Android : Kotlin : MVVM : Why viewModel.onButtonClicked() causes the app crash?

Question

Logcat Message :

java.lang.IllegalStateException: Could not find method @={() -> viewModel.onBtnClicked()(View) in a parent or ancestor Context for android:onClick attribute defined on view class androidx.appcompat.widget.AppCompatButton with id 'button'

File1 : activity_main.xml

<data>
    <variable
        name="viewModel"
        type="com.wingsquare.databindingdemo.MainViewModel" />
</data>

<androidx.constraintlayout.widget.ConstraintLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:onClick = "@={() -> viewModel.onBtnClicked()"
        />
</androidx.constraintlayout.widget.ConstraintLayout>

File 2 : MainActivity.kt

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

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

    // val binding = ActivityMainBinding.inflate(layoutInflater)
    val mainViewModel = ViewModelProvider(this).get(MainViewModel::class.java)
    binding.viewModel = mainViewModel
    binding.lifecycleOwner = this
}

}

File 3 : MainViewModel.kt

class MainViewModel : ViewModel() {

fun onBtnClicked() {
    Log.d("MainViewModel", "MainViewModel")
}

}

Answer 1

The Logcat Message is some how misleading. I was facing the same error for couple of hours checking everything else I thought it be the root cause. But for this particular error, all you have to do is to keep an eye on "{ }". This is one of the drawbacks of data binding in android. lots of the times you don't get any error on compile times, And if you do! that's not really helpful !

and another thing to consider is that '=' in

android:onClick = "@={() -> viewModel.onBtnClicked()}"

is used for a two way binding. you don't need it in this case. you can read about it here on android official docs

Answer 2

android:onClick="@{() -> viewModel.onBtnClicked()}"

• Add a parenthesis at the end.
• also you may want to remove the equal sign after the @