How to save and read data structures using User Defaults?

User Defaults only stores a limited set of types: Data, String, Number, Date, Array, Dictionary. To store custom data structures, they need to be encoded into Data and decoded back.

The Codable protocol (a combination of Encodable and Decodable) is used.

1. Defining the structure: The structure must conform to the Codable protocol.

   struct MyData: Codable {
       let id: Int
       let name: String
       let isActive: Bool
   }
   

2. Saving: Encode the structure into Data using JSONEncoder and save it in User Defaults.

   let myStruct = MyData(id: 1, name: "Example", isActive: true)
   
   // Encode to Data
   let encoder = JSONEncoder()
   if let encodedData = try? encoder.encode(myStruct) {
       // Save Data with key
       UserDefaults.standard.set(encodedData, forKey: "myCustomData")
   } else {
       // Handle encoding error
       print("Failed to encode struct")
   }
   

3. Reading: Retrieve Data from User Defaults and decode it back into the structure using JSONDecoder.

   // Retrieve Data by key
   if let savedData = UserDefaults.standard.data(forKey: "myCustomData") {
       // Decode Data back into structure
       let decoder = JSONDecoder()
       if let loadedStruct = try? decoder.decode(MyData.self, from: savedData) {
           // Use the loaded structure
           print("Loaded struct: \(loadedStruct)")
       } else {
           // Handle decoding error
           print("Failed to decode data")
       }
   } else {
       // No data found for key
       print("No data found for key")
   }
   

Key points:

• User Defaults is not intended for storing large amounts of data or confidential information.
• Using Codable with JSONEncoder/JSONDecoder is the standard way to save custom types as Data.
• Use try? and handle potential encoding/decoding errors.
• For saving arrays of structures, Codable is also used.

// Example of saving an array of structures
let dataArray: [MyData] = [
    MyData(id: 1, name: "A", isActive: true),
    MyData(id: 2, name: "B", isActive: false)
]

let encoder = JSONEncoder()
if let encodedArrayData = try? encoder.encode(dataArray) {
    UserDefaults.standard.set(encodedArrayData, forKey: "myCustomArray")
}

// Example of reading an array of structures
if let savedArrayData = UserDefaults.standard.data(forKey: "myCustomArray") {
    let decoder = JSONDecoder()
    if let loadedArray = try? decoder.decode([MyData].self, from: savedArrayData) {
        print("Loaded array: \(loadedArray)")
    }
}