当其他属性发生变化时如何添加一个可观察的属性

Question

我有以下模型对象，我用它来填充List，每行都有一个Toggle，它绑定到measurement.isSelected

final class Model: ObservableObject {

    struct Measurement: Identifiable {
        var id = UUID()
        let name: String
        var isSelected: Binding<Bool>

        var selected: Bool = false

        init(name: String) {
            self.name = name

            let selected = CurrentValueSubject<Bool, Never>(false)
            self.isSelected = Binding<Bool>(get: { selected.value }, set: { selected.value = $0 })
        }
    }

    @Published var measurements: [Measurement]
    @Published var hasSelection: Bool = false  // How to set this?

    init(measurements: [Measurement]) {
        self.measurements = measurements
    }
}

只要任何measurement.isSelected 是true，我希望hasSelection 属性为真。我猜想Model 需要观察measurements 的变化，然后更新其hasSelection 属性……但我不知道从哪里开始！

这个想法是 hasSelection 将绑定到 Button 以启用或禁用它。

---

Model的用法如下……

struct MeasurementsView: View {

    @ObservedObject var model: Model

    var body: some View {
        NavigationView {
            List(model.measurements) { measurement in
                MeasurementView(measurement: measurement)
            }
            .navigationBarTitle("Select Measurements")
            .navigationBarItems(trailing: NavigationLink(destination: NextView(), isActive: $model.hasSelection, label: {
                Text("Next")
            }))
        }
    }
}

struct MeasurementView: View {
    let measurement: Model.Measurement
    var body: some View {
        HStack {
            Text(measurement.name)
                .font(.subheadline)
            Spacer()
            Toggle(measurement.name, isOn: measurement.isSelected)
                .labelsHidden()
        }
    }
}

---

有关信息，这是我正在尝试实现的屏幕截图。可选择项目的列表，带有一个导航链接，当一个或多个被选中时启用，当没有项目被选中时禁用。

Answer 1

@user3441734 hasSelection 理想情况下应该是一个只能获取的属性，即 如果有任何measurement.isSelected 为真，则为真

struct Data {
    var bool: Bool
}
class Model: ObservableObject {
    @Published var arr: [Data] = []
    var anyTrue: Bool {
        arr.map{$0.bool}.contains(true)
    }
}

示例（如前）复制 - 粘贴 - 运行

import SwiftUI

struct Data: Identifiable {
    let id = UUID()
    var name: String
    var on_off: Bool
}

class Model: ObservableObject {
    @Published var data = [Data(name: "alfa", on_off: false), Data(name: "beta", on_off: false), Data(name: "gama", on_off: false)]
    var bool: Bool {
        data.map {$0.on_off} .contains(true)
    }
}



struct ContentView: View {
    @ObservedObject var model = Model()
    var body: some View {
        VStack {
        List(0 ..< model.data.count) { idx in
            HStack {
                Text(verbatim: self.model.data[idx].name)
                Toggle(isOn: self.$model.data[idx].on_off) {
                    EmptyView()
                }
            }
        }
            Text("\(model.bool.description)").font(.largeTitle).padding()
        }
    }
}

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

model.data 更新时

@Published var data ....

它的发布者在ObservableObject 上调用objectWillChange。

Next SwiftUI 认识到ObservedObject 需要“更新”视图。视图被重新创建，这将强制model.bool.description 具有新的价值。

最后更新

修改这部分代码

struct ContentView: View {
    @ObservedObject var model = Model()
    var body: some View {
        NavigationView {
        List(0 ..< model.data.count) { idx in
            HStack {
                Text(verbatim: self.model.data[idx].name)
                Toggle(isOn: self.$model.data[idx].on_off) {
                    EmptyView()
                }
            }
            }.navigationBarTitle("List")
            .navigationBarItems(trailing:

                NavigationLink(destination: Text("next"), label: {
                    Text("Next")
                }).disabled(!model.bool)

            )
        }
    }
}

这正是您更新后的问题中的内容

在真机上试试，否则 NavigationLink 只能使用一次（这是当前 Xcode 11.3.1 (11C504) 中众所周知的模拟器错误）。

Answer 2

目前您的代码的问题是即使您观察到measurements的更改，它们也不会在选择更新时更新，因为您将var isSelected: Binding<Bool>作为绑定。这意味着 SwiftUI 将其存储在您的结构之外，并且结构本身不会更新（保持不可变）。

你可以尝试在你的模型上声明@Published var selectedMeasurementId: UUID? = nil 所以你的代码会是这样的：

import SwiftUI
import Combine

struct NextView: View {
    var body: some View {
        Text("Next View")

    }
}

struct MeasurementsView: View {

    @ObservedObject var model: Model

    var body: some View {

        let hasSelection = Binding<Bool> (
            get: {
                self.model.selectedMeasurementId != nil
            },
            set: { value in
                self.model.selectedMeasurementId = nil
            }
        )

        return NavigationView {
            List(model.measurements) { measurement in
                MeasurementView(measurement: measurement, selectedMeasurementId: self.$model.selectedMeasurementId)
            }
            .navigationBarTitle("Select Measurements")
            .navigationBarItems(trailing: NavigationLink(destination: NextView(), isActive: hasSelection, label: {
                Text("Next")
            }))
        }
    }
}

struct MeasurementView: View {


    let measurement: Model.Measurement
    @Binding var selectedMeasurementId: UUID?

    var body: some View {

        let isSelected = Binding<Bool>(
            get: {
                self.selectedMeasurementId == self.measurement.id
            },
            set: { value in
                if value {
                    self.selectedMeasurementId = self.measurement.id
                } else {
                    self.selectedMeasurementId = nil
                }
            }
        )

        return HStack {
            Text(measurement.name)
                .font(.subheadline)
            Spacer()
            Toggle(measurement.name, isOn: isSelected)
                .labelsHidden()
        }
    }
}

final class Model: ObservableObject {

    @Published var selectedMeasurementId: UUID? = nil

    struct Measurement: Identifiable {
        var id = UUID()
        let name: String

        init(name: String) {
            self.name = name
        }
    }

    @Published var measurements: [Measurement]


    init(measurements: [Measurement]) {
        self.measurements = measurements
    }
}

我不确定您希望导航栏中的导航按钮如何表现。现在我只是在点击时将选择设置为 nil 。您可以根据自己的需要进行修改。

如果你想支持多选，你可以使用一组选定的id来代替。

另外，iOS 模拟器似乎在导航方面存在一些问题，但我在物理设备上进行了测试，并且可以正常工作。