错误无法在属性初始值设定项中使用实例成员 'xxx'

Question

26-07-19

我会在观看 WWDC 视频的过程中更新我的代码。我的数据模型是：

struct Egg: Identifiable {
    var id = UUID()
    var thumbnailImage: String
    var day: String
    var date: String
    var text: String
    var imageDetail: String
    var weight: Double
}

#if DEBUG
let testData = [

    Egg(thumbnailImage: "Dag-1", day: "1.circle", date: "7 augustus 2019", text: "Kippen leggen iedere dag een ei.", imageDetail: "Day-1", weight: 35.48),

    Egg(thumbnailImage: "Dag-2", day: "2.circle", date: "8 augustus 2019", text: "Kippen leggen iedere dag een ei.", imageDetail: "Day-2", weight: 35.23),

    Egg(thumbnailImage: "Dag-3", day: "3.circle", date: "9 augustus 2019", text: "Kippen leggen iedere dag een ei.", imageDetail: "Day-3", weight: 34.92)

Etc, etc
]

我有一个 TabbedView、一个 ContentView、一个 ContentDetail 和几个其他视图（用于设置等）。 ContentView 的代码是：

struct ContentView : View {
    var eggs : [Egg] = []

    var body: some View {
        NavigationView {
            List(eggs) { egg in
                EggCell(egg: egg)
            }
            .padding(.top, 10.0)
            .navigationBarTitle(Text("Egg management"), displayMode: .inline)
        }
    }
}

#if DEBUG
struct ContentView_Previews : PreviewProvider {
    static var previews: some View {
        ContentView(eggs: testData)
    }
}
#endif

struct EggCell : View {
    let egg: Egg

    var body: some View {

        return NavigationLink(destination: ContentDetail(egg: egg)) {

            ZStack {

                HStack(spacing: 8.0) {

                    Image(egg.thumbnailImage)
                        .resizable()
                        .aspectRatio(contentMode: .fit)
                        .padding(.leading, -25)
                        .padding(.top, -15)
                        .padding(.bottom, -15)
                        .padding(.trailing, -25)
                        .frame(width: 85, height: 61)

                    VStack {
                        Image(systemName: egg.day)
                            .resizable()
                            .frame(width: 30, height: 22)
                            .padding(.leading, -82)

                        Spacer()
                    }
                    .padding(.leading)

                    VStack {
                        Text(egg.date)
                            .font(.headline)
                            .foregroundColor(Color.gray)
                        Text(egg.weight.clean)
                            .font(.title)

                    }

                }
            }
        }
    }
}

extension Double {
    var clean: String {
        return self.truncatingRemainder(dividingBy: 1) == 0 ? String(format: "%.0f", self) : String(format: "%.2f", self)
    }
}

ContentDetail 的代码是：

struct ContentDetail : View {
    let egg: Egg

    @State private var photo = true
    @State private var calculated = false
    @Binding var weight: Double

    var body: some View {

        VStack (alignment: .center, spacing: 10) {

            Text(egg.date)
                .font(.title)
                .fontWeight(.medium)
                .navigationBarTitle(Text(egg.date), displayMode: .inline)

            ZStack (alignment: .topLeading) {

                Image(photo ? egg.imageDetail : egg.thumbnailImage)
                    .resizable()
                    .aspectRatio(contentMode: .fill)
                    .background(Color.black)
                    .padding(.trailing, 0)
                    .tapAction { self.photo.toggle() }

                VStack {

                    HStack {
                        Image(systemName: egg.day)
                            .resizable()
                            .padding(.leading, 10)
                            .padding(.top, 10)
                            .frame(width: 50, height: 36)
                            .foregroundColor(.white)

                        Spacer()

                        Image(systemName: photo ?  "photo" : "wand.and.stars")
                            .resizable()
                            .padding(.trailing, 10)
                            .padding(.top, 10)
                            .frame(width: 50, height: 36)
                            .foregroundColor(.white)

                    }

                    Spacer()

                    HStack {
                        Image(systemName: "arrow.left.circle")
                            .resizable()
                            .padding(.leading, 10)
                            .padding(.bottom, 10)
                            .frame(width: 50, height: 50)
                            .foregroundColor(.white)

                        Spacer()

