Implement Room Database Singleton for Todo App

.gitignore

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+# Android
+*.iml
+.gradle/
+build/
+local.properties
+.idea/
+.DS_Store
+captures/
+.externalNativeBuild
+.cxx
+
+# Kotlin
+*.class
+*.kotlin_module
+
+# Dependency directories
+/node_modules
+/app/build
+
+# Logging
+*.log
+
+# Testing
+/app/reports
+/app/test-results
+
+# Sensitive information
+*.env
+*.keystore
+
+# System files
+.DS_Store
+Thumbs.db

app/src/main/java/com/todoapp/data/dao/TodoDao.kt

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+package com.todoapp.data.dao
+
+import androidx.room.Dao
+import androidx.room.Insert
+import androidx.room.Query
+import com.todoapp.data.entity.TodoItem
+import kotlinx.coroutines.flow.Flow
+
+/**
+ * Data Access Object for Todo items.
+ * Defines database operations for TodoItem.
+ */
+@Dao
+interface TodoDao {
+    /**
+     * Retrieve all todo items.
+     *
+     * @return Flow of list of TodoItems
+     */
+    @Query("SELECT * FROM todo_items")
+    fun getAllTodoItems(): Flow<List<TodoItem>>
+
+    /**
+     * Insert a new todo item.
+     *
+     * @param todoItem TodoItem to be inserted
+     * @return Long ID of the inserted item
+     */
+    @Insert
+    suspend fun insertTodoItem(todoItem: TodoItem): Long
+}

app/src/main/java/com/todoapp/data/database/TodoDatabase.kt

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+package com.todoapp.data.database
+
+import android.content.Context
+import androidx.room.Database
+import androidx.room.Room
+import androidx.room.RoomDatabase
+import com.todoapp.data.dao.TodoDao
+import com.todoapp.data.entity.TodoItem
+
+/**
+ * Room Database for storing Todo items.
+ * This is a singleton database instance to ensure only one connection is maintained.
+ */
+@Database(entities = [TodoItem::class], version = 1, exportSchema = false)
+abstract class TodoDatabase : RoomDatabase() {
+    // Abstract method to get the TodoDao
+    abstract fun todoDao(): TodoDao
+
+    companion object {
+        // Volatile ensures the instance is always up-to-date across threads
+        @Volatile
+        private var INSTANCE: TodoDatabase? = null
+
+        /**
+         * Get or create the database instance.
+         * Uses synchronized block to ensure thread safety during instance creation.
+         *
+         * @param context Application context
+         * @return TodoDatabase instance
+         */
+        fun getDatabase(context: Context): TodoDatabase {
+            // If instance already exists, return it
+            return INSTANCE ?: synchronized(this) {
+                // Double-checked locking pattern
+                INSTANCE ?: buildDatabase(context).also { INSTANCE = it }
+            }
+        }
+
+        /**
+         * Create the Room database instance.
+         *
+         * @param context Application context
+         * @return Created TodoDatabase
+         */
+        private fun buildDatabase(context: Context): TodoDatabase {
+            return Room.databaseBuilder(
+                context.applicationContext,
+                TodoDatabase::class.java,
+                "todo_database"
+            )
+            // Optional: Add migration strategy if needed in future
+            .fallbackToDestructiveMigration() // Simplified for this example
+            .build()
+        }
+    }
+}

app/src/main/java/com/todoapp/data/entity/TodoItem.kt

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+package com.todoapp.data.entity
+
+import androidx.room.Entity
+import androidx.room.PrimaryKey
+
+/**
+ * Represents a Todo item in the database.
+ *
+ * @property id Unique identifier for the todo item
+ * @property title Title of the todo item
+ * @property description Optional description of the todo item
+ * @property isCompleted Flag indicating whether the todo item is completed
+ * @property createdAt Timestamp of item creation
+ */
+@Entity(tableName = "todo_items")
+data class TodoItem(
+    @PrimaryKey(autoGenerate = true)
+    val id: Int = 0,
+    val title: String,
+    val description: String? = null,
+    val isCompleted: Boolean = false,
+    val createdAt: Long = System.currentTimeMillis()
+)

app/src/test/java/com/todoapp/data/TodoDatabaseTest.kt

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+package com.todoapp.data
+
+import android.content.Context
+import androidx.room.Room
+import androidx.test.core.app.ApplicationProvider
+import com.todoapp.data.database.TodoDatabase
+import com.todoapp.data.entity.TodoItem
+import kotlinx.coroutines.flow.first
+import kotlinx.coroutines.runBlocking
+import org.junit.After
+import org.junit.Assert.*
+import org.junit.Before
+import org.junit.Test
+import java.io.IOException
+
+class TodoDatabaseTest {
+    private lateinit var database: TodoDatabase
+
+    @Before
+    fun createDatabase() {
+        val context = ApplicationProvider.getApplicationContext<Context>()
+        database = Room.inMemoryDatabaseBuilder(
+            context, TodoDatabase::class.java
+        ).build()
+    }
+
+    @After
+    @Throws(IOException::class)
+    fun closeDatabase() {
+        database.close()
+    }
+
+    @Test
+    @Throws(Exception::class)
+    fun insertAndRetrieveTodoItem() = runBlocking {
+        val todoItem = TodoItem(
+            title = "Test Todo",
+            description = "Test Description",
+            isCompleted = false
+        )
+
+        // Insert todo item
+        val insertedId = database.todoDao().insertTodoItem(todoItem)
+        assertTrue(insertedId > 0)
+
+        // Retrieve todo items
+        val todoItems = database.todoDao().getAllTodoItems().first()
+
+        // Verify
+        assertEquals(1, todoItems.size)
+        assertEquals("Test Todo", todoItems[0].title)
+        assertEquals("Test Description", todoItems[0].description)
+        assertFalse(todoItems[0].isCompleted)
+    }
+}