**Atlas Device SDK (之前叫做 Realm)**
https://www.mongodb.com/docs/atlas/device-sdks/

Data layer 管理資料的一個選擇，有提供各個平台的 SDK

**TOML**
https://toml.io/

## 在 Currency Converter app 中儲存基本設定與數值

在 `build.gradle` 中加入

implementation("androidx.datastore:datastore-preferences:1.1.0")

`MainActivity.kt`

import androidx.datastore.core.DataStore
import androidx.datastore.preferences.core.Preferences
import androidx.datastore.preferences.preferencesDataStore

val Context.dataStore: DataStore<Preferences> by preferencesDataStore(name = "preferences")

class PreferenceHelper(private val context: Context) {
    companion object {
        val EXCHANGE_RATE = stringPreferencesKey("exchange_rate")
        val USE_CUSTOM_EXCHANGE_RATE = booleanPreferencesKey("use_custom_exchange_rate")
    }
    
    val exchangeRateFlow: Flow<String> = context.dataStore.data
        .map { preferences ->
            preferences[EXCHANGE_RATE] ?: "0.0"
        }
    
    val useCustomExchangeRateFlow: Flow<Boolean> = context.dataStore.data
        .map { preferences ->
            preferences[USE_CUSTOM_EXCHANGE_RATE] ?: false
        }
    
    suspend fun setExchangeRate(exchangeRate: String) {
        context.dataStore.edit { preferences -> 
            preferences[EXCHANGE_RATE] = exchangeRate
        }
    }

    suspend fun setUseCustomExchangeRate(useCustomExchangeRate: Boolean) {
        context.dataStore.edit { preferences ->
            preferences[USE_CUSTOM_EXCHANGE_RATE] = useCustomExchangeRate
        }
    }
}

`MainActivity.kt` 完整程式碼

package com.example.currencyconverterapp

import android.content.Context
import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.activity.enableEdgeToEdge
import androidx.compose.animation.AnimatedVisibility
import androidx.compose.animation.fadeIn
import androidx.compose.animation.fadeOut
import androidx.compose.animation.slideInHorizontally
import androidx.compose.animation.slideOutHorizontally
import androidx.compose.foundation.layout.Arrangement
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.layout.Row
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.layout.fillMaxWidth
import androidx.compose.foundation.layout.padding
import androidx.compose.foundation.layout.safeDrawingPadding
import androidx.compose.foundation.layout.size
import androidx.compose.foundation.layout.statusBarsPadding
import androidx.compose.foundation.layout.wrapContentWidth
import androidx.compose.foundation.rememberScrollState
import androidx.compose.foundation.text.KeyboardOptions
import androidx.compose.foundation.verticalScroll
import androidx.compose.material.icons.Icons
import androidx.compose.material.icons.filled.Settings
import androidx.compose.material3.Icon
import androidx.compose.material3.IconButton
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.material3.Switch
import androidx.compose.material3.Text
import androidx.compose.material3.TextField
import androidx.compose.runtime.Composable
import androidx.compose.runtime.LaunchedEffect
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.saveable.rememberSaveable
import androidx.compose.runtime.setValue
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.platform.LocalContext
import androidx.compose.ui.res.stringResource
import androidx.compose.ui.text.input.ImeAction
import androidx.compose.ui.text.input.KeyboardType
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.dp
import androidx.datastore.core.DataStore
import androidx.datastore.preferences.core.Preferences
import androidx.datastore.preferences.core.booleanPreferencesKey
import androidx.datastore.preferences.core.edit
import androidx.datastore.preferences.core.stringPreferencesKey
import androidx.datastore.preferences.preferencesDataStore
import androidx.navigation.NavController
import androidx.navigation.compose.NavHost
import androidx.navigation.compose.composable
import androidx.navigation.compose.rememberNavController
import com.example.currencyconverterapp.ui.theme.CurrencyConverterAppTheme
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.map
import kotlinx.coroutines.launch
import org.jetbrains.annotations.VisibleForTesting
import retrofit2.Retrofit
import retrofit2.converter.gson.GsonConverterFactory
import retrofit2.http.GET

