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

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.LaunchedEffect
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
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.launch
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
}

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 {
                Surface(
                    modifier = Modifier.fillMaxSize(),
                ) {
                    CurrencyConverterLayout(api)
                }
            }
        }
    }
}

@Composable
fun CurrencyConverterLayout(api: CurrencyApi) {
    var amountInput by remember { mutableStateOf("") }
    var exchangeRateInput by remember { mutableStateOf("") }

    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
        )
    }

    LaunchedEffect(Unit) {
        CoroutineScope(Dispatchers.IO).launch {
            try {
                val response = api.getExchangeRate()
                exchangeRateInput = response.jpy.twd.toString()
            } catch (e: Exception) {
                // Handle the error
            }
        }
    }
}

@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 {
        CurrencyConverterLayout(api)
    }
}

## 加入 Switch

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.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.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.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.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
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.launch
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
}

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 {
                Surface(
                    modifier = Modifier.fillMaxSize(),
                ) {
                    CurrencyConverterLayout(api)
                }
            }
        }
    }
}

@Composable
fun CurrencyConverterLayout(api: CurrencyApi) {
    var amountInput by remember { mutableStateOf("") }
    var exchangeRateInput by remember { mutableStateOf("0.21") }
    var exchangeRateShowUp by remember { mutableStateOf(false) }

    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()
        )
        if (exchangeRateShowUp) {
            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()
            )
        }
        CustomExchangeRateRow(
            exchangeRateShowUp = exchangeRateShowUp,
            onExchangeRateChanged = { exchangeRateShowUp = it },
            modifier = Modifier.padding(bottom = 32.dp)
        )
        Text(
            text = stringResource(R.string.twd_amount, targetAmount),
            style = MaterialTheme.typography.displayMedium
        )
    }

    LaunchedEffect(Unit) {
        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
        )
    }
}

@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 {
        CurrencyConverterLayout(api)
    }
}

## 增加動畫效果

package com.example.currencyconverterapp

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.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.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.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.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
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.launch
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
}

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 {
                Surface(
                    modifier = Modifier.fillMaxSize(),
                ) {
                    CurrencyConverterLayout(api)
                }
            }
        }
    }
}

@Composable
fun CurrencyConverterLayout(api: CurrencyApi) {
    var amountInput by remember { mutableStateOf("") }
    var exchangeRateInput by remember { mutableStateOf("0.21") }
    var exchangeRateShowUp by remember { mutableStateOf(false) }

    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()
        )
        AnimatedVisibility(
            exchangeRateShowUp,
            enter = fadeIn() + slideInHorizontally(),
            exit = fadeOut() + slideOutHorizontally()
            ) {
            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()
            )
        }
        CustomExchangeRateRow(
            exchangeRateShowUp = exchangeRateShowUp,
            onExchangeRateChanged = { exchangeRateShowUp = it },
            modifier = Modifier.padding(bottom = 32.dp)
        )
        Text(
            text = stringResource(R.string.twd_amount, targetAmount),
            style = MaterialTheme.typography.displayMedium
        )
    }

    LaunchedEffect(Unit) {
        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
        )
    }
}

@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 {
        CurrencyConverterLayout(api)
    }
}

加入設定圖示在畫面右上角

package com.example.currencyconverterapp

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.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
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.launch
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
}

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 {
                Surface(
                    modifier = Modifier.fillMaxSize(),
                ) {
                    CurrencyConverterLayout(api)
                }
            }
        }
    }
}

@Composable
fun CurrencyConverterLayout(api: CurrencyApi) {
    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 = 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 },
                    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 },
                modifier = Modifier.padding(bottom = 32.dp)
            )
            Text(
                text = stringResource(R.string.twd_amount, targetAmount),
                style = MaterialTheme.typography.displayMedium
            )
        }
        IconButton(
            onClick = { /* Handle settings click */ },
            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) {
        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
        )
    }
}

@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 {
        CurrencyConverterLayout(api)
    }
}

## 使用 Navigation 來切換 Screen
在 `build.gradle` 中加入

implementation("androidx.navigation:navigation-compose:2.7.7")

檔案 `MainActivity.kt`

package com.example.currencyconverterapp

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.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.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.launch
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
}

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) {
    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 = 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 },
                    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 },
                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) {
        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
        )
    }
}

@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)
    }
}

檔案 `SettingScreen.kt`

package com.example.currencyconverterapp

import androidx.compose.foundation.layout.Arrangement
import androidx.compose.foundation.layout.Column
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.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.unit.dp

@Composable
fun SettingsScreen() {
    Column(
        modifier = Modifier
            .statusBarsPadding()
            .padding(horizontal = 40.dp)
            .verticalScroll(rememberScrollState())
            .safeDrawingPadding(),
        horizontalAlignment = Alignment.CenterHorizontally,
        verticalArrangement = Arrangement.Center
    ) {
        Text(text = "Settings Screen")
    }
}

## Testing

`MainActivity.kt`

    val targetAmount = formatCurrency(amount * exchangeRate)

@VisibleForTesting
fun formatCurrency(amount: Double): String {
    return String.format("%.2f", amount)
}

`ExampleUnitTest.kt`

class ExampleUnitTest {
    @Test
    fun addition_isCorrect() {
        assertEquals(4, 2 + 2)
    }

    @Test
    fun currencyFormat_isCorrect() {
        assertEquals("1000.00", formatCurrency(1000.0))
        assertEquals("1000.50", formatCurrency(1000.50))
        assertEquals("1000.31", formatCurrency(1000.31415))
        assertEquals("1000.62", formatCurrency(1000.61742))
    }
}

## 課後練習
在 Currency Converter app 的設定畫面中自行設計想要的功能吧