Introduction:
The Model-View-ViewModel (MVVM) pattern is integral to Android development, offering a clear separation of concerns and promoting maintainable code. However, common mistakes can lead to bloated, unmanageable, or buggy implementations. This article delves into the top 10 MVVM mistakes, offering deep insights into why they are problematic and how to fix them with practical examples.
1. Overloading the ViewModel:
Mistake:
Developers often treat the ViewModel as a catch-all for business logic, UI logic, and data handling, resulting in bloated and complex classes.
Bad Example:
class MyViewModel : ViewModel() {
fun fetchData() {
// Complex business logic here
val result = performComplexCalculation()
// Fetch data from API
val data = api.getData()
// Update UI directly
textView.text = data.toString()
}
}
Why It’s Bad: In this example, the ViewModel is responsible for business logic, data fetching, and even updating the UI. This violates the single responsibility principle, making the ViewModel difficult to test, maintain, and extend. Additionally, directly interacting with the UI from the ViewModel breaks the separation of concerns and tightly couples the ViewModel to specific UI elements.
Correct Example:
class MyViewModel(private val repository: MyRepository) : ViewModel() {
private val _userData = MutableLiveData<User>()
val userData: LiveData<User> get() = _userData
fun fetchUserData() {
viewModelScope.launch {
val data = repository.getUserData()
_userData.postValue(data)
}
}
}
Why It’s Better: This version keeps the ViewModel focused solely on managing UI-related data. The business logic and data fetching are delegated to the repository, which adheres to the separation of concerns. The ViewModel exposes LiveData to the UI, ensuring that the UI updates itself in response to data changes without direct manipulation.
2. Ignoring Separation of Concerns:
Mistake:
Directly referencing UI elements in the ViewModel or performing UI updates within it.
Bad Example:
class MyViewModel : ViewModel() {
fun updateUI(data: String) {
// Updating UI element directly
textView.text = data
}
}
Why It’s Bad: This approach violates the MVVM principle by tightly coupling the ViewModel with specific UI components. It not only makes the ViewModel harder to test but also reduces the flexibility of the UI, as the ViewModel now depends on the presence of certain UI elements.
Correct Example:
class MyViewModel : ViewModel() {
private val _uiState = MutableLiveData<UIState>()
val uiState: LiveData<UIState> = _uiState
fun fetchData() {
_uiState.value = UIState.Loading
// Perform data fetching
_uiState.value = UIState.Success(data)
}
}
Why It’s Better: Here, the ViewModel doesn’t directly manipulate UI components. Instead, it exposes a LiveData object representing the UI state. The UI observes this LiveData and updates itself accordingly. This approach maintains a clean separation between the ViewModel and UI, promoting better testability and flexibility.
3. Mismanaging LiveData:
Mistake:
Creating multiple LiveData objects for each UI component, leading to a fragmented and difficult-to-maintain codebase.
Bad Example:
class MyViewModel : ViewModel() {
val name = MutableLiveData<String>()
val age = MutableLiveData<Int>()
val loading = MutableLiveData<Boolean>()
}
Why It’s Bad: This approach results in a proliferation of LiveData objects, making the ViewModel cluttered and hard to manage. It also complicates the UI logic, which now has to observe multiple LiveData objects, increasing the risk of inconsistencies and errors.
Correct Example:
data class UserProfileUIState(
val name: String = "",
val age: Int = 0,
val loading: Boolean = false
)
class UserProfileViewModel : ViewModel() {
private val _uiState = MutableLiveData<UserProfileUIState>()
val uiState: LiveData<UserProfileUIState> = _uiState
fun loadUserProfile() {
_uiState.value = UserProfileUIState(loading = true)
// Fetch user profile data
_uiState.value = UserProfileUIState(name = "John Doe", age = 30, loading = false)
}
}
Why It’s Better: By grouping related UI state into a single data class, this approach simplifies the ViewModel and makes it easier to manage. The UI only needs to observe one LiveData object, reducing complexity and making the code more maintainable and less error-prone.
