SwiftUI：将存储的值（通过 userdefaults）传递给另一个视图答案

【问题标题】：SwifUI : pass a stored value (via user defaults) to another viewSwiftUI：将存储的值（通过 userdefaults）传递给另一个视图
【发布时间】：2021-04-21 22:45:15
【问题描述】：

我使用用户默认值来保存表单中的值。我希望将其中一个值（“国籍”）传递给另一个视图。

我尝试使用视图模型来执行此操作，但我在希望传递此数据的文本中得到一个空值。

首先看一下表格

import SwiftUI

struct ProfilePageView: View {
    @ObservedObject var viewModel = ViewModel()
    
    var body: some View {
        Form {
            Section(header: Text("Personal Ifo")) {
                TextField("User Name", text: $viewModel.userName)
                
            }
            //
            Section(header: Text("Your Nationality")) {
                Picker(selection: $viewModel.nationality, label: Text("You were born in")) {
                    ForEach(viewModel.nationalities, id: \.self) { nationality in
                        Text(nationality)
                    }
                    
                }
            } //nationality section end
            //
            Section(header: Text("Country of residence")) {
                Picker(selection: $viewModel.countryOfResidence, label: Text("Where do you live ?")) {
                    ForEach(viewModel.countriesOfResidence, id: \.self) { residence in
                        Text(residence)
                    }
                }
            } // country of residence section end
            //
            
        }
        .navigationTitle("Profile")
        //form end
        
    }
}

第二个是我的视图模型，它使我能够将表单值保存为用户默认值

import Foundation

class ViewModel: ObservableObject {
    @Published var userName: String {
        didSet {
            UserDefaults.standard.setValue(userName, forKey: "username")
        }
    }
    //nationality
    @Published var nationality: String {
        
        didSet {
            UserDefaults.standard.setValue(nationality, forKey: "nationality")
        }

    }
    public var nationalities = ["USA", "Japan", "Senegal", "France"]
    
    //country of residence
    @Published var countryOfResidence: String {
        didSet {
            UserDefaults.standard.setValue(countryOfResidence, forKey: "country of residence")
        }
    }
    public var countriesOfResidence = ["USA", "Japan", "Senegal", "France"]
    
    init() {
        self.userName = UserDefaults.standard.object(forKey: "username") as? String ?? ""
        self.nationality = UserDefaults.standard.object(forKey: "nationality") as? String ?? ""
        self.countryOfResidence = UserDefaults.standard.object(forKey: "Coutry of residence") as? String ?? ""
    }
}

最后是我希望附加此值的视图。我正在使用@observedObject 来尝试从我的视图模型中检索“国籍”值。但是该值未在文本中传递

import SwiftUI

struct VisaPolicyDetailView: View {
    
    @Binding var country: Country
    @ObservedObject var viewmodel = ViewModel()
    
    var body: some View {
        VStack(alignment: .leading,spacing: 20) {
            //a dismiss button
            HStack {
                Spacer()
                Image(systemName: "xmark")
                    .renderingMode(.original)
                    .resizable()
                    .aspectRatio(contentMode: .fit)
                    .frame(width: 40, height: 40)
            }
            
            Spacer()
            
            Text(country.countryName)
                .font(.title)
                .fontWeight(.medium)
            Text("Visa policy for \(viewmodel.nationality) : visa policy")
                .font(.title2)
            
            Spacer()
        }
        .padding()
        .navigationTitle("Visa Policy Detail")
    }
}

【问题讨论】：

标签： swift forms swiftui userdefaults

【解决方案1】：

在提供的场景中，最好使用共享ViewModel，比如

class ViewModel: ObservableObject {
   static let shared = ViewModel()

   // .. other code
}

并在所有受影响的视图中使用 shared

struct ProfilePageView: View {
    @ObservedObject var viewModel = ViewModel.shared  // << here !!
...

和

struct VisaPolicyDetailView: View {
    
    @Binding var country: Country
    @ObservedObject var viewmodel = ViewModel.shared  // << here !!
...

那么两个视图都会观察到ViewModel 的同一个实例。

SwiftUI 2.0：使用AppStorage 自动存储/观察用户默认值，例如

@AppStorage("nationality") var nationality = "some default here"

【讨论】：