Published标记需要监听的state属性，确保属性变化时能通知视图更新。
2. 在ContentView中用@StateObject（iOS 14+）持有ProximityObserver实例，确保SwiftUI能监听其内部状态变化并管理实例生命周期。

修改后的完整代码：

import SwiftUI
import UIKit

class ProximityObserver: ObservableObject {
    @Published var state = false
    
    @objc func didChange(notification: NSNotification) {
        print("MyView::ProximityObserver.didChange")
        if let device = notification.object as? UIDevice {
            print(device.proximityState)
            state = device.proximityState
            print(state)
        }
    }
}

struct ContentView: View {
    @StateObject private var proximityObserver = ProximityObserver()
        
    func activateProximitySensor() {
        print("MyView::activateProximitySensor")
        if !UIDevice.current.isProximityMonitoringEnabled {
            UIDevice.current.isProximityMonitoringEnabled = true
            
            if UIDevice.current.isProximityMonitoringEnabled {
                NotificationCenter.default.addObserver(proximityObserver, selector: #selector(proximityObserver.didChange), name: UIDevice.proximityStateDidChangeNotification, object: UIDevice.current)
            }
        }
    }
    
    func deactivateProximitySensor() {
        print("MyView::deactivateProximitySensor")
        UIDevice.current.isProximityMonitoringEnabled = false
        NotificationCenter.default.removeObserver(proximityObserver, name: UIDevice.proximityStateDidChangeNotification, object: UIDevice.current)
    }

    var body: some View {
        Text(proximityObserver.state ? "true" : "false" )
            .animation(.linear(duration: 20).delay(20), value: proximityObserver.state)
            .onAppear() {
                activateProximitySensor()
            }
            .onDisappear() {
                deactivateProximitySensor()
            }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

关键改动说明 #

• ProximityObserver继承ObservableObject并使用@Published：@Published会在属性值变化时自动发送通知，让监听该对象的SwiftUI视图触发更新。
• ContentView使用@StateObject：@StateObject专门用于创建和管理引用类型的生命周期，确保视图在整个生命周期内复用同一个ProximityObserver实例，且能响应其@Published属性的变化。若需兼容iOS 13，可替换为@ObservedObject，但需注意实例的创建时机和生命周期管理。

内容的提问来源于stack exchange，提问作者user1329928