data class CurrencyResponse(
    val jpy: CurrencyRate
)

data class CurrencyRate(
    val twd: Double
)

interface CurrencyApi {
    @GET("v1/currencies/jpy.json")
    suspend fun getExchangeRate(): CurrencyResponse
}

val Context.dataStore: DataStore<Preferences> by preferencesDataStore(name = "preferences")

class PreferenceHelper(private val context: Context) {
    companion object {
        val EXCHANGE_RATE = stringPreferencesKey("exchange_rate")
        val USE_CUSTOM_EXCHANGE_RATE = booleanPreferencesKey("use_custom_exchange_rate")
    }

    val exchangeRateFlow: Flow<String> = context.dataStore.data
        .map { preferences ->
            preferences[EXCHANGE_RATE] ?: "0.0"
        }

    val useCustomExchangeRateFlow: Flow<Boolean> = context.dataStore.data
        .map { preferences ->
            preferences[USE_CUSTOM_EXCHANGE_RATE] ?: false
        }

    suspend fun setExchangeRate(exchangeRate: String) {
        context.dataStore.edit { preferences ->
            preferences[EXCHANGE_RATE] = exchangeRate
        }
    }

    suspend fun setUseCustomExchangeRate(useCustomExchangeRate: Boolean) {
        context.dataStore.edit { preferences ->
            preferences[USE_CUSTOM_EXCHANGE_RATE] = useCustomExchangeRate
        }
    }
}

class MainActivity : ComponentActivity() {
    private val api by lazy {
        Retrofit.Builder()
            .baseUrl("https://cdn.jsdelivr.net/npm/@fawazahmed0/currency-api@latest/")
            .addConverterFactory(GsonConverterFactory.create())
            .build()
            .create(CurrencyApi::class.java)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        enableEdgeToEdge()
        super.onCreate(savedInstanceState)
        setContent {
            CurrencyConverterAppTheme {
                val navController = rememberNavController()
                NavHost(navController = navController, startDestination = "main") {
                    composable("main") {
                        Surface(
                            modifier = Modifier.fillMaxSize(),
                        ) {
                            CurrencyConverterLayout(navController, api)
                        }
                    }
                    composable("settings") {
                        Surface(
                            modifier = Modifier.fillMaxSize(),
                        ) {
                            SettingsScreen()
                        }
                    }
                }
            }
        }
    }
}

@Composable
fun CurrencyConverterLayout(navController: NavController, api: CurrencyApi) {
    val context = LocalContext.current
    val preferences = PreferenceHelper(context)

    var amountInput by remember { mutableStateOf("") }
    var exchangeRateInput by rememberSaveable { mutableStateOf("0.21") }
    var exchangeRateShowUp by rememberSaveable { mutableStateOf(false) }

    val amount = amountInput.toDoubleOrNull() ?: 0.0
    val exchangeRate = exchangeRateInput.toDoubleOrNull() ?: 0.0
    val targetAmount = formatCurrency(amount * exchangeRate)

