✅Day 11 – ViewModels in Android – Everything You Need to Know - HandsOnAndroid — Build Better Android Apps, One Project at a Time

📘 Introduction

Welcome to Day 11 of the HandsOnAndroid 90-Day Android Journey! After introducing MVVM Architecture on Day 10, it’s time to explore the ViewModel—a core component of modern Android development.

The ViewModel allows your app to handle UI-related data in a lifecycle-conscious manner, making your apps more robust and responsive to configuration changes like screen rotations.

🧩 What is a ViewModel?

The ViewModel is part of the Jetpack Architecture Components introduced to help developers separate concerns and write cleaner, testable, and scalable code.

Simply put, a ViewModel stores and manages UI-related data that needs to survive configuration changes.

It acts as a communication layer between the UI (Activity/Fragment) and the data layer (Repository, Use Cases, etc.).

💡 Benefits of ViewModel

✅ Survives configuration changes like rotation
✅ Separates UI logic from business logic
✅ Works seamlessly with LiveData, StateFlow, or MutableState
✅ Helps in unit testing your business logic without involving Android components

🧑‍💻 How to Use ViewModel

Let’s look at a simple ViewModel setup in Kotlin:

class UserViewModel : ViewModel() {
    private val _username = MutableLiveData<String>()
    val username: LiveData<String> = _username

    fun updateName(newName: String) {
        _username.value = newName
    }
}

In your Fragment or Activity:

val viewModel: UserViewModel by viewModels()

viewModel.username.observe(viewLifecycleOwner) { name ->
    textView.text = name
}

🔗 Internal Learning Continuity

Here’s how this fits into your ongoing learning:

🧠 Best Practices for ViewModel

Practice	Why It Matters
✅ Use `ViewModelProvider`	Ensures lifecycle-aware creation
🚫 Don’t store context	Leads to memory leaks
✅ Use `SavedStateHandle`	For restoring state after process death
✅ Share ViewModel across Fragments	Useful in navigation or tabbed UIs

🛠 When to Use Shared ViewModel?

A shared ViewModel is useful when multiple fragments under the same activity need to share or react to the same data:

val sharedViewModel: UserViewModel by activityViewModels()

Perfect for bottom navigation, master-detail layouts, or wizard-style screens.

🔍 ViewModel + StateFlow (Modern Approach)

For more modern reactive UIs, use StateFlow or MutableStateFlow inside ViewModel:

class UserViewModel : ViewModel() {
    private val _userName = MutableStateFlow("Guest")
    val userName: StateFlow<String> = _userName
}

Jetpack Compose integrates seamlessly with this approach using collectAsState().

Here’s a diagram that visually explains the ViewModel role in Android’s MVVM architecture:

Key Concepts:

UI Layer: Displays data, observes ViewModel state.
ViewModel: Acts as the bridge between UI and data, holds logic & data state.
Repository: Handles data operations, abstracts sources.
Data Sources: Can be remote (API) or local (Room DB, SharedPreferences).

🎯 Summary

The ViewModel class is essential in MVVM-based Android apps. It separates UI logic, preserves state, and makes your app resilient and testable. Whether you’re using LiveData, StateFlow, or Jetpack Compose, ViewModel is a must-know tool.

👉 Up next: We’ll integrate LiveData and observe changes in the UI in Day 12: LiveData and StateFlow in Android.