BindingAdapters for separation of viewmodels and Android framework for resource loading

_BindingAdapters for resources.md

Creating binding adapters that take resource ids allow view models to ignore the loading of resources like strings, colors, and drawables

• View models are then free to deal in resource ids which are independent of the Android framework and are easy to test
• Define globally applicable binding adapters at the root level in BindingAdapters.kt
• Define feature/package specific adapters within the package as _BindingAdapters.kt
  • the prefix allows that file to appear at the top of the package contents list which makes it easier to find, and remember

activity_main.xml

<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>

        <variable
            name="viewModel"
            type="com.n8ebel.databindingsandbox.main.MainViewModel" />

    </data>

        <FrameLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:backgroundColor="@{viewModel.itemBackgroundId}"
            >

            <LinearLayout
                android:orientation="vertical"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_gravity="center"
                android:padding="16dp"
                android:gravity="center"
                app:backgroundDrawable="@{viewModel.screenBackgroundId}"
                >

                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:layout_gravity="center"
                    android:text="@{viewModel.messageResId}"
                    app:textColor="@{viewModel.messageColorId}"
                    />

                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:layout_gravity="center"
                    app:mainSubtitleText="@{viewModel.number}"
                    />

                <Button
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="Click Me"
                    android:onClick="@{() -> viewModel.handleClick()}"
                    />

        </LinearLayout>

    </FrameLayout>
</layout>

BindingAdapters.kt

// I would typically place this adapter in main/_BindingAdapters.kt
// The gist tool won't allow that filename structure
//
@BindingAdapter("mainSubtitleText")
fun setText(view: TextView, value:Int) {
    view.text = view.context.getString(R.string.main_subtitle_format, value)
}

@BindingAdapter("textColor")
fun setTextColor(view: TextView, @ColorRes resId: Int) = view.setTextColor(ContextCompat.getColor(view.context, resId))

@BindingAdapter("backgroundColor")
fun setBackground(view: View, @ColorRes resId: Int) {
    view.setBackgroundColor(ContextCompat.getColor(view.context, resId))
}

@BindingAdapter("backgroundDrawable")
fun setBackgroundDrawable(view: View, @DrawableRes resId: Int) {
    view.background = ContextCompat.getDrawable(view.context, resId)
}

MainActivity.kt

class MainActivity : AppCompatActivity(), ViewModelListener {

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

        val binding = DataBindingUtil.setContentView<ActivityMainBinding>(this, R.layout.activity_main).bindData {
            viewModel = MainViewModel(R.string.hello, 5, R.color.colorAccent, this@MainActivity)
        }
    }

    // region implements ViewModelListener

    override fun showMessage(message: String) {
        Toast.makeText(this, message, Toast.LENGTH_SHORT).show()
    }

    // endregion implements ViewModelListener
}

MainViewModel.kt

class MainViewModel(@StringRes msgId:Int, num:Int, @ColorRes msgColorId:Int, val listener: ViewModelListener) {

    val messageResId = ObservableInt(msgId)
    val number = ObservableInt(num)
    val messageColorId = ObservableInt(msgColorId)
    val screenBackgroundId = ObservableInt(R.drawable.background_gradient)
    val itemBackgroundId = ObservableInt(R.color.colorPrimaryDark)

    @BoundFunction
    fun handleClick() {
        listener.showMessage("You Clicked It!!")
    }
}