Implemented expense creation functionality.

A new `ExpenseCreationScreen` and `ExpenseCreationViewModel` have been added to allow users to create new expenses. This includes fields for the title, amount, and a participant selection using filter chips.

Key changes:
- Added `ExpenseCreationViewModel` to handle user loading and persistent storage of expenses and shares.
- Created `ExpenseCreationScreen` UI with validation for the "Save" button.
- Updated `DashboardScreen` to include a Floating Action Button (FAB) for navigating to the expense creation flow.
- Added `expenseCreate` API endpoint and related data types (`ExpenseCreationRequest`, `ExpenseCreationResponse`).
- Integrated `ExpenseCreationViewModel` into `MainActivity` and `AppContent`.

app/src/main/java/de/miaurizius/shap_planner/activities/MainActivity.kt

@@ -8,11 +8,13 @@ import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
 import de.miaurizius.shap_planner.TokenStorage
 import de.miaurizius.shap_planner.UserPreferences
 import de.miaurizius.shap_planner.room.AppDatabase
 import de.miaurizius.shap_planner.ui.AppContent
 import de.miaurizius.shap_planner.ui.theme.ShapPlannerTheme
+import de.miaurizius.shap_planner.viewmodels.ExpenseCreationViewModel
 import de.miaurizius.shap_planner.viewmodels.ExpenseDetailViewModel
 import de.miaurizius.shap_planner.viewmodels.LoginViewModel
 import de.miaurizius.shap_planner.viewmodels.MainViewModel
@@ -38,6 +40,12 @@ class MainActivity : ComponentActivity() {
             database.userDao(),
             tokenStorage
         )
+        val creationViewModel = ExpenseCreationViewModel(
+            database.userDao(),
+            database.expenseDao(),
+            database.expenseShareDao(),
+            tokenStorage
+        )
 
 
         setContent {
@@ -67,7 +75,8 @@ class MainActivity : ComponentActivity() {
                     onSessionInvalid = { mainViewModel.logoutFromAccount() },
                     onExpenseClick = { expense -> println("Clicked: ${expense.title}") },
                     viewModel = mainViewModel,
-                    detailViewModel = detailViewModel
+                    detailViewModel = detailViewModel,
+                    creationViewModel = creationViewModel,
                 )
             }
         }

app/src/main/java/de/miaurizius/shap_planner/network/APIService.kt

@@ -25,7 +25,7 @@ interface APIService {
     @GET("api/expenses")
     suspend fun expensesGet(@Header("Authorization") token: String): Response<ExpensesResponse>
     @POST("api/expenses")
-    suspend fun expenseCreate(@Header("Authorization") token: String)
+    suspend fun expenseCreate(@Header("Authorization") token: String, @Body req: ExpenseCreationRequest): Response<ExpenseCreationResponse>
     @PUT("api/expenses")
     suspend fun expenseUpdate(@Header("Authorization") token: String)
     @DELETE("api/expenses")

app/src/main/java/de/miaurizius/shap_planner/network/ComDataTypes.kt

@@ -2,7 +2,6 @@ package de.miaurizius.shap_planner.network
 
 import de.miaurizius.shap_planner.entities.Expense
 import de.miaurizius.shap_planner.entities.ExpenseShare
-import de.miaurizius.shap_planner.entities.User
 
 // Login
 data class LoginRequest(val username: String, val password: String)
@@ -15,6 +14,8 @@ data class RefreshResponse(val access_token: String, val refresh_token: String)
 
 // Expenses
 data class ExpensesResponse(val expenses: List<Expense>)
+data class ExpenseCreationRequest(val expense: Expense, val shares: List<ExpenseShare>)
+data class ExpenseCreationResponse(val expense: Expense, val shares: List<ExpenseShare>)
 
 // ExpenseShares
 data class ExpenseSharesResponse(val shares: List<ExpenseShare>)

app/src/main/java/de/miaurizius/shap_planner/ui/AppContent.kt

@@ -1,5 +1,7 @@
 package de.miaurizius.shap_planner.ui
 
+import androidx.activity.compose.BackHandler
+import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
@@ -10,8 +12,10 @@ import de.miaurizius.shap_planner.entities.Expense
 import de.miaurizius.shap_planner.network.SessionState
 import de.miaurizius.shap_planner.ui.screens.AccountSelectionScreen
 import de.miaurizius.shap_planner.ui.screens.DashboardScreen
+import de.miaurizius.shap_planner.ui.screens.ExpenseCreationScreen
 import de.miaurizius.shap_planner.ui.screens.ExpenseDetailScreen
 import de.miaurizius.shap_planner.ui.screens.LoginScreen
+import de.miaurizius.shap_planner.viewmodels.ExpenseCreationViewModel
 import de.miaurizius.shap_planner.viewmodels.ExpenseDetailViewModel
 import de.miaurizius.shap_planner.viewmodels.MainViewModel
 
