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MainActivityTest.java

package io.github.wingyplus.calculator;


import android.support.test.espresso.ViewInteraction;
import android.support.test.rule.ActivityTestRule;
import android.support.test.runner.AndroidJUnit4;
import android.test.suitebuilder.annotation.LargeTest;
import android.view.View;
import android.view.ViewGroup;
import android.view.ViewParent;

import org.hamcrest.Description;
import org.hamcrest.Matcher;
import org.hamcrest.TypeSafeMatcher;
import org.junit.Rule;
import org.junit.Test;
import org.junit.runner.RunWith;

import static android.support.test.espresso.Espresso.onView;
import static android.support.test.espresso.action.ViewActions.click;
import static android.support.test.espresso.action.ViewActions.closeSoftKeyboard;
import static android.support.test.espresso.action.ViewActions.replaceText;
import static android.support.test.espresso.assertion.ViewAssertions.matches;
import static android.support.test.espresso.matcher.ViewMatchers.isDisplayed;
import static android.support.test.espresso.matcher.ViewMatchers.withId;
import static android.support.test.espresso.matcher.ViewMatchers.withParent;
import static android.support.test.espresso.matcher.ViewMatchers.withText;
import static org.hamcrest.Matchers.allOf;

@LargeTest
@RunWith(AndroidJUnit4.class)
public class MainActivityTest {

  @Rule
  public ActivityTestRule<MainActivity> mActivityTestRule = new ActivityTestRule<>(MainActivity.class);

  @Test
  public void mainActivityTest() {
    ViewInteraction appCompatEditText = onView(
      allOf(withId(R.id.etLeftNumber), isDisplayed()));
    appCompatEditText.perform(replaceText("20"), closeSoftKeyboard());

    ViewