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Lesson 10: Data Storage and Network Requests

Objective:

Learn how to store data locally and make network requests.

Content:

1. UserDefaults

The UserDefaults class provides a way to store simple data persistently.

Storing and Retrieving Data:

@State private var username = UserDefaults.standard.string(forKey: "username") ?? ""

var body: some View {
    TextField("Username", text: $username)
        .onDisappear {
            UserDefaults.standard.set(username, forKey: "username")
        }
}Copy Code

Explanation:

• @State private var username = UserDefaults.standard.string(forKey: "username") ?? "": Declares a state variable to hold the username, retrieving it from UserDefaults if it exists.
• TextField("Username", text: $username): Creates a text field for entering the username.
• .onDisappear { UserDefaults.standard.set(username, forKey: "username") }: Saves the username to UserDefaults when the view disappears.

2. CoreData Integration

CoreData is Apple's framework for managing object graphs and persisting data.

Setting up CoreData:

// Assume CoreData setup is done in the AppDelegate or SceneDelegate
@Environment(\.managedObjectContext) private var viewContext

var body: some View {
    // Example of fetching and displaying CoreData entities
    Text("CoreData Example")
}Copy Code

Explanation:

• @Environment(\.managedObjectContext) private var viewContext: Declares an environment variable to access the CoreData managed object context.
• Text("CoreData Example"): Placeholder for displaying CoreData entities.

3. Network Requests with URLSession

Using URLSession to perform network requests.

Fetching Data from the Network:

@State private var data: String = ""

var body: some View {
    VStack {
        Text(data)
        Button("Fetch Data") {
            fetchData()
        }
    }
}

func fetchData() {
    guard let url = URL(string: "https://api.example.com/data") else { return }
    URLSession.shared.dataTask(with: url) { data, response, error in
        if let data = data, let fetchedData = String(data: data, encoding: .utf8) {
            DispatchQueue.main.async {
                self.data = fetchedData
            }
        }
    }.resume()
}Copy Code

Explanation:

• @State private var data: String = "": Declares a state variable to hold the fetched data.
• Text(data): Displays the fetched data.
• Button("Fetch Data") { fetchData() }: Fetches the data when the button is tapped.
• func fetchData(): Defines the function to fetch data from the network.
• guard let url = URL(string: "https://api.example.com/data") else { return }: Creates a URL from the string and ensures it is valid.
• URLSession.shared.dataTask(with: url) { data, response, error in ... }.resume(): Creates a data task to fetch the data and resumes it.
• if let data = data, let fetchedData = String(data: data, encoding: .utf8) { DispatchQueue.main.async { self.data = fetchedData } }: Converts the data to a string and updates the state variable on the main thread.

4. Processing JSON Data

Parsing JSON data into Swift models.

Parsing JSON into Swift Models:

struct User: Codable {
    let id: Int
    let name: String
}

@State private var users: [User] = []

var body: some View {
    List(users) { user in
        Text(user.name)
    }
    .onAppear {
        fetchUsers()
    }
}

func fetchUsers() {
    guard let url = URL(string: "https://api.example.com/users") else { return }
    URLSession.shared.dataTask(with: url) { data, response, error in
        if let data = data {
            let decoder = JSONDecoder()
            if let fetchedUsers = try? decoder.decode([User].self, from: data) {
                DispatchQueue.main.async {
                    self.users = fetchedUsers
                }
            }
        }
    }.resume()
}Copy Code

Explanation:

• struct User: Codable: Defines a model that conforms to the Codable protocol for JSON decoding.
• @State private var users: [User] = []: Declares a state variable to hold the list of users.
• List(users) { user in Text(user.name) }: Creates a list view with dynamic rows. Each row displays the user's name.
• .onAppear { fetchUsers() }: Fetches the users when the view appears.
• func fetchUsers(): Defines the function to fetch users from the network.
• guard let url = URL(string: "https://api.example.com/users") else { return }: Creates a URL from the string and ensures it is valid.
• URLSession.shared.dataTask(with: url) { data, response, error in ... }.resume(): Creates a data task to fetch the data and resumes it.
• if let data = data { let decoder = JSONDecoder(); if let fetchedUsers = try? decoder.decode([User].self, from: data) { DispatchQueue.main.async { self.users = fetchedUsers } } }: Decodes the JSON data into an array of User objects and updates the state variable on the main thread.

5. Synchronizing Online and Offline Data

Implementing strategies for data synchronization.

Strategies for Synchronization:

// Placeholder for detailed synchronization logic and implementation
Text("Synchronization Example")Copy Code

Explanation:

• // Placeholder for detailed synchronization logic and implementation: Placeholder for implementing data synchronization logic between online and offline states.
• Text("Synchronization Example"): Placeholder for displaying synchronized data.