What the actual meaning `Caused by: java.lang.RuntimeException: view must have a tag`?

Question

Please inform me if knowing whats tag that wanted.

Caused by: java.lang.RuntimeException: view must have a tag

__BaseActivity.java

    @Override
    public void setContentView(int layoutResID) {

        mBinding.contentParent.removeAllViews();
        DataBindingUtil.inflate(LayoutInflater.from(this), layoutResID, mBinding.contentParent, true);
        super.setContentView(mBinding.getRoot());
    }

__ChildActivity.java

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mBinding = DataBindingUtil.setContentView(this, R.layout.my_wallet);
}

ERROR logcat

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mydev}: java.lang.RuntimeException: view must have a tag
        at <more...>
     Caused by: java.lang.RuntimeException: view must have a tag
        at android.databinding.DataBinderMapperImpl.getDataBinder(DataBinderMapperImpl.java:121)

Answer 1

This usually happens when attempting to use DataBindingUtil.inflate() to inflate a layout that does not support data binding. In other words, the layout you're attempting to inflate does not have its root element as <layout>.

I have run into this problem when refactoring an Activity to use data binding, and the Activity has multiple layouts. I successfully refactored one of the layouts to include the <layout> element at its root, but I didn't refactor all the other layouts (layouts for other screen densities, languages, modules, etc.).

Check to make sure ALL the possible matching layouts are configured with <layout> as their root element.

See this developer doc Layouts and binding expressions

Answer 2

Another scenario where this error occurs is in a RecyclerView's ViewHolder.

Avoid initialising a binding instance in the ViewHolder's bind method

class BindingAdapter(private val items: List<Any>): RecyclerView.Adapter<BindingHolder>() {
      override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): BindingHolder {}

      override fun onBindViewHolder(holder: BindingHolder, position: Int) {
           holder.bindItem(items[position])
      }
 }

class BindingHolder(view: View): RecyclerView.ViewHolder(view) {
    fun bindItem(item: Any) {
        //Don't do this
        val binding = ItemSampleBinding.bind(itemView)
    }
}

The databinding instance should be initialised outside the bind method because ViewHolders could be recycled and in the code above we could be trying to create a binding instance from a view that's already bound.

Instead create the binding instance in initialisation block of the ViewHolder (this can be in init{} block or just after the class declaration as shown below)

class BindingHolder(view: View): RecyclerView.ViewHolder(view) {
    val binding = ItemSampleBinding.bind(view)

    fun bindItem(item: Any) {
        //Rest of ViewHolder logic
        //binding.textView.text = "Something nice"
    }
}

Answer 3

This happened to me due to the library's layout file (the one for which the error was flagged) having the same name as another one in the app module. Both used data binding.