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

## 貨幣轉換 app

package com.example.currencyconverterapp

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.activity.enableEdgeToEdge
import androidx.compose.foundation.layout.Arrangement
import androidx.compose.foundation.layout.Column
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.statusBarsPadding
import androidx.compose.foundation.rememberScrollState
import androidx.compose.foundation.verticalScroll
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.material3.Text
import androidx.compose.material3.TextField
import androidx.compose.runtime.Composable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.setValue
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.dp
import com.example.currencyconverterapp.ui.theme.CurrencyConverterAppTheme

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        enableEdgeToEdge()
        super.onCreate(savedInstanceState)
        setContent {
            CurrencyConverterAppTheme {
                // A surface container using the 'background' color from the theme
                Surface(
                    modifier = Modifier.fillMaxSize(),
                    //color = MaterialTheme.colorScheme.background
                ) {
                    CurrencyConverterLayout()
                }
            }
        }
    }
}

@Composable
fun CurrencyConverterLayout() {
    var amountInput by remember { mutableStateOf("") }
    val amount = amountInput.toDoubleOrNull() ?: 0.0
    val exchangeRate = 0.21
    val targetAmount = amount * exchangeRate

    Column(
        modifier = Modifier
            .statusBarsPadding()
            .padding(horizontal = 40.dp)
            .verticalScroll(rememberScrollState())
            .safeDrawingPadding(),
        horizontalAlignment = Alignment.CenterHorizontally,
        verticalArrangement = Arrangement.Center
    ) {
        Text(
            text = "Currency Converter",
            modifier = Modifier
                .padding(bottom = 16.dp, top = 40.dp)
                .align(alignment = Alignment.Start)
        )
        TextField(
            value = amountInput,
            onValueChange = { amountInput = it},
            label = { Text("JPY") },
            modifier = Modifier
                .padding(bottom = 32.dp)
                .fillMaxWidth()
        )
        Text(
            text = "TWD: $targetAmount",
            style = MaterialTheme.typography.displayMedium
        )
    }

}

@Composable
fun Greeting(name: String, modifier: Modifier = Modifier) {
    Text(
        text = "Hello $name!",
        modifier = modifier
    )
}

@Preview(showBackground = true)
@Composable
fun GreetingPreview() {
    CurrencyConverterAppTheme {
        CurrencyConverterLayout()
    }
}

增加可以修改匯率與調整使用體驗

package com.example.currencyconverterapp

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.activity.enableEdgeToEdge
import androidx.compose.foundation.layout.Arrangement
import androidx.compose.foundation.layout.Column
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.statusBarsPadding
import androidx.compose.foundation.rememberScrollState
import androidx.compose.foundation.text.KeyboardOptions
import androidx.compose.foundation.verticalScroll
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.material3.Text
import androidx.compose.material3.TextField
import androidx.compose.runtime.Composable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.setValue
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
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 com.example.currencyconverterapp.ui.theme.CurrencyConverterAppTheme

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        enableEdgeToEdge()
        super.onCreate(savedInstanceState)
        setContent {
            CurrencyConverterAppTheme {
                // A surface container using the 'background' color from the theme
                Surface(
                    modifier = Modifier.fillMaxSize(),
                    //color = MaterialTheme.colorScheme.background
                ) {
                    CurrencyConverterLayout()
                }
            }
        }
    }
}

@Composable
fun CurrencyConverterLayout() {
    var amountInput by remember { mutableStateOf("") }
    var exchangeRateInput by remember { mutableStateOf("0.21") }

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

    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 = ImeAction.Next
            ),
            modifier = Modifier
                .padding(bottom = 32.dp)
                .fillMaxWidth()
        )
        TextField(
            value = exchangeRateInput,
            onValueChange = { exchangeRateInput = 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()
        )
        Text(
            text = stringResource(R.string.twd_amount, targetAmount),
            style = MaterialTheme.typography.displayMedium
        )
    }
}

@Preview(showBackground = true)
@Composable
fun CurrencyConverterLayoutPreview() {
    CurrencyConverterAppTheme {
        CurrencyConverterLayout()
    }
}

## 匯率換算 API

* Free Currency Exchange Rates API
* https://github.com/fawazahmed0/exchange-api
* `https://cdn.jsdelivr.net/npm/@fawazahmed0/currency-api@latest/v1/currencies/jpy.json`

## 課後練習
透過 Free Currency Exchage Rates API 取得 JPY to TWD 的預設匯率