ViewModel constructor should be annotated with @Inject instead of @AssistedInject

ViewModel Constructor Annotations: @Inject vs. @AssistedInject

In Android development using Dagger 2, there’s a common dilemma when injecting dependencies into ViewModels: should you use @Inject or @AssistedInject? This article delves into the differences between these annotations and why @Inject is generally the preferred choice for ViewModel constructors.

Understanding the Differences

@Inject: Standard Dependency Injection

• Used for injecting dependencies that are provided by Dagger’s graph.
• Provides direct dependency injection without the need for additional parameters.
• Suitable for dependencies that are always available and don’t require customization during ViewModel creation.

@AssistedInject: Customizing ViewModel Creation

• Used for injecting dependencies that need to be provided at ViewModel instantiation time.
• Requires an additional @Assisted parameter in the constructor to receive custom values.
• Suitable for scenarios where ViewModels require unique configurations based on specific parameters.

Why @Inject is Preferred for ViewModels

While @AssistedInject provides flexibility, using @Inject for ViewModel constructors offers several benefits:

Simplicity and Maintainability

• @Inject keeps the ViewModel constructor clean and focused on dependency injection.
• It eliminates the need for an additional @Assisted parameter, reducing complexity.

Improved Testability

• ViewModels annotated with @Inject can be easily tested using Dagger’s test module.
• Direct dependency injection simplifies mock object creation and integration.

Enhanced Reusability

• ViewModels using @Inject are more reusable, as they don’t rely on custom parameters during creation.
• They can be easily shared across different activities or fragments without requiring modification.

Example: Using @Inject for ViewModel Construction

@HiltViewModel
class MyViewModel @Inject constructor(
    private val repository: MyRepository
) : ViewModel() {
    // ViewModel logic
}

Example: Using @AssistedInject for ViewModel Construction

@HiltViewModel
class MyViewModel @AssistedInject constructor(
    @Assisted private val customParameter: String,
    private val repository: MyRepository
) : ViewModel() {
    // ViewModel logic
}

In this example, MyViewModel requires a custom parameter customParameter. Therefore, we need to use @AssistedInject to receive this parameter at instantiation time. For simpler cases, where no custom parameter is needed, using @Inject with a direct dependency injection from Dagger’s graph is the preferred approach.

Conclusion

For ViewModel construction, @Inject is generally the preferred annotation over @AssistedInject. @Inject promotes simplicity, testability, and reusability while fulfilling the dependency injection needs of ViewModels. Use @AssistedInject only when custom parameters are required during ViewModel creation, ensuring proper implementation and adhering to best practices for maintainability.