SwiftUI，为什么部分视图没有刷新？ @State 变量不更新

Question

这是对我在 SwiftUI 代码中遇到的问题的再现。假设有一个设备列表，并且每个设备都有一些连接信息（启用或不启用）。

使用 SwiftUI 中的经典列表，对于每个设备，都会显示一个 DeviceView。为了方便起见，我想在顶部添加几个箭头，以便在设备之间切换而不必每次都返回列表（理解，就像 iOS 中的 Mail 应用程序）。因此，我没有从列表中调用 DeviceView，而是在调用存储当前设备信息的 MotherDeviceView 之前。当我点击顶部的箭头时，MotherDeviceView 再次调用 DeviceView 并更新视图。

struct Device: Identifiable {
    var id = Int()
    var deviceName = String()

    var isWifiOn = Bool()
    var isCellularOn = Bool()
    var isBluetoothOn = Bool()
}

let devices: [Device] = [
    Device(id: 0, deviceName: "iPhone", isWifiOn: true, isCellularOn: false, isBluetoothOn: true),
    Device(id: 1, deviceName: "iPod", isWifiOn: false, isCellularOn: false, isBluetoothOn: true),
    Device(id: 2, deviceName: "iPad", isWifiOn: false, isCellularOn: true, isBluetoothOn: false)
]

// main view, with list of devices
struct DeviceTab: View {
    var body: some View {
        NavigationView {
            List(devices) { device in
                NavigationLink(destination: MotherDeviceView(device: device)){
                    Text(device.deviceName)
                }
            }.navigationBarTitle(Text("Devices"))
        }
    }
}

// the list call this view and pass the device to DeviceView.
// Using this view is possible to go to the other device using the arrows on the top
// without returning to the list
struct MotherDeviceView: View {
    @State var device: Device

    var body: some View {
        VStack{
            DeviceView(device: $device)
        }
        .navigationBarItems(trailing: arrows)
    }

    private var arrows: some View {
        HStack{
            Button(action: { self.previous() },
                    label: { Image(systemName: "chevron.up") } )
                .padding(.horizontal, 10)
            Button(action: { self.next() },
                   label: { Image(systemName: "chevron.down") } )
        }
    }

    func previous() {
        if device.id == 0 { return }
        device = devices[device.id - 1]
    }

    func next() {
        if device.id == 2 { return }
        device = devices[device.id + 1]
    }

}

// DeviceView
struct DeviceView: View {
    @Binding var device: Device

    var body: some View {
        VStack{
            Spacer()
            Text("This is the device number: " + String(device.id))
            Spacer()
            ToggleSection(dev: $device)
            Spacer()
        }.navigationBarTitle(device.deviceName)
    }
}

// Toggle part of DeviceView
struct ToggleSection: View {
    @Binding var dev: Device

    @State var toggleOn: [Bool] = [false, false, false]

    var body: some View {

        VStack{

            Text(dev.deviceName)
                .fontWeight(.bold)

            Toggle(isOn: $toggleOn[0], label: { Text("Wifi") } )
                .padding()

            Toggle(isOn: $toggleOn[1], label: { Text("Cellular") } )
                .padding()

            Toggle(isOn: $toggleOn[2], label: { Text("Bluetooth") } )
                .padding()

        }
        .onAppear{ self.initialConfigToggle() }
        // with onAppear is okay, but when I use the top arrows, this section does not update
    }

    private func initialConfigToggle() {
        self.toggleOn[0] = self.dev.isWifiOn
        self.toggleOn[1] = self.dev.isCellularOn
        self.toggleOn[2] = self.dev.isBluetoothOn
    }

}

但是DeviceView里面有一个区段ToggleSection不会更新，不知道怎么解决。也许强制更新？但我认为这不是正确的答案（但我什至不知道如何强制更新）。

我认为如果在 ToggleSection 中有 @Binding 而不是 @State 可能是正确的答案，但这不起作用。此示例将起作用（除了未更新的切换）

提前致谢！

Answer 1

这是一种可能的方法，它为设备存储添加了显式视图模型，并加入所有视图以准确地在该存储中进行修改（避免应对设备值）。

测试 Xcode 11.4 / iOS 13.4

修改后的代码：

import SwiftUI
import Combine

struct Device: Identifiable {
    var id = Int()
    var deviceName = String()

    var isWifiOn = Bool()
    var isCellularOn = Bool()
    var isBluetoothOn = Bool()
}

class DeviceStorage: ObservableObject {
    @Published var devices: [Device] = [
        Device(id: 0, deviceName: "iPhone", isWifiOn: true, isCellularOn: false, isBluetoothOn: true),
        Device(id: 1, deviceName: "iPod", isWifiOn: false, isCellularOn: false, isBluetoothOn: true),
        Device(id: 2, deviceName: "iPad", isWifiOn: false, isCellularOn: true, isBluetoothOn: false)
    ]
}

// main view, with list of devices
struct DeviceTab: View {
    @ObservedObject var vm = DeviceStorage() // for demo, can be injected via init
    var body: some View {
        NavigationView {
            List(Array(vm.devices.enumerated()), id: \.element.id) { i, device in
                    NavigationLink(destination: MotherDeviceView(vm: self.vm, selectedDevice: i)) {
                        Text(device.deviceName)
                }
            }.navigationBarTitle(Text("Devices"))
        }
    }
}

struct MotherDeviceView: View {
    @ObservedObject var vm: DeviceStorage // reference type, so have same
    @State private var selected: Int      // selection index

    init(vm: DeviceStorage, selectedDevice: Int) {
        self.vm = vm
        self._selected = State<Int>(initialValue: selectedDevice)
    }

    var body: some View {
        VStack{
            DeviceView(device: $vm.devices[selected]) // bind to selected
        }
        .navigationBarItems(trailing: arrows)
    }

    private var arrows: some View {
        HStack{
            Button(action: { self.previous() },
                    label: { Image(systemName: "chevron.up") } )
                .padding(.horizontal, 10)
            Button(action: { self.next() },
                   label: { Image(systemName: "chevron.down") } )
        }
    }

    func previous() {
        if vm.devices[selected].id == 0 { return }
        selected -= 1                              // change selection
    }

    func next() {
        if vm.devices[selected].id == 2 { return }
        selected += 1                              // change selection
    }

}

// DeviceView
struct DeviceView: View {
    @Binding var device: Device

    var body: some View {
        VStack{
            Spacer()
            Text("This is the device number: " + String(device.id))
            Spacer()
            ToggleSection(dev: $device)
            Spacer()
        }.navigationBarTitle(device.deviceName)
    }
}

// Toggle part of DeviceView
struct ToggleSection: View {
    @Binding var dev: Device

    var body: some View {

        VStack{

            Text(dev.deviceName)
                .fontWeight(.bold)

            // all below bound directly to model !!!

            Toggle(isOn: $dev.isWifiOn, label: { Text("Wifi") } )
                .padding()

            Toggle(isOn: $dev.isCellularOn, label: { Text("Cellular") } )
                .padding()

            Toggle(isOn: $dev.isBluetoothOn, label: { Text("Bluetooth") } )
                .padding()

        }
    }
}

SwiftUI，为什么部分视图没有刷新？ @State 变量不更新

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