如何在 SwiftUI 中使用 UserDefaults？

Question

struct ContentView: View {
@State var settingsConfiguration: Settings
    struct Settings {
        var passwordLength: Double = 20
        var moreSpecialCharacters: Bool = false
        var specialCharacters: Bool = false
        var lowercaseLetters: Bool = true
        var uppercaseLetters: Bool = true
        var numbers: Bool = true
        var space: Bool = false
    }
  var body: some View {
    VStack {
                HStack {
                    Text("Password Length: \(Int(settingsConfiguration.passwordLength))")
                    Spacer()
                    Slider(value: $settingsConfiguration.passwordLength, from: 1, through: 512)
                }
                Toggle(isOn: $settingsConfiguration.moreSpecialCharacters) {
                    Text("More Special Characters")
                }
                Toggle(isOn: $settingsConfiguration.specialCharacters) {
                    Text("Special Characters")
                }
                Toggle(isOn: $settingsConfiguration.space) {
                    Text("Spaces")
                }
                Toggle(isOn: $settingsConfiguration.lowercaseLetters) {
                    Text("Lowercase Letters")
                }
                Toggle(isOn: $settingsConfiguration.uppercaseLetters) {
                    Text("Uppercase Letters")
                }
                Toggle(isOn: $settingsConfiguration.numbers) {
                    Text("Numbers")
                }
                Spacer()
                }
                .padding(.all)
                .frame(width: 500, height: 500)
  }
}

So I have all this code here and I want to use UserDefaults to save settings whenever a switch is changed or a slider is slid and to retrieve all this data when the app launches but I have no idea how I would go about using UserDefaults with SwiftUI (Or UserDefaults in general, I've just started looking into it so I could use it for my SwiftUI app but all the examples I see are for UIKit and when I try implementing them in SwiftUI I just run into a ton of errors).

Answer 1

The approach from caram is in general ok but there are so many problems with the code that SmushyTaco did not get it work. Below you will find an "Out of the Box" working solution.

1. UserDefaults propertyWrapper

import Foundation
import Combine

@propertyWrapper
struct UserDefault<T> {
    let key: String
    let defaultValue: T
    
    init(_ key: String, defaultValue: T) {
        self.key = key
        self.defaultValue = defaultValue
    }
    
    var wrappedValue: T {
        get {
            return UserDefaults.standard.object(forKey: key) as? T ?? defaultValue
        }
        set {
            UserDefaults.standard.set(newValue, forKey: key)
        }
    }
}

2. UserSettings class

final class UserSettings: ObservableObject {

    let objectWillChange = PassthroughSubject<Void, Never>()

    @UserDefault("ShowOnStart", defaultValue: true)
    var showOnStart: Bool {
        willSet {
            objectWillChange.send()
        }
    }
}

3. SwiftUI view

struct ContentView: View {

@ObservedObject var settings = UserSettings()

var body: some View {
    VStack {
        Toggle(isOn: $settings.showOnStart) {
            Text("Show welcome text")
        }
        if settings.showOnStart{
            Text("Welcome")
        }
    }
}

Answer 2

Starting from Xcode 12.0 (iOS 14.0) you can use @AppStorage property wrapper for such types: Bool, Int, Double, String, URL and Data . Here is example of usage for storing String value:

struct ContentView: View {
    
    static let userNameKey = "user_name"
    
    @AppStorage(Self.userNameKey) var userName: String = "Unnamed"
    
    var body: some View {
        VStack {
            Text(userName)
            
            Button("Change automatically ") {
                userName = "Ivor"
            }
            
            Button("Change manually") {
                UserDefaults.standard.setValue("John", forKey: Self.userNameKey)
            }
        }
    }
}

Here you are declaring userName property with default value which isn't going to the UserDefaults itself. When you first mutate it, application will write that value into the UserDefaults and automatically update the view with the new value.

Also there is possibility to set custom UserDefaults provider if needed via store parameter like this:

@AppStorage(Self.userNameKey, store: UserDefaults.shared) var userName: String = "Mike"

and

extension UserDefaults {
    static var shared: UserDefaults {
        let combined = UserDefaults.standard
        combined.addSuite(named: "group.myapp.app")
        return combined
    }
}

Notice: ff that value will change outside of the Application (let's say manually opening the plist file and changing value), View will not receive that update.

PS Also there is new Extension on View which adds func defaultAppStorage(_ store: UserDefaults) -> some View which allows to change the storage used for the View. This can be helpful if there are a lot of @AppStorage properties and setting custom storage to each of them is cumbersome to do.

