Data Binding + LIve Data Form Validation

form_validation_activity.xml

<layout>
    <data>
        <variable
            name="viewModel"
            type="works.wever.mvvm.formvalidation.FormValidationViewModel" />
    </data>
    <LinearLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:padding="16dp">

        <EditText
            android:id="@+id/emailAddress"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/email"
            android:text="@={viewModel.emailAddress}" />

        <Button
            android:id="@+id/saveButton"
            style="@style/Widget.AppCompat.Button.Borderless.Colored"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:enabled="@{viewModel.valid}"
            android:text="@string/save" />
    </LinearLayout>

</layout>

FormValidationActivity.kt

import android.os.Bundle
import android.util.Log
import android.util.Patterns
import androidx.appcompat.app.AppCompatActivity
import androidx.databinding.DataBindingUtil
import androidx.lifecycle.MediatorLiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProviders
import works.wever.mvvm.R
import works.wever.mvvm.databinding.FormValidationActivityBinding

class FormValidationActivity: AppCompatActivity() {

    lateinit var binding: FormValidationActivityBinding
    // 1. Create the view model
    val viewModel: FormValidationViewModel by lazy {
        ViewModelProviders.of(this).get(FormValidationViewModel::class.java)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        title = "My Cool Form"
        // 2. Inflate the layout with data binding
        binding = DataBindingUtil.setContentView(this, R.layout.form_validation_activity)
        binding.setLifecycleOwner(this)
        // 3. Set the viewModel instance
        binding.viewModel = viewModel
    }
}

class FormValidationViewModel: ViewModel() {

    val emailAddress = MutableLiveData<String>("")

    val valid = MediatorLiveData<Boolean>().apply {
        addSource(emailAddress) {
            val valid = isFormValid(it)
            Log.d(it, valid.toString())
            value = valid
        }
    }

    fun isFormValid(emailAddress: String): Boolean {
        return Patterns.EMAIL_ADDRESS.matcher(emailAddress).matches()
    }
}

FormValidationViewModelTest.kt

import androidx.arch.core.executor.testing.InstantTaskExecutorRule
import androidx.lifecycle.Observer
import org.junit.Assert.*
import org.junit.Before
import org.junit.Rule
import org.junit.Test
import org.junit.runner.RunWith
import org.robolectric.RobolectricTestRunner

@RunWith(RobolectricTestRunner::class)
class FormValidationViewModelTest {

    @get:Rule
    val instantTaskExecutorRule = InstantTaskExecutorRule()
    lateinit var viewModel: FormValidationViewModel

    @Before
    fun setUp() {
        viewModel = FormValidationViewModel()
    }

    @Test
    fun testIsFormValid() {
        val valid = viewModel.isFormValid("sundar@google.com")
        assertTrue(valid)
        val invalid = viewModel.isFormValid("not an email address")
        assertFalse(invalid)
    }

    @Test
    fun testValidMediator() {
        viewModel.emailAddress.postValue("sundar@google.com")
        val validObserver = Observer<Boolean> {
            assertTrue(it)
        }
        viewModel.valid.observeForever(validObserver)
        viewModel.valid.removeObserver(validObserver)

        viewModel.emailAddress.postValue("not an email address")
        val invalidObserver = Observer<Boolean> {
            assertFalse(it)
        }
        viewModel.valid.observeForever(invalidObserver)
        viewModel.valid.removeObserver(invalidObserver)
    }
}