Shared ViewModel lifecycle for Android JetPack

Shared ViewModel Lifecycle for Android JetPack

In Android Jetpack, ViewModels are designed to hold and manage UI-related data. However, there are situations where multiple UI components (Activities, Fragments, or Compose screens) need to share the same data and state. This is where the concept of Shared ViewModel comes into play.

Understanding Shared ViewModel

What is a Shared ViewModel?

A Shared ViewModel is a ViewModel instance that can be accessed and shared across multiple UI components within the same scope. It allows you to maintain a single source of truth for data and state, even if multiple UI elements are displaying and interacting with it.

Benefits of Shared ViewModels:

• Data Synchronization: Ensures consistent data across multiple screens.
• State Management: Simplifies state management for shared data.
• Improved Communication: Provides a clean way for UI components to communicate.
• Reduced Boilerplate Code: Eliminates the need for complex data passing mechanisms.

Sharing ViewModel: Key Concepts

ViewModelFactory

The ViewModelFactory plays a crucial role in creating and providing shared ViewModels. You can customize it to inject dependencies or share specific ViewModel instances.

ViewModelStoreOwner

To share a ViewModel, you need a common ViewModelStoreOwner. This can be an Activity, Fragment, or a shared component like a Navigation Graph.

ViewModelProvider

The ViewModelProvider is used to retrieve ViewModel instances, including shared ones. You pass the ViewModelStoreOwner and the ViewModel class to obtain the desired instance.

Implementation Example

Scenario: Shopping Cart

Imagine an e-commerce app where users can add items to their shopping cart. We need to maintain the cart data across different screens (product list, cart details, checkout).

Code Example:

// SharedViewModel for the shopping cart
class CartViewModel : ViewModel() {
  private val _cartItems = MutableLiveData>()
  val cartItems: LiveData> = _cartItems

  fun addItem(item: CartItem) {
    _cartItems.value = (_cartItems.value ?: emptyList()) + item
  }

  fun removeItem(item: CartItem) {
    _cartItems.value = (_cartItems.value ?: emptyList()).filter { it != item }
  }
}

// Activity or Fragment where the Shared ViewModel is used
class ProductListActivity : AppCompatActivity() {
  private lateinit var cartViewModel: CartViewModel

  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    // Create an instance of SharedViewModel
    cartViewModel = ViewModelProvider(this)[CartViewModel::class.java] 
    // Access and use the cartItems LiveData
    cartViewModel.cartItems.observe(this) { items ->
      // Update UI based on cart items
    }
  }
}

Lifecycle Management

Shared ViewModel Lifecycle:

Shared ViewModels have the following lifecycle characteristics:

• Associated with the ViewModelStoreOwner: Their lifecycle is tied to the component (Activity, Fragment) that owns the ViewModelStore.
• Survive Configuration Changes: Shared ViewModels persist across configuration changes like screen rotation.
• Cleared when the ViewModelStoreOwner is Destroyed: When the owning component is destroyed, the shared ViewModel is also cleared.

Comparison: Shared vs Non-Shared ViewModels

Best Practices

• Define a Clear Scope: Carefully choose the ViewModelStoreOwner based on your requirements.
• Use LiveData for Observability: Use LiveData to observe changes in shared data and update UI accordingly.
• Avoid Complex Logic: Keep the Shared ViewModel focused on data and state management, not business logic.
• Consider Alternative Solutions: For complex interactions, explore solutions like Jetpack Compose’s remember() function or other state management libraries.

Conclusion

Shared ViewModels are a powerful feature in Android Jetpack that simplifies data sharing and state management for multiple UI components. By understanding their lifecycle, implementation, and best practices, you can leverage Shared ViewModels to create more robust and efficient Android applications.