CustomSaverActivity.kt

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.layout.fillMaxWidth
import androidx.compose.material.MaterialTheme
import androidx.compose.material.Text
import androidx.compose.runtime.Composable
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.saveable.Saver
import androidx.compose.runtime.saveable.SaverScope
import androidx.compose.runtime.saveable.rememberSaveable
import androidx.compose.ui.Modifier
import androidx.compose.ui.tooling.preview.Preview
import com.google.gson.Gson

class CustomSaverActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MaterialTheme {
                BookItem()
            }
        }
    }
}

@Composable
fun BookItem() {
    val book = rememberSaveable(stateSaver = BookSaverMapApproach) {
        mutableStateOf(Book("A123", "Title 1", 4.6))
    }

    Column(modifier = Modifier.fillMaxWidth()) {
        Text(text = book.value.isbn)
        Text(text = book.value.title)
        Text(text = book.value.rating.toString())
    }
}

// Custom object
data class Book(val isbn: String, val title: String, val rating: Double)

// Custom saver implementing Saver interface
// Saving data into map
object BookSaverMapApproach : Saver<Book, Map<String, Any>> {
    override fun restore(value: Map<String, Any>): Book {
        return Book(
            isbn = value["isbn"] as String,
            title = value["title"] as String,
            rating = value["rating"] as Double
        )
    }

    override fun SaverScope.save(value: Book): Map<String, Any> {
        return mapOf(
            "isbn" to value.isbn,
            "title" to value.title,
            "rating" to value.rating
        )
    }
}

// Custom saver implementing Saver interface
// Saving data as string using gson
object BookSaverGsonApproach : Saver<Book, String> {
    override fun restore(value: String): Book? {
        return Gson().fromJson(value, Book::class.java)
    }

    override fun SaverScope.save(value: Book): String {
        return Gson().toJson(value)
    }
}

@Preview
@Composable
fun BookItemPreview() {
    BookItem()
}