SwiftUI中@Published属性更新未在UI中生效的问题排查

你的SimpleViewModel用@Published修饰了predictor，但@Published只会在对象本身被替换时发送视图更新通知。而SimplePredictor内部的@Published var label变化时，predictor对象本身并没有被替换，所以SimpleViewModel不会触发objectWillChange通知，导致SwiftUI视图没有重新计算body，Text自然不会更新显示新值。

虽然onReceive能订阅到predictor.$label的变化（这是直接监听SimplePredictor内部的发布者），但这个回调不会触发视图整体刷新，Text还是会使用旧的body计算结果。

方法1：让ViewModel转发Predictor的更新通知 #

修改SimpleViewModel，订阅predictor的更新事件，当predictor内部有变化时，手动发送ViewModel自己的更新通知：

import SwiftUI
import Combine // 需要导入Combine框架

class SimplePredictor: ObservableObject {
    @Published var label: String = "N/A"
    
    func updateLabel() {
        DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
            self.label = "Updated Label"
            print("Label set to: \(self.label)")
        }
    }
}

class SimpleViewModel: ObservableObject {
    let predictor = SimplePredictor() // 此处不再需要@Published
    private var cancellables = Set<AnyCancellable>()
    
    init() {
        // 订阅predictor的更新事件，转发给ViewModel的objectWillChange
        predictor.objectWillChange
            .sink { [weak self] _ in
                self?.objectWillChange.send()
            }
            .store(in: &cancellables)
    }
}

struct SimpleContentView: View {
    @StateObject var viewModel = SimpleViewModel()
    
    var body: some View {
        VStack {
            Text(viewModel.predictor.label)
                .onReceive(viewModel.predictor.$label) { newLabel in
                    print("Label received: \(newLabel)")
                }
        }
        .padding()
        .onAppear {
            viewModel.predictor.updateLabel()
        }
    }
}

方法2：将Label属性提升到ViewModel中 #

如果业务逻辑允许，直接让ViewModel持有label属性，避免嵌套ObservableObject的监听问题：

class SimplePredictor {
    func updateLabel(completion: @escaping (String) -> Void) {
        DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
            let newLabel = "Updated Label"
            print("Label set to: \(newLabel)")
            completion(newLabel)
        }
    }
}

class SimpleViewModel: ObservableObject {
    @Published var label: String = "N/A"
    private let predictor = SimplePredictor()
    
    func updateLabel() {
        predictor.updateLabel { [weak self] newLabel in
            self?.label = newLabel
        }
    }
}

struct SimpleContentView: View {
    @StateObject var viewModel = SimpleViewModel()
    
    var body: some View {
        VStack {
            Text(viewModel.label)
                .onReceive(viewModel.$label) { newLabel in
                    print("Label received: \(newLabel)")
                }
        }
        .padding()
        .onAppear {
            viewModel.updateLabel()
        }
    }
}

方法3：让视图直接观察Predictor #

在视图中直接将predictor声明为@ObservedObject，绕过ViewModel的嵌套监听：

struct SimpleContentView: View {
    @StateObject var viewModel = SimpleViewModel()
    @ObservedObject var predictor: SimplePredictor
    
    init() {
        let vm = SimpleViewModel()
        self.predictor = vm.predictor
        self._viewModel = StateObject(wrappedValue: vm)
    }
    
    var body: some View {
        VStack {
            Text(predictor.label)
                .onReceive(predictor.$label) { newLabel in
                    print("Label received: \(newLabel)")
                }
        }
        .padding()
        .onAppear {
            predictor.updateLabel()
        }
    }
}

内容的提问来源于stack exchange，提问作者Megool