Answer 3

The code below adapts Mohammad Azam's excellent solution in this video :

import SwiftUI

struct ContentView: View {
    @ObservedObject var userDefaultsManager = UserDefaultsManager()

    var body: some View {
        VStack {
            Toggle(isOn: self.$userDefaultsManager.firstToggle) {
                Text("First Toggle")
            }

            Toggle(isOn: self.$userDefaultsManager.secondToggle) {
                Text("Second Toggle")
            }
        }
    }
}

class UserDefaultsManager: ObservableObject {
    @Published var firstToggle: Bool = UserDefaults.standard.bool(forKey: "firstToggle") {
        didSet { UserDefaults.standard.set(self.firstToggle, forKey: "firstToggle") }
    }

    @Published var secondToggle: Bool = UserDefaults.standard.bool(forKey: "secondToggle") {
        didSet { UserDefaults.standard.set(self.secondToggle, forKey: "secondToggle") }
    }
}

Answer 4

First, create a property wrapper that will allow us to easily make the link between your Settings class and UserDefaults:

import Foundation

@propertyWrapper
struct UserDefault<Value: Codable> {    
    let key: String
    let defaultValue: Value

    var value: Value {
        get {
            let data = UserDefaults.standard.data(forKey: key)
            let value = data.flatMap { try? JSONDecoder().decode(Value.self, from: $0) }
            return value ?? defaultValue
        }
        set {
            let data = try? JSONEncoder().encode(newValue)
            UserDefaults.standard.set(data, forKey: key)
        }
    }
}

Then, create a data store that holds your settings:

import Combine
import SwiftUI

final class DataStore: BindableObject {
    let didChange = PassthroughSubject<DataStore, Never>()

    @UserDefault(key: "Settings", defaultValue: [])
    var settings: [Settings] {
        didSet {
            didChange.send(self)
        }
    }
}

Now, in your view, access your settings:

import SwiftUI

struct SettingsView : View {
    @EnvironmentObject var dataStore: DataStore

    var body: some View {
        Toggle(isOn: $settings.space) {
            Text("\(settings.space)")
        }
    }
}

Answer 5

If you are persisting a one-off struct such that a property wrapper is overkill, you can encode it as JSON. When decoding, use an empty Data instance for the no-data case.

final class UserData: ObservableObject {
    @Published var profile: Profile? = try? JSONDecoder().decode(Profile.self, from: UserDefaults.standard.data(forKey: "profile") ?? Data()) {
        didSet { UserDefaults.standard.set(try? JSONEncoder().encode(profile), forKey: "profile") }
    }
}

Answer 6

Another great solution is to use the unofficial static subscript API of @propertyWrapper instead of the wrappedValue which simplifies a lot the code. Here is the definition:

@propertyWrapper
struct UserDefault<Value> {
    let key: String
    let defaultValue: Value

    init(wrappedValue: Value, _ key: String) {
        self.key = key
        self.defaultValue = wrappedValue
    }

    var wrappedValue: Value {
        get { fatalError("Called wrappedValue getter") }
        set { fatalError("Called wrappedValue setter") }
    }

    static subscript(
        _enclosingInstance instance: Preferences,
        wrapped wrappedKeyPath: ReferenceWritableKeyPath<Preferences, Value>,
        storage storageKeyPath: ReferenceWritableKeyPath<Preferences, Self>
    ) -> Value {
        get {
            let wrapper = instance[keyPath: storageKeyPath]
            return instance.userDefaults.value(forKey: wrapper.key) as? Value ?? wrapper.defaultValue
        }

        set {
            instance.objectWillChange.send()
            let key = instance[keyPath: storageKeyPath].key
            instance.userDefaults.set(newValue, forKey: key)
        }
    }
}

Then you can define your settings object like this:

final class Settings: ObservableObject {
  let userDefaults: UserDefaults

  init(defaults: UserDefaults = .standard) {
    userDefaults = defaults
  }

  @UserDefaults("yourKey") var yourSetting: SettingType
  ...
}

However, be careful with this kind of implementation. Users tend to put all their app settings in one of such object and use it in every view that depends on one setting. This can result in slow down caused by too many unnecessary objectWillChange notifications in many view. You should definitely separate concerns by breaking down your settings in many small classes.

The @AppStorage is a great native solution but the drawback is that is kind of break the unique source of truth paradigm as you must provide a default value for every property.

Answer 7

I'm supriced no one wrote the new way, anyway, Apple migrated to this method now and you don't need all the old code, you can read and write to it like this:

@AppStorage("example") var example: Bool = true

that's the equivalent to read/write in the old UserDefaults . You can use it as a regular variable.