@@ -39,11 +43,21 @@ fun AppContent(
 
     //Important
     viewModel: MainViewModel,
-    detailViewModel: ExpenseDetailViewModel
+    detailViewModel: ExpenseDetailViewModel,
+    creationViewModel: ExpenseCreationViewModel
 ) {
     var selectedExpense by remember { mutableStateOf<Expense?>(null) }
+    var showAddExpenseScreen by remember { mutableStateOf(false) }
 
     when {
+        showAddExpenseScreen -> {
+            ExpenseCreationScreen(
+                account = selectedAccount!!,
+                viewModel = creationViewModel,
+                onBack = { showAddExpenseScreen = false },
+                onSaved = { showAddExpenseScreen = false }
+            )
+        }
         selectedExpense != null -> {
             ExpenseDetailScreen(
                 expense = selectedExpense!!,
@@ -65,7 +79,8 @@ fun AppContent(
             onDelete = onDeleteAccount,
             sessionState = sessionState,
             onValidate = onValidateSession,
-            onSessionInvalid = onSessionInvalid
+            onSessionInvalid = onSessionInvalid,
+            onAddExpenseClick = { showAddExpenseScreen = true },
         )
         else -> AccountSelectionScreen(
             accounts = accountList,

app/src/main/java/de/miaurizius/shap_planner/ui/screens/DashboardScreen.kt

@@ -22,6 +22,7 @@ import androidx.compose.material3.Button
 import androidx.compose.material3.CircularProgressIndicator
 import androidx.compose.material3.ExperimentalMaterial3Api
 import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Scaffold
 import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
@@ -45,6 +46,7 @@ fun DashboardScreen(
     // Data and regarding Methods
     account: Account,
     onExpenseClick: (Expense) -> Unit,
+    onAddExpenseClick: () -> Unit,
 
     // Default Methods
     mainViewModel: MainViewModel,
@@ -69,9 +71,19 @@ fun DashboardScreen(
             BackHandler {
                 onBack()
             }
+            Scaffold(floatingActionButton = {
+                androidx.compose.material3.FloatingActionButton(
+                    onClick = onAddExpenseClick,
+                    containerColor = MaterialTheme.colorScheme.primary,
+                    contentColor = MaterialTheme.colorScheme.onPrimary
+                ) {
+                    Text("+", style = MaterialTheme.typography.headlineSmall)
+                }
+            }) { paddingValues ->
                 Column(
                     modifier = Modifier
                         .fillMaxSize()
+                        .padding(paddingValues)
                         .padding(16.dp)
                         .statusBarsPadding()
                         .navigationBarsPadding()
@@ -130,6 +142,7 @@ fun DashboardScreen(
                     }
                 }
             }
+        }
 
