first off all sorry for my bad english. Im trying to recieve Data from my self-written python Backend(REST-API) in my Android APP.

ApiService.kt:

    private const val Base_URL = "http://192.168.178.93:5000/api/"
private val moshi = Moshi.Builder()
    .add(KotlinJsonAdapterFactory())
    .build()

private val retrofit = Retrofit.Builder()
    .addConverterFactory(MoshiConverterFactory.create(moshi))
    .baseUrl(Base_URL)
    .build()

interface TodoApiService{
    @GET("todo")
    suspend fun getToDo(): List<ToDo>
}

object ToDoApi{
    val retrofitService : TodoApiService by lazy {
        retrofit.create(TodoApiService::class.java)
    }
}

MainActivityViewModel.kt:

class MainActivityViewModel : ViewModel() {
    of the most recent request
    private val _status = MutableLiveData<String>()


val status: LiveData<String> = _status

private val _toDo = MutableLiveData<List<ToDo>>()

val toDo: LiveData<List<ToDo>> = _toDo

init {
    getToDo()
}

private fun getToDo() {
    viewModelScope.launch{
        try {
            _toDo.value = ToDoApi.retrofitService.getToDo()
            _status.value = "Success $_toDo"
        }catch (e: Exception){
            _status.value = "Failure ${e.message}"
        }
    }

}
}

activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>

<FrameLayout 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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    >


    <androidx.recyclerview.widget.RecyclerView

        android:scrollbars="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@ id/recycler_view"
        tools:listitem="@layout/todo_item"/>
</FrameLayout>

todo_item.xml:

<?xml version="1.0" encoding="utf-8"?>
<androidx.cardview.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_marginEnd="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginTop="8dp">

    <RelativeLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:padding="8dp">
        <TextView
            android:id="@ id/text_view_name"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="1"/>

    </RelativeLayout>

</androidx.cardview.widget.CardView>

TodoAdapter.kt

class TodoAdapter(private val context: Context, private val Items: List<ToDo>):RecyclerView.   Adapter<TodoAdapter.TodoViewHolder>(){

    class TodoViewHolder(private val view: View) : RecyclerView.ViewHolder(view){
        val textView: TextView = view.findViewById(R.id.text_view_name)

    }

    override fun getItemCount() = Items.size

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): TodoViewHolder {
        val adapterLayout = LayoutInflater.from(parent.context)
            .inflate(R.layout.todo_item, parent, false)
        return TodoViewHolder(adapterLayout)
    }

    override fun onBindViewHolder(holder: TodoViewHolder, position: Int) {
        val ToDo = Items.get(position)
        holder.textView.text =  context.resources.getString(ToDo.Id.toInt())

    }
}

MainActivity.kt:

class MainActivity : AppCompatActivity() {


    private val viewModel = MainActivityViewModel()
    private lateinit var binding: ActivityMainBinding
    private lateinit var linearLayoutManager: LinearLayoutManager

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)
        linearLayoutManager = LinearLayoutManager(this)
        binding.recyclerView.layoutManager = linearLayoutManager
        binding.recyclerView.adapter = TodoAdapter(this, viewModel.toDo.value)

    }
}

I know that i need an adapter to connect my LiveData to my recyclerView. But im not able to implement it right. Android studio tells, i cant use my MutableLiveData<List> for my Adapter that just only need a normal List(Required: List Found:List?. I cant Cast cause the data could be null. Im gratefully for Hints.

CodePudding user response：

This is because you are sending a possible nullable list from the activity to the Adapter. You must do something like this:

binding.recyclerView.adapter = TodoAdapter(this, viewModel.toDo?.value ?: listOf())

CodePudding user response：

Your use of LiveData is incorrect. You use it as a simple variable, passing its current value (which is null) to the adapter. LiveData is intended for data stream and need to be observed.

I like to add a setter for the data on the Adapter class:

fun setData(data: List<ToDo>) {
    Items = data
    notifyDataSetChanged()
}

And in the Activity, observe the ViewModel's live data and update the adapter when new data arrives:

class MainActivity : AppCompatActivity() {

    private lateinit var adapter: TodoAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        ...
        adapter = TodoAdapter(this)
        viewModel.todo.observe(this, { todos ->
            adapter.setData(todos)
        } 
    }

    ...
}

Now when you set value to your LiveData in the ViewModel, the adapter will be notified.