4. Inconsistent Data Handling:
Mistake:
Fetching data directly within the ViewModel instead of using a repository or data source layer.
Bad Example:
class MyViewModel : ViewModel() {
private val _userData = MutableLiveData<User>()
val userData: LiveData<User> get() = _userData
init {
// Fetching data directly in the ViewModel
val data = api.getUserData()
_userData.value = data
}
}
Why It’s Bad: Fetching data directly in the ViewModel mixes concerns, as the ViewModel now handles both data fetching and UI logic. This makes the ViewModel more complex and harder to test, and it creates a dependency on specific data sources, reducing flexibility.
Correct Example:
class MyRepository {
fun getUserData(): User {
// Fetch data from local database or network
}
}
class MyViewModel(private val repository: MyRepository) : ViewModel() {
val userData: LiveData<User> = liveData {
emit(repository.getUserData())
}
}
Why It’s Better: By delegating data fetching to a repository, the ViewModel remains focused on its primary role: managing UI-related data. The repository handles data retrieval, making the code more modular, easier to test, and allowing for greater flexibility in changing data sources without affecting the ViewModel.
5. Neglecting Testing:
Mistake:
Skipping unit tests for ViewModels due to perceived complexity or lack of time.
Bad Example:
// No tests written for ViewModel logic
class MyViewModel : ViewModel() {
fun fetchData() {
// Business logic
}
}
Why It’s Bad: Without tests, there’s no way to verify that the ViewModel’s logic works as intended, especially as the application grows. This increases the risk of bugs and makes refactoring more dangerous, as there’s no safety net to catch errors.
Correct Example:
@Test
fun `test loading state`() {
val viewModel = MyViewModel(repository)
viewModel.fetchData()
assertEquals(UIState.Loading, viewModel.uiState.value)
}
Why It’s Better: Testing the ViewModel ensures that your logic works correctly under different scenarios. It also makes your codebase more maintainable, as you can confidently refactor knowing that tests will catch any regressions. Testing is crucial for ensuring the long-term quality and reliability of your code.
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6. Misusing Coroutines or RxJava:
Mistake:
Launching coroutines or subscribing to observables directly in the ViewModel without proper scope management, leading to memory leaks or crashes.
Bad Example:
class MyViewModel : ViewModel() {
fun fetchData() {
// Launching coroutine without proper scope
GlobalScope.launch {
val data = repository.getData()
_uiState.value = UIState.Success(data)
}
}
}
Why It’s Bad: Using GlobalScope for coroutines within the ViewModel can lead to memory leaks and unexpected behavior because the coroutine’s lifecycle isn’t tied to the ViewModel’s lifecycle. If the ViewModel is cleared (e.g., when the user navigates away), the coroutine continues running, potentially leading to crashes or data inconsistencies.
Correct Example:
class MyViewModel : ViewModel() {
fun fetchData() {
viewModelScope.launch {
val data = repository.getData()
_uiState.value = UIState.Success(data)
}
}
}
Why It’s Better: viewModelScope is tied to the ViewModel’s lifecycle, ensuring that any coroutines are automatically canceled when the ViewModel is cleared. This prevents memory leaks and ensures that coroutines don’t continue running when they’re no longer needed, leading to safer and more predictable code.
7. Poor Error Handling:
Mistake:
Not handling exceptions within the ViewModel, leading to crashes or a poor user experience.
Bad Example:
class MyViewModel : ViewModel() {
fun fetchData() {
val data = repository.getData()
_uiState.value = UIState.Success(data)
}
}
Why It’s Bad: This approach assumes that data fetching will always succeed, which is rarely the case in real-world applications. Without proper error handling, any exception will crash the app or lead to an undefined state, resulting in a poor user experience.