         SessionState.Invalid -> {
             LaunchedEffect(Unit) {

app/src/main/java/de/miaurizius/shap_planner/ui/screens/ExpenseCreationScreen.kt

@@ -0,0 +1,87 @@
+package de.miaurizius.shap_planner.ui.screens
+
+import androidx.compose.foundation.layout.*
+import androidx.compose.foundation.text.KeyboardOptions
+import androidx.compose.material3.*
+import androidx.compose.runtime.*
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.input.KeyboardType
+import androidx.compose.ui.unit.dp
+import de.miaurizius.shap_planner.entities.Account
+import de.miaurizius.shap_planner.viewmodels.ExpenseCreationViewModel
+import java.util.UUID
+
+@OptIn(ExperimentalMaterial3Api::class, ExperimentalLayoutApi::class)
+@Composable
+fun ExpenseCreationScreen(
+    account: Account,
+    viewModel: ExpenseCreationViewModel,
+    onSaved: () -> Unit,
+    onBack: () -> Unit
+) {
+    var title by remember { mutableStateOf("") }
+    var amountStr by remember { mutableStateOf("") }
+    val users by viewModel.users.collectAsState()
+    val selectedUsers = remember { mutableStateListOf<UUID>() }
+
+    LaunchedEffect(Unit) { viewModel.loadUsers() }
+
+    Scaffold(
+        topBar = {
+            TopAppBar(title = { Text("New Expense") })
+        }
+    ) { padding ->
+        Column(modifier = Modifier.padding(padding).padding(16.dp)) {
+            OutlinedTextField(
+                value = title,
+                onValueChange = { title = it },
+                label = { Text("What was bought?") },
+                modifier = Modifier.fillMaxWidth()
+            )
+
+            Spacer(modifier = Modifier.height(8.dp))
+
+            OutlinedTextField(
+                value = amountStr,
+                onValueChange = { amountStr = it },
+                label = { Text("Amount in €") },
+                keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Decimal),
+                modifier = Modifier.fillMaxWidth()
+            )
+
+            Spacer(modifier = Modifier.height(16.dp))
+            Text("Who participated?", style = MaterialTheme.typography.titleMedium)
+
+            FlowRow(modifier = Modifier.padding(vertical = 8.dp), horizontalArrangement = Arrangement.spacedBy(8.dp)) {
+                users.forEach { user ->
+                    FilterChip(
+                        selected = selectedUsers.contains(user.id),
+                        onClick = {
+                            if (selectedUsers.contains(user.id)) selectedUsers.remove(user.id)
+                            else selectedUsers.add(user.id)
+                        },
+                        label = { Text(user.name) }
+                    )
+                }
+            }
+
+            Spacer(modifier = Modifier.weight(1f))
+
+            Button(
+                onClick = {
+                    val cents = (amountStr.replace(",", ".").toDoubleOrNull() ?: 0.0) * 100
+                    viewModel.saveExpense(account, title, cents.toInt(), selectedUsers.toList())
+                    onSaved()
+                },
+                modifier = Modifier.fillMaxWidth(),
+                enabled = title.isNotBlank() && amountStr.isNotBlank() && selectedUsers.isNotEmpty()
+            ) {
+                Text("Save")
+            }
+
+            TextButton(onClick = onBack, modifier = Modifier.fillMaxWidth()) {
+                Text("Cancel")
+            }
+        }
+    }
+}

app/src/main/java/de/miaurizius/shap_planner/viewmodels/ExpenseCreationViewModel.kt

@@ -0,0 +1,64 @@
+package de.miaurizius.shap_planner.viewmodels
+
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
+import de.miaurizius.shap_planner.TokenStorage
+import de.miaurizius.shap_planner.entities.Account
+import de.miaurizius.shap_planner.entities.Expense
+import de.miaurizius.shap_planner.entities.ExpenseDao
+import de.miaurizius.shap_planner.entities.ExpenseShare
+import de.miaurizius.shap_planner.entities.ExpenseShareDao
+import de.miaurizius.shap_planner.entities.User
+import de.miaurizius.shap_planner.entities.UserDao
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.launch
+import java.util.UUID
+
+class ExpenseCreationViewModel(
+    private val userDao: UserDao,
+    private val expenseDao: ExpenseDao,
+    private val shareDao: ExpenseShareDao,
+    private val tokenStorage: TokenStorage
+) : ViewModel() {
+    private val _users = MutableStateFlow<List<User>>(emptyList())
+    val users: StateFlow<List<User>> = _users
+
+    fun loadUsers() {
+        viewModelScope.launch {
+            userDao.getAllUsers().collect { _users.value = it }
+        }
+    }
+
+    fun saveExpense(account: Account, title: String, amountCents: Int, selectedUserIds: List<UUID>) {
+        viewModelScope.launch {
+            val expenseId = UUID.randomUUID() //TODO: Backend  has to generate UUID
+            val newExpense = Expense(
+                id = expenseId,
+                payer_id = account.id,
+                amount = amountCents,
+                title = title,
+                description = "",
+                attachments = null,
+                created_at = (System.currentTimeMillis() / 1000).toInt(),
+                last_updated_at = 0
+            )
+
+            expenseDao.insertExpense(newExpense)
+
+            val shareAmount = amountCents / selectedUserIds.size
+            selectedUserIds.forEach { userId ->
+                shareDao.insertShare(
+                    ExpenseShare(
+                        UUID.randomUUID(), //TODO: Backend has to generate UUID
+                        expenseId,
+                        userId,
+                        shareAmount
+                    )
+                )
+            }
+
+            // API Calls
+        }
+    }
+}