我正在使用底部导航和导航架构组件。当用户通过底部导航从一个项目导航到另一个项目，然后再返回时，视图模型将调用存储库函数重新获取数据。因此，如果用户来回切换10次，则会获取相同的数据10次。如何在片段重新创建数据已经存在的情况下避免重新获取数据？。
片段
class HomeFragment : Fragment() {
@Inject
lateinit var viewModelFactory: ViewModelProvider.Factory
private lateinit var productsViewModel: ProductsViewModel
private lateinit var productsAdapter: ProductsAdapter
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?): View? {
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_home, container, false)
}
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
initViewModel()
initAdapters()
initLayouts()
getData()
}
private fun initViewModel() {
(activity!!.application as App).component.inject(this)
productsViewModel = activity?.run {
ViewModelProviders.of(this, viewModelFactory).get(ProductsViewModel::class.java)
}!!
}
private fun initAdapters() {
productsAdapter = ProductsAdapter(this.context!!, From.HOME_FRAGMENT)
}
private fun initLayouts() {
productsRecyclerView.layoutManager = LinearLayoutManager(this.activity)
productsRecyclerView.adapter = productsAdapter
}
private fun getData() {
val productsFilters = ProductsFilters.builder().sortBy(SortProductsBy.NEWEST).build()
//Products filters
productsViewModel.setInput(productsFilters, 2)
//Observing products data
productsViewModel.products.observe(viewLifecycleOwner, Observer {
it.products()?.let { products -> productsAdapter.setData(products) }
})
//Observing loading
productsViewModel.networkState.observe(viewLifecycleOwner, Observer {
//Todo showing progress bar
})
}
}
ViewModel
class ProductsViewModel
@Inject constructor(private val repository: ProductsRepository) : ViewModel() {
private val _input = MutableLiveData<PInput>()
fun setInput(filters: ProductsFilters, limit: Int) {
_input.value = PInput(filters, limit)
}
private val getProducts = map(_input) {
repository.getProducts(it.filters, it.limit)
}
val products = switchMap(getProducts) { it.data }
val networkState = switchMap(getProducts) { it.networkState }
}
data class PInput(val filters: ProductsFilters, val limit: Int)
仓库
@Singleton
class ProductsRepository @Inject constructor(private val api: ApolloClient) {
val networkState = MutableLiveData<NetworkState>()
fun getProducts(filters: ProductsFilters, limit: Int): ApiResponse<ProductsQuery.Data> {
val products = MutableLiveData<ProductsQuery.Data>()
networkState.postValue(NetworkState.LOADING)
val request = api.query(ProductsQuery
.builder()
.filters(filters)
.limit(limit)
.build())
request.enqueue(object : ApolloCall.Callback<ProductsQuery.Data>() {
override fun onFailure(e: ApolloException) {
networkState.postValue(NetworkState.error(e.localizedMessage))
}
override fun onResponse(response: Response<ProductsQuery.Data>) = when {
response.hasErrors() -> networkState.postValue(NetworkState.error(response.errors()[0].message()))
else -> {
networkState.postValue(NetworkState.LOADED)
products.postValue(response.data())
}
}
})
return ApiResponse(data = products, networkState = networkState)
}
}
导航主文件 main.xml
<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/mobile_navigation.xml"
app:startDestination="@id/home">
<fragment
android:id="@+id/home"
android:name="com.nux.ui.home.HomeFragment"
android:label="@string/title_home"
tools:layout="@layout/fragment_home"/>
<fragment
android:id="@+id/search"
android:name="com.nux.ui.search.SearchFragment"
android:label="@string/title_search"
tools:layout="@layout/fragment_search" />
<fragment
android:id="@+id/my_profile"
android:name="com.nux.ui.user.MyProfileFragment"
android:label="@string/title_profile"
tools:layout="@layout/fragment_profile" />
</navigation>
ViewModelFactory
@Singleton
class ViewModelFactory @Inject
constructor(private val viewModels: MutableMap<Class<out ViewModel>, Provider<ViewModel>>) : ViewModelProvider.Factory {
@Suppress("UNCHECKED_CAST")
override fun <T : ViewModel> create(modelClass: Class<T>): T {
val creator = viewModels[modelClass]
?: viewModels.asIterable().firstOrNull { modelClass.isAssignableFrom(it.key) }?.value
?: throw IllegalArgumentException("unknown model class $modelClass")
return try {
creator.get() as T
} catch (e: Exception) {
throw RuntimeException(e)
}
}
}
ViewModelProvider.Factory是如何创建实例的吗？ - Jeel Vankhederepository.getProducts(it.filters, it.limit)每次我看到这个 fragment 的时候都会被调用。这是我第一次使用断点，所以我不知道我是否使用得正确，但我可以确定它被调用了，因为当应用程序启动时，线程会被挂起，直到我按下播放按钮（这个 fragment 是主页）。当我导航到另一个 fragment 并返回时，线程再次被挂起。我希望我做得对！那么这意味着什么？提前致谢。 - Nux