Why does my data-binding with EditText not work?

Question

I have this class which I use for Room:

data class Piece(
    @ColumnInfo(name = "title")
    var title: String
)

Then I have a form class, which simply holds string values which I want to be shown in EditText .

class CreateEditPieceForm {
    var title: String = ""
}

My ViewModel holds instances of these classes:

class EditPieceViewModel(...) : AndroidViewModel(application) {
    val piece : LiveData<Piece?> = database.getMyPiece() // valid Piece with title set
    val form = CreateEditPieceForm()
}

In my fragment I observe the piece:

viewModel.piece.observe(this, Observer {piece ->
   piece?.let {
       viewModel.updateInputValues(piece)
    }
})

updateInputValues function in the ViewModel simply sets values in the form:

   fun updateInputValues(piece: Piece) {
        Log.d("mylog", "value: " + piece.title) // logs correct value
        form.title = piece.title // setting this does not change EditText
   }

And finally, in my layout, I try to use data binding to show the text from form.title in EditText:

<data>

  <variable
            name="viewModel"
            type="com.example.tutorial.createedit.CreateEditPieceViewModel" />

</data>

<!-- ... -->

  <EditText
            android:id="@+id/input_title"
            <!-- ... -->
            android:inputType="text"

            android:text="@={viewModel.form.title}" />

When I open the Screen with this fragment, EditText is empty. I know that the query for piece title is correct, because I log it before I set EditText's text attribute.

When I type something in the empty field, value of viewModel.form.title is being set with that value.

Why does it not set right at the beginning?

Answer 1

Databinding is not to be confused with View binding .

Like in the guide, make EditPieceViewModel implement Observable and make form.title @Bindable.

Answer 2

Now it works. I still do not fully understand the Databinding library, so any advice to improve the code is welcomed!

I changed my ViewModel to implement Observable and added some methods to it:

// Make sure to import the correct Observable interface
import androidx.databinding.Observable
// ...

class EditPieceViewModel(...) : AndroidViewModel(application), Observable {

   // ...

   // New getter and setter methods for my title field:

    @Bindable
    fun getTitle() : String {
        return form.title
    }

    fun setTitle(value: String) {
        if(form.title != value) {
            form.title = value
            notifyPropertyChanged(BR.title) // This line is important for EditText' value to update
        }
    }


   // New methods added for Observable:

    override fun addOnPropertyChangedCallback(
        callback: Observable.OnPropertyChangedCallback) {
        callbacks.add(callback)
    }

    override fun removeOnPropertyChangedCallback(
        callback: Observable.OnPropertyChangedCallback) {
        callbacks.remove(callback)
    }

    /**
     * Notifies observers that a specific property has changed. The getter for the
     * property that changes should be marked with the @Bindable annotation to
     * generate a field in the BR class to be used as the fieldId parameter.
     *
     * @param fieldId The generated BR id for the Bindable field.
     */
    fun notifyPropertyChanged(fieldId: Int) {
        callbacks.notifyCallbacks(this, fieldId, null)
    }

}

In layout xml I changed title to observe viewModel.title instead of viewModel.form.title:

        <EditText
            // ...
            android:text="@={viewModel.title}" />

Finally in updateInputValues I call my custom setter:

fun updateInputValues(piece: Piece) {
        setTitle(piece.title)
}