android - 使用 Dagger 2 将属性注入(inject) ViewModel

Question

我尝试学习如何使用 Dagger 2。请帮助解决以下异常:

Exception: UninitializedPropertyAccessException: lateinit property trips has not been initialized

主 Activity View 模型:

class MainActivityViewModel : ViewModel() {
    private lateinit var tripsLiveData: MutableLiveData<List<Trip>>

    @Inject
    lateinit var trips : List<Trip>

    fun getTrips() : LiveData<List<Trip>> {
        if (!::tripsLiveData.isInitialized){
            tripsLiveData = MutableLiveData()
            tripsLiveData.value = trips
        }
        return tripsLiveData
    }
}

旅行模块:

@Module
class TripModule{
    @Provides
    fun provideTrips(): List<Trip> {

        var list = ArrayList<Trip>()
        list.add(Trip(100,10))
        list.add(Trip(200,20))
        return list
    }
}

应用组件:

@Singleton
@Component(modules = [
    AndroidSupportInjectionModule::class,
    ActivityBuilder::class,
    TripModule::class])
interface AppComponent{
    @Component.Builder
    interface Builder {
        @BindsInstance
        fun application(application: Application): Builder

        fun build(): AppComponent
    }

    fun inject(app: MyApplication)
}

主要 Activity :

class MainActivity : AppCompatActivity() {

    @Inject
    lateinit var tripsAdapter: TripsAdapter

    override fun onCreate(savedInstanceState: Bundle?) {

        // Inject external dependencies
        AndroidInjection.inject(this)

        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        setupRecyclerView();
        setUpViewModel();
    }

    private fun setupRecyclerView() {
        recycler_view.apply {
            layoutManager = LinearLayoutManager(context)
            adapter = tripsAdapter
        }
    }

    private fun setUpViewModel(){
        val model = ViewModelProviders.of(this).get(MainActivityViewModel::class.java)
        model.getTrips().observe(this, Observer { tripsAdapter.trips = it!! })
    }
}

Answer 1

如果您希望您的 View 模型成为 Dagger 图的一部分，您需要做几件事 - 使用 Dagger 的多重绑定(bind)(只需一次，对于较新的 View 模型，它会更容易)。您将创建新的 View 模型工厂，它将负责实例化 View 模型。该工厂将成为 Dagger 图的一部分，因此将引用通过 Dagger 提供的任何内容。然后，您可以通过 @Inject constructor(anyParameterFromDagger: Param) 或 @Inject lateinit var someParam: Param 在 vi​​ewmodel 体内注入(inject)构造函数。

1) 为 View 模型类创建限定符

@MustBeDocumented
@Target(AnnotationTarget.FUNCTION)
@Retention(AnnotationRetention.RUNTIME)
@MapKey
annotation class ViewModelKey(val value: KClass<out ViewModel>)

2) 创建从 Dagger 的多重绑定(bind)中获取值的 View 模型工厂

@Singleton
class DaggerViewModelFactory @Inject constructor(
    private val creators: Map<Class<out ViewModel>, @JvmSuppressWildcards Provider<ViewModel>>
) : ViewModelProvider.Factory {

    @Suppress("UNCHECKED_CAST")
    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        var creator: Provider<out ViewModel>? = creators[modelClass]
        if (creator == null) {
            for ((key, value) in creators) {
                if (modelClass.isAssignableFrom(key)) {
                    creator = value
                    break
                }
            }
        }
        if (creator == null) {
            throw IllegalArgumentException("unknown model class $modelClass")
        }
        try {
            return creator.get() as T
        } catch (e: Exception) {
            throw RuntimeException(e)
        }
    }
}

3) 有 dagger 模块，它将提供工厂(从第 2 点开始)，然后是你的 View 模型

abstract class YourDaggerModuleWhichThenNeedToBePartOfYourGraphAsIncluded {

    @Binds
    abstract fun bindViewModelFactory(factory: DaggerViewModelFactory): ViewModelProvider.Factory // this needs to be only one for whole app (therefore marked as `@Singleton`)

    @Binds
    @IntoMap
    @ViewModelKey(MainActivityViewModel::class)
    abstract fun bindMainActivityViewModel(vm: MainActivityViewModel): ViewModel // for every viewmodel you have in your app, you need to bind them to dagger
}

4) 在您的 Activity 中，当您获得 View 模型时，您需要使用 dagger 中的工厂:(在下面的代码中标记为 //TODO 的地方已更改)

class MainActivity : AppCompatActivity() {

    @Inject
    lateinit var tripsAdapter: TripsAdapter

    @Inject
    lateinit var viewModelFactory: ViewModelProvider.Factory // TODO this was added to the activity

    override fun onCreate(savedInstanceState: Bundle?) {

        // Inject external dependencies
        AndroidInjection.inject(this)

        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        setupRecyclerView();
        setUpViewModel();
    }

    private fun setupRecyclerView() {
        recycler_view.apply {
            layoutManager = LinearLayoutManager(context)
            adapter = tripsAdapter
        }
    }

    private fun setUpViewModel(){
        val model = ViewModelProviders.of(this, viewModelFactory)[MainActivityViewModel::class.java] // TODO this was changed

        model.getTrips().observe(this, Observer { tripsAdapter.trips = it!! })
    }
}

我没有提供将模块包含到 Dagger 组件的代码，因为我希望这是您已经做过的事情。

您可以阅读更多相关信息，例如在这个medium article (我不是文章的作者):