How to make databinding with MutableLiveData<Boolean> and button with android:enabled?

Question

I have a registration form where I have a big condition to continue. For simplicity, I have an editText and a Button. I write with MVVM so I have a ViewModel() where
val stroke = MutableLiveData<String>() is stored. So when stroke length is more than 5 the button has to be activated. I do it like this: Activity 'class MainActivity : AppCompatActivity() {

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)

    val binding = DataBindingUtil.setContentView<ActivityMainBinding>(this, R.layout.activity_main)

    val viewModel = ViewModelProviders.of(this).get(MainViewModel::class.java)
    binding.setLifecycleOwner(this)


    binding.model = viewModel

}}

View Model

class MainViewModel: ViewModel(){
val stroke = MutableLiveData<String>()

val enableButton = MutableLiveData<Boolean>()
    get(){
        field.value = (stroke.value ?: "").length > 5
        return field
    }}

xml

<?xml version="1.0" encoding="utf-8"?>
<layout 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">
<data>
    <variable
        name="model"
        type="nekono.inno.mvvmsample.MainViewModel"/>
</data>
<android.support.constraint.ConstraintLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <EditText
    android:id="@+id/editText"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginEnd="8dp"
    android:layout_marginLeft="8dp"
    android:layout_marginRight="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginTop="8dp"
    android:ems="10"
    android:inputType="textPersonName"
    android:text="@={model.stroke}"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

    <Button
    android:id="@+id/button"
    android:layout_width="wrap_content"
    android:layout_height="49dp"
    android:layout_marginEnd="8dp"
    android:layout_marginLeft="8dp"
    android:layout_marginRight="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginTop="8dp"
    android:enabled="@{model.enableButton}"
    android:text="Button"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/editText" />


</android.support.constraint.ConstraintLayout>
</layout>

It seems like enabled only asks variable ones, at the moment of binding, after that it stops asking and holds the state that was at the moment of binding. How to fix this?

Answer 1

You could use a Transformations on stroke to achieve that.

val stroke = MutableLiveData<String>()
val enableButton: LiveData<Boolean> = Transformations.map(stroke) {  it.length > 5 }

When stroke changes also the enableButton value will change