android – Navigation Architecture Component – Action Navigation with lambda expression-ThrowExceptions

Exception or error:

I have added a new action Navigation.xml:

 <fragment
    android:id="@+id/launcher_home"
    android:name="com.example.android.codelabs.navigation.MainFragment"
    android:label="@string/home"
    tools:layout="@layout/main_fragment">
    <action
        android:id="@+id/go_to_step_one"
        app:destination="@id/flow_step_one" />
</fragment>

When calling the navigation action, if i use, the navigation works correctly :

 view.findViewById<Button>(R.id.navigate_action_bt)?.setOnClickListener(
            Navigation.createNavigateOnClickListener(R.id.go_to_step_one, null)
    )

But when calling it with a lambda, it does not work :

 view.findViewById<Button>(R.id.navigate_action_bt)?.setOnClickListener {
        Navigation.createNavigateOnClickListener(R.id.go_to_step_one, null)
    }
How to solve:

Well, that’s the proper way of working. The method Navigation.createNavigateOnClickListener() returns a View.OnClickListener and according to the docs the proper way of assiging it is by using, even in Kotlin:

button.setOnClickListener(Navigation.createNavigateOnClickListener(R.id.next_fragment, null));

If you using the method inside the lambda, the lambda itself is the click listener so you’re returning a click listener inside the click listener, that’s why it isn’t working. It’s like doing this:

button.setOnClickListener {
            View.OnClickListener {
                Toast.makeText(this, "hi", Toast.LENGTH_LONG).show()
            }
        }

What you might be intrested in is this:

 view.findViewById<Button>(R.id.navigate_action_bt)?.setOnClickListener { view ->
   view.findNavController().navigate(R.id.go_to_step_one)
}

Which will perform the navigation transition on button click.

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