    Box(modifier = Modifier.fillMaxSize(), contentAlignment = Alignment.Center) {
        Column(
            modifier = Modifier
                .statusBarsPadding()
                .padding(horizontal = 40.dp)
                .verticalScroll(rememberScrollState())
                .safeDrawingPadding(),
            horizontalAlignment = Alignment.CenterHorizontally,
            verticalArrangement = Arrangement.Center
        ) {
            Text(
                text = stringResource(R.string.currency_converter),
                modifier = Modifier
                    .padding(bottom = 16.dp, top = 40.dp)
                    .align(alignment = Alignment.Start)
            )
            TextField(
                value = amountInput,
                onValueChange = { amountInput = it },
                label = { Text(stringResource(R.string.jpy)) },
                singleLine = true,
                keyboardOptions = KeyboardOptions(
                    keyboardType = KeyboardType.Number,
                    imeAction = if (exchangeRateShowUp) ImeAction.Next else ImeAction.Done
                ),
                modifier = Modifier
                    .padding(bottom = 32.dp)
                    .fillMaxWidth()
            )
            AnimatedVisibility(
                exchangeRateShowUp,
                enter = fadeIn() + slideInHorizontally(),
                exit = fadeOut() + slideOutHorizontally()
            ) {
                TextField(
                    value = exchangeRateInput,
                    onValueChange = {
                        exchangeRateInput = it
                        CoroutineScope(Dispatchers.IO).launch {
                            preferences.setExchangeRate(it)
                        }
                    },
                    label = { Text(stringResource(R.string.jpy_to_twd)) },
                    singleLine = true,
                    keyboardOptions = KeyboardOptions(
                        keyboardType = KeyboardType.Number,
                        imeAction = ImeAction.Done
                    ),
                    modifier = Modifier
                        .padding(bottom = 32.dp)
                        .fillMaxWidth()
                )
            }
            CustomExchangeRateRow(
                exchangeRateShowUp = exchangeRateShowUp,
                onExchangeRateChanged = {
                    exchangeRateShowUp = it
                    CoroutineScope(Dispatchers.IO).launch {
                        preferences.setUseCustomExchangeRate(it)
                        if (!it) {
                            try {
                                val response = api.getExchangeRate()
                                exchangeRateInput = response.jpy.twd.toString()
                            } catch (e: Exception) {
                                // Handle the error
                            }
                        }
                    }
                },
                modifier = Modifier.padding(bottom = 32.dp)
            )
            Text(
                text = stringResource(R.string.twd_amount, targetAmount),
                style = MaterialTheme.typography.displayMedium
            )
        }
        IconButton(
            onClick = { navController.navigate("settings") },
            modifier = Modifier
                .statusBarsPadding()
                .safeDrawingPadding()
                .padding(top = 10.dp)
                .padding(horizontal = 10.dp)
                .align(Alignment.TopEnd)
        ) {
            Icon(Icons.Filled.Settings, contentDescription = null, modifier = Modifier.size(36.dp))
        }
    }

    LaunchedEffect(Unit) {
        preferences.useCustomExchangeRateFlow.collect {
            exchangeRateShowUp = it
            if (it) {
                preferences.exchangeRateFlow.collect { exchangeRateInput = it }
            } else {
                CoroutineScope(Dispatchers.IO).launch {
                    try {
                        val response = api.getExchangeRate()
                        exchangeRateInput = response.jpy.twd.toString()
                    } catch (e: Exception) {
                        // Handle the error
                    }
                }
            }
        }
    }
}

@Composable
fun CustomExchangeRateRow(
    exchangeRateShowUp: Boolean,
    onExchangeRateChanged: (Boolean) -> Unit,
    modifier: Modifier = Modifier
) {
    Row(
        modifier = modifier.fillMaxWidth(),
        verticalAlignment = Alignment.CenterVertically
    ) {
        Text(text = stringResource(R.string.custom_exchange_rate))
        Switch(
            modifier = Modifier
                .fillMaxWidth()
                .wrapContentWidth(Alignment.End),
            checked = exchangeRateShowUp,
            onCheckedChange = onExchangeRateChanged
        )
    }
}
@VisibleForTesting
fun formatCurrency(amount: Double): String {
    return String.format("%.2f", amount)
}

@Preview(showBackground = true)
@Composable
fun CurrencyConverterLayoutPreview() {
    val api by lazy {
        Retrofit.Builder()
            .baseUrl("https://cdn.jsdelivr.net/npm/@fawazahmed0/currency-api@latest/")
            .addConverterFactory(GsonConverterFactory.create())
            .build()
            .create(CurrencyApi::class.java)
    }

    CurrencyConverterAppTheme {
        val navController = rememberNavController()
        CurrencyConverterLayout(navController, api)
    }
}

## 課後練習

* 繼續完善 Currency Converter app 的設定畫面吧～
* 把自家研發好的剪刀石頭布遊戲移植到 Android 上吧！