NullPointerException on generated binding class for dynamic feature modules when minSdkVersion is >= 26

Question

I am trying to update my MinSdkVersion from 25 to 26, but I receive NullPointerExceptions when accessing properties of the generated binding classes.

The following snippet shows which line of code causes a NPE:

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

        val binding: ActivityDynamicBinding = DataBindingUtil.setContentView(
            this, R.layout.activity_dynamic)

        // Access a random binding to do a random thing
        binding.observableFieldsActivityButton.setOnClickListener { // This is the line it will crash
            Log.i("Tag", "I did a random thing, it works")
        }
        binding.viewmodelActivityButton.setOnClickListener {
            Log.i("Tag", "I did another random thing, it works")
        }
    }
}

The binding.observableFieldsActivityButton is null.

This is the XML file:

<?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/>

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

                <Button
                    android:id="@+id/observable_fields_activity_button"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:layout_marginTop="8dp"
                    android:text="@string/observable_fields_activity_text"
                    app:layout_constraintEnd_toEndOf="parent"
                    app:layout_constraintHorizontal_bias="0.5"
                    app:layout_constraintStart_toStartOf="parent"
                    app:layout_constraintTop_toBottomOf="@+id/observableactivity_activity_label"/>

<!-- The rest -->

And this is the stacktrace:

FATAL EXCEPTION: main
    Process: com.example.android.databinding.twowaysample, PID: 27268
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android.databinding.twowaysample/com.example.dynamicfeature.DynamicActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.example.dynamicfeature.DynamicActivity.onCreate(DynamicActivity.kt:17)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)

This only seems to happen when using databinding in a dynamic feature (this DynamicActivity is the activity in my dynamic feature module)

I have already debugged what happens in the databinding library internally, and I have also found the cause of these issues.

In the class ViewDataBinding.java on line 1020 it checks if the generated view id is > 0 (http://androidxref.com/9.0.0_r3/xref/frameworks/data-binding/extensions/library/src/main/java/android/databinding/ViewDataBinding.java#1020)

When upgrading the minSdkVersion to 26 or above, the generated view ids for dynamic feature modules are negative, whereas these are positive when setting minSdkVersion to 25 or lower.

This causes the DataBinding to skip these views, causing them to be null later on.

Does someone have a solution to this? I am thinking of something to force the build process to only generate ids with positive integers.

It appears this bug is already created on the issue tracker of google: https://issuetracker.google.com/issues/123304430

Answer 1

As mentioned in the question this is a known bug. Workarounds I have found are:

1. using findViewById on the root instead of the direct access to the field.

    val view = binding.root.findViewById<View>(R.id.my_view)
    // val view = binding.MyView // Original (now broken) code

2. Binding a dummy value in xml. Not sure if this a good or a terrible idea but it seems to me that the bug only appears when you don't actually bind any data to your view in your xml but access it in your code instead. You can trick databinding to create the implementation differently by binding a dummy value e.g. by adding this to the view:

    android:tag="@{null}"

#2 relies on the databinding tooling implementation and might break in future. However, it is also easier to clean up afterwards (search / replace).