                        Image(systemName: "arrow.right.circle")
                            .resizable()
                            .padding(.trailing, 10)
                            .padding(.bottom, 10)
                            .frame(width: 50, height: 50)
                            .foregroundColor(.white)

                    }
                }
            }

            Text("the weight is: \(egg.weight) gram")
                .font(.headline)
                .fontWeight(.bold)

            ZStack {

                RoundedRectangle(cornerRadius: 10)
                    .padding(.top, 45)
                    .padding(.bottom, 45)
                    .border(Color.gray, width: 5)
                    .opacity(0.1)

                HStack {

                    Spacer()

                    DigitPicker(digitName: "tens", digit: $weight.tens)
                    DigitPicker(digitName: "ones", digit: $weight.ones)

                    Text(".")
                        .font(.largeTitle)
                        .fontWeight(.black)
                        .padding(.bottom, 10)

                    DigitPicker(digitName: "tenths", digit: $weight.tenths)
                    DigitPicker(digitName: "hundredths", digit: $weight.hundredths)

                    Spacer()

                    }
                }

            Toggle(isOn: $calculated) {
                Text(calculated ? "This weight is calculated." : "This weight is measured.")
            }

            Text(egg.text)
                .lineLimit(2)
                .frame(minWidth: 0, maxWidth: .infinity, alignment: .leading)
                .padding(.leading, 6)

            Spacer()

        }
        .padding(6)

    }
}

#if DEBUG
struct ContentDetail_Previews : PreviewProvider {
    static var previews: some View {
        NavigationView { ContentDetail(egg: testData[0]) }
    }
}
#endif


struct DigitPicker: View {
    var digitName: String
    @Binding var digit: Int

    var body: some View {
        VStack {
            Picker(selection: $digit, label: Text(digitName)) {
                ForEach(0 ... 9) {
                    Text("\([=14=])").tag([=14=])
                }
            }.frame(width: 60, height: 110).clipped()
        }
    }
}

fileprivate extension Double {
    var tens: Int {
        get { sigFigs / 1000 }
        set { replace(tens: newValue)  }
    }

    var ones: Int {
        get { (sigFigs / 100) % 10 }
        set { replace(ones: newValue) }
    }

    var tenths: Int {
        get { (sigFigs / 10) % 10 }
        set { replace(tenths: newValue) }
    }

    var hundredths: Int {
        get { sigFigs % 10 }
        set { replace(hundredths: newValue) }
    }

    private mutating func replace(tens: Int? = nil, ones: Int? = nil, tenths: Int? = nil, hundredths: Int? = nil) {
        self = Double(0
            + 1000 * (tens ?? self.tens)
            + 100 * (ones ?? self.ones)
            + 10 * (tenths ?? self.tenths)
            + (hundredths ?? self.hundredths)) / 100.0
    }

    private var sigFigs: Int {
        return Int((self * 100).rounded(.toNearestOrEven))
    }
}

我仍然遇到的编译器错误是：

在 ContentView 中，NavigationLink 下方：缺少参数调用
在 NavigationView 的 ContentDetail 中：缺少参数参数 'weight' 通话中
在 ContentDetail 中，#endif 之后：参数缺少参数 'weight' 通话中

25-07-19

以下代码是列表详细信息视图的一部分。 var 'weight' 通过 'NavigationLink' 语句来自列表。在此代码中，我将其声明为“35.48”，但 NavigationLink 填写了它的实际值。

我想用 compactMap 语句创建一个数组 [3, 5, 4, 8]。这在 Playground 中工作正常。这些值转到 4 个不同的选择器（在 HStack 中）。

import SwiftUI
import Foundation

    struct ContentDetail : View {

        var weight : Double = 35.48
        var weightArray = "\(weight)".compactMap { Int("\([=15=])") }

        @State var digit1 = weightArray[0] // error
        @State var digit2 = weightArray[1] // error
        @State var digit3 = weightArray[2] // error
        @State var digit4 = weightArray[3] // error

        var body: some View {

            VStack (alignment: .center, spacing: 10) {

                Text(weight)
                    .font(.title)
                    .fontWeight(.medium)

    etc etc

我收到错误“无法在属性初始值设定项中使用实例成员 'weightArray'；属性初始值设定项运行在 'self' 可用之前'。

如果我使用下面的代码，它工作正常（对于第一个列表元素）：

import SwiftUI
import Foundation

    struct ContentDetail : View {

        var weight : Double = 35.48
        var weightArray = [3, 5, 4, 8]

        @State var digit1 = 3
        @State var digit2 = 5
        @State var digit3 = 4
        @State var digit4 = 8

        var body: some View {

            VStack (alignment: .center, spacing: 10) {

                Text(weight)
                    .font(.title)
                    .fontWeight(.medium)

    etc etc

什么是正确的 SwiftUI 方法，为什么？

Answer 1

确实，属性初始化程序不能引用同一容器中的另一个属性。您必须在 init 中初始化您的属性。

struct ContentDetail: View {

    var weight: Double
    var weightArray: [Int]

    @State var digit1: Int
    @State var digit2: Int
    @State var digit3: Int
    @State var digit4: Int

    init(weight: Double) {
        self.weight = weight
        weightArray = "\(weight)".compactMap { Int("\([=10=])") }
        _digit1 = .init(initialValue: weightArray[0])
        _digit2 = .init(initialValue: weightArray[1])
        _digit3 = .init(initialValue: weightArray[2])
        _digit4 = .init(initialValue: weightArray[3])
    }

但我怀疑您要拆分数字是因为您想让用户单独编辑它们，如下所示：

如果那是你想要的，你不应该为每个数字单独设置 @State 属性。相反，weight 应该是 @Binding 并且每个数字应该有一个单独的可变属性。

首先，扩展 Double 以允许您访问数字：

fileprivate extension Double {
    var tens: Int {
        get { sigFigs / 1000 }
        set { replace(tens: newValue)  }
    }

    var ones: Int {
        get { (sigFigs / 100) % 10 }
        set { replace(ones: newValue) }
    }

    var tenths: Int {
        get { (sigFigs / 10) % 10 }
        set { replace(tenths: newValue) }
    }

    var hundredths: Int {
        get { sigFigs % 10 }
        set { replace(hundredths: newValue) }
    }

    private mutating func replace(tens: Int? = nil, ones: Int? = nil, tenths: Int? = nil, hundredths: Int? = nil) {
        self = Double(0
            + 1000 * (tens ?? self.tens)
            + 100 * (ones ?? self.ones)
            + 10 * (tenths ?? self.tenths)
            + (hundredths ?? self.hundredths)) / 100.0
    }

    private var sigFigs: Int {
        return Int((self * 100).rounded(.toNearestOrEven))
    }
}

然后，将 ContentDetail 的 weight 属性更改为 @Binding 并去掉其他属性：

struct ContentDetail: View {
    @Binding var weight: Double

    var body: some View {
        HStack {
            DigitPicker(digitName: "tens", digit: $weight.tens)
            DigitPicker(digitName: "ones", digit: $weight.ones)
            DigitPicker(digitName: "tenths", digit: $weight.tenths)
            DigitPicker(digitName: "hundredths", digit: $weight.hundredths)
        }
    }
}

struct DigitPicker: View {
    var digitName: String
    @Binding var digit: Int

    var body: some View {
        VStack {
            Picker(selection: $digit, label: Text(digitName)) {
                ForEach(0 ... 9) {
                    Text("\([=12=])").tag([=12=])
                }
            }.frame(width: 60, height: 110).clipped()
        }
    }
}

下面是在操场上测试它所需的其余代码，这就是我生成上面那张图片的方式：

import PlaygroundSupport

struct TestView: View {
    @State var weight: Double = 35.48

    var body: some View {
        VStack(spacing: 0) {
            Text("Weight: \(weight)")
            ContentDetail(weight: $weight)
                .padding()
        }
    }
}

let host = UIHostingController(rootView: TestView())
host.preferredContentSize = .init(width: 320, height: 240)
PlaygroundPage.current.liveView = host

错误无法在属性初始值设定项中使用实例成员 'xxx'

Error Cannot use instance member 'xxx' within property initializer

swiftui

错误无法在 属性 初始值设定项中使用实例成员 'xxx'

Error Cannot use instance member 'xxx' within property initializer

swiftui

错误无法在属性初始值设定项中使用实例成员 'xxx'