feat: Implement MainViewModel and UiState for user data display

This commit introduces the `MainViewModel` to manage user data fetching and a `UiState` sealed class to represent the different states of the UI. It also adds Koin dependency injection for the `MainViewModel`.

Changes:

-   Created `MainViewModel` to manage the user data fetching and UI state.
    -   It fetches users from the `UserRepository` and updates the `UiState`.
    -   It exposes a `uiState` as a `StateFlow` to be observed by the UI.
    -   It contains the `fetchUsers()` function that calls `userRepository.getUsers()`
-   Created `UiState` to represent the loading, success, and error states of the UI.
    -   `UiState.Loading`: Represents the loading state.
    -   `UiState.Success`: Represents the success state with a list of users.
    -   `UiState.Error`: Represents the error state with an error message.
-   Added `viewModelOf(::MainViewModel)` in `AppModule` to enable Koin dependency injection for `MainViewModel`.

Author: CGreenP

app/src/main/java/com/example/techexactly/di/AppModule.kt

@@ -3,10 +3,14 @@ package com.example.techexactly.di
 import com.example.techexactly.model.network.provideRetrofit
 import com.example.techexactly.model.network.provideUserApi
 import com.example.techexactly.model.repository.UserRepository
+import com.example.techexactly.viewmodel.MainViewModel
+import org.koin.core.module.dsl.viewModelOf
 import org.koin.dsl.module
 
 val appModule = module {
     single { provideRetrofit() }
     single { provideUserApi(get()) }
     single { UserRepository(get()) }
+
+    viewModelOf(::MainViewModel)
 }

app/src/main/java/com/example/techexactly/viewmodel/MainViewModel.kt

@@ -0,0 +1,36 @@
+package com.example.techexactly.viewmodel
+
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
+import com.example.techexactly.model.repository.UserRepository
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.launch
+
+class MainViewModel(private val userRepository: UserRepository) : ViewModel() {
+    private val _uiState = MutableStateFlow<UiState>(UiState.Loading)
+    val uiState: StateFlow<UiState> = _uiState.asStateFlow()
+
+    init {
+        fetchUsers()
+    }
+
+    fun fetchUsers() {
+        viewModelScope.launch {
+            userRepository.getUsers().collect { result ->
+                when (result.isSuccess) {
+                    true -> {
+                        val users = result.getOrNull() ?: emptyList()
+                        _uiState.value = UiState.Success(users)
+                    }
+
+                    false -> {
+                        val exception = result.exceptionOrNull()
+                        _uiState.value = UiState.Error(exception?.message ?: "Unknown Error")
+                    }
+                }
+            }
+        }
+    }
+}

app/src/main/java/com/example/techexactly/viewmodel/UiState.kt

@@ -0,0 +1,9 @@
+package com.example.techexactly.viewmodel
+
+import com.example.techexactly.model.dataclass.User
+
+sealed class UiState {
+    data object Loading : UiState()
+    data class Success(val users: List<User>) : UiState()
+    data class Error(val errorMessage: String) : UiState()
+}