Correct Example:
class MyViewModel : ViewModel() {
private val _errorState = MutableLiveData<String>()
val errorState: LiveData<String> = _errorState
fun fetchData() {
viewModelScope.launch {
try {
val data = repository.getData()
_uiState.value = UIState.Success(data)
} catch (e: Exception) {
_errorState.value = "Failed to fetch data: ${e.message}"
}
}
}
}
Why It’s Better: This approach ensures that errors are caught and handled gracefully within the ViewModel. The user can be informed of the error through the UI, and the app can continue running without crashing. This leads to a more robust and user-friendly application.
8. Tight Coupling Between ViewModel and Repository:
Mistake:
Hardcoding dependencies within the ViewModel, making it difficult to test or swap out implementations.
Bad Example:
class MyViewModel : ViewModel() {
private val repository = MyRepository()
fun fetchData() {
val data = repository.getData()
_uiState.value = UIState.Success(data)
}
}
Why It’s Bad: Hardcoding the repository directly in the ViewModel creates a tight coupling between the two, making it difficult to mock the repository in tests or replace it with a different implementation. This reduces flexibility and makes the code harder to maintain.
Correct Example:
@HiltViewModel
class MyViewModel @Inject constructor(
private val repository: MyRepository
) : ViewModel() {
// ViewModel logic
}
Why It’s Better: By using dependency injection (e.g., with Hilt), the ViewModel is decoupled from the specific repository implementation. This makes the ViewModel easier to test, as you can inject a mock or fake repository during testing. It also enhances flexibility, allowing you to swap out the repository without modifying the ViewModel.
9. Unclear ViewModel Responsibilities:
Mistake:
Blurring the lines between what the ViewModel should handle versus what should be handled by the View, leading to confusion and harder maintenance.
Bad Example:
class MyViewModel : ViewModel() {
fun updateUI() {
// Formatting data for UI display directly in ViewModel
val formattedDate = SimpleDateFormat("yyyy-MM-dd", Locale.getDefault()).format(Date())
textView.text = formattedDate
}
}
Why It’s Bad: In this example, the ViewModel is performing UI-related tasks, such as formatting data and directly updating UI elements. This blurs the responsibilities between the ViewModel and the View, making the code harder to maintain and less flexible.
Correct Example:
class MyViewModel : ViewModel() {
fun getFormattedDate(): String {
val date = repository.getDate()
return SimpleDateFormat("yyyy-MM-dd", Locale.getDefault()).format(date)
}
}
Why It’s Better: This approach keeps the ViewModel focused on preparing data for the UI, without directly manipulating UI elements. The View can handle the display logic, ensuring a clear separation of concerns and making both the ViewModel and View easier to maintain.
10. Ignoring Lifecycle Awareness:
Mistake:
Failing to make the ViewModel lifecycle-aware, which can lead to memory leaks or unintended behavior.
Bad Example:
class MyViewModel : ViewModel() {
init {
// Starting a process without lifecycle awareness
startProcess()
}
fun startProcess() {
// Process that may lead to memory leaks
}
}
Why It’s Bad: Initiating processes in the ViewModel without considering its lifecycle can lead to memory leaks and other issues. For example, if the ViewModel is cleared (e.g., when the user navigates away), ongoing processes may continue, consuming resources and potentially causing crashes.
Correct Example:
class MyViewModel : ViewModel() {
init {
// Observe lifecycle-aware data sources or components
}
override fun onCleared() {
super.onCleared()
// Cleanup resources
}
}
Why It’s Better: By leveraging the onCleared() method, you ensure that resources are properly cleaned up when the ViewModel is no longer needed. This prevents memory leaks and ensures that your app remains performant and free from unintended behavior.
Conclusion:
Understanding and avoiding these common MVVM mistakes can significantly enhance the maintainability, testability, and overall quality of your Android applications. By following these best practices, you can ensure that your MVVM architecture is robust, scalable, and easier to manage in the long run.
Dobri Kostadinov
Android Consultant | Trainer
Email me | Follow me on LinkedIn | Follow me on Medium | Buy me a coffee
This article is previously published on proandroiddev.com
