Assisted ViewModel

Existing solution

When we work with ViewModel from the Architecture Components we spend many efforts to write factory for ViewModel when it has non default constructor. Fow now it's look like:

class SampleViewModel(private val repository: Repository): ViewModel {

    class Factory @Inject constructor(
        repository:Provider<Repository>
    ): ViewModelProvider.Factory {

        override fun <T : ViewModel> get(modelClass: Class<T>): T {
            if (modelClass == SampleViewModel::class.java) {
                return SampleViewModel(mRepository.get()) as T
            }
            throw IllegalArgumentException(
                "ViewModel of type ${modelClass.simpleName} isn't supported by the factory."
            )
        }
    }
}

class SampleFragment : Fragment {

    private lateinit var viewModel: SampleViewModel

    override fun onAttach(context:Context) {
        super.onAttach(context)
        // Inject dependencies
    }

    @Inject
    fun initViewModel(viewModelFactory: SampleViewModel.Factory) {
        mViewModel = 
            ViewModelProviders.of(this, viewModelFactory).get(SampleViewModel::class.java)
    }
}

Although this approach is robust, it requires writing a large number of boilerplate code.

Usage of ViewModelInject

In order to use ViewModel Inject library in the code a few simple steps should be done:

Add gradle dependency, also you can use kapt instead of annotationProcessor:

dependencies {
    implementation 'com.epam.inject.viewmodel:annotation:1.0.0-alpha02'
    annotationProcessor 'com.epam.inject.viewmodel:processor:1.0.0-alpha02'
}

Mark corresponding constructors in the ViewModel implementation with @AssistedViewModel annotation:

class SampleViewModel @AssistedViewModel constructor(repository:Repository): ViewModel

Perform the build to generate corresponding dagger module and ViewModel factory.
Add generated module to the root (app) module:

@Module(include = [ViewModelInjectModule::class])
class AppModule

Inject a `AssistedViewModelFactory' implementation in the usual way:

class SampleFragment: Fragment {

    private lateinit var viewModel: SampleViewModel

    override fun onAttach(context: Context) {
        super.onAttach(context)
        // Inject dependencies
    }

    @Inject
    fun initViewModel(viewModelFactory: AssistedViewModelFactory) {
        mViewModel = 
            ViewModelProviders.of(this, viewModelFactory).get(SampleViewModel::class.java)
    }
}

Current limitations

For the proper work minimum dagger version should be 2.11.
Constructor's parameters that aren't in DI graph aren't supported.
ViewModel from non application (root) scope aren't supported.
For the kotlin primary constructor which has default values for all parameters @AssistedViewModel annotation can't be used.
In case of kapt usage generated module should be specified in the dagger's [Module] annotation with full name, e.g.

@Module(include = [com.epam.inject.viewmodel.ViewModelInjectModule::class])

ViewmodelInjectDagger

General

Additional

Blurb

Assisted ViewModel

Existing solution

Usage of ViewModelInject

Current limitations