SwiftUI与Combine问题：无法获取含前后值的Publisher流

你的代码中前值始终为nil的核心原因是：每次视图因count变化而刷新时，publisher都会被重新创建。Just(count)只会发送当前count的单次值，且scan的初始状态每次都重置为nil，导致每次新生成的流都无法累积之前的状态，因此previous永远是nil。

要解决这个问题，需要让监听的流持续存在且能累积状态，最合适的方式是将count封装到ObservableObject中，通过@Published属性提供持续的数据流，避免视图刷新时重建整个流。

步骤1：创建ViewModel #

import Combine
import SwiftUI

class CounterViewModel: ObservableObject {
    @Published var count: Int = 0
}

步骤2：重构ContentView #

struct ContentView: View {
    // iOS13需使用@ObservedObject，iOS14+可替换为@StateObject
    @ObservedObject private var viewModel = CounterViewModel()
    @State private var previous: Int?
    @State private var current: Int?

    var body: some View {
        VStack {
            Button("Increment") {
                viewModel.count += 1
            }
            Text("Previous: \(previous?.description ?? "nil")")
            Text("Current: \(current?.description ?? "nil")")
        }
        .onReceive(publisher) { prev, curr in
            self.previous = prev
            self.current = curr
        }
    }

    private var publisher: AnyPublisher<(previous: Int?, current: Int), Never> {
        viewModel.$count
            // 初始状态设为(nil, 初始count)，确保第一次能获取初始值
            .scan((previous: Int?, current: Int)?(nil, viewModel.count)) { lastState, newCount in
                // 用上次的current作为本次的previous，新值作为current
                (lastState?.current, newCount)
            }
            .compactMap { $0 }
            .eraseToAnyPublisher()
    }
}

方案说明 #

1. ObservableObject + @Published：viewModel.$count是一个持续发送count变化的Publisher，不会因视图刷新而重建，确保scan能持续累积状态。
2. scan的初始值：设置为(nil, viewModel.count)，第一次流触发时就能正确输出初始的previous: nil和current: 0。
3. iOS13兼容：使用@ObservedObject而非@StateObject（后者是iOS14+引入），保证在iOS13上正常运行。

备选方案（手动管理Subject） #

如果不想使用ObservableObject，也可以用CurrentValueSubject手动包装count的变化，但需要手动发送值，代码如下：

struct ContentView: View {
    @State private var count: Int = 0
    @State private var countSubject = CurrentValueSubject<Int, Never>(0)
    @State private var previous: Int?
    @State private var current: Int?

    var body: some View {
        VStack {
            Button("Increment") {
                count += 1
                countSubject.send(count)
            }
            Text("Previous: \(previous?.description ?? "nil")")
            Text("Current: \(current?.description ?? "nil")")
        }
        .onReceive(publisher) { prev, curr in
            previous = prev
            current = curr
        }
        .onAppear {
            countSubject.send(count)
        }
    }

    private var publisher: AnyPublisher<(previous: Int?, current: Int), Never> {
        countSubject
            .scan((previous: Int?, current: Int)?(nil, count)) { last, new in
                (last?.current, new)
            }
            .compactMap { $0 }
            .eraseToAnyPublisher()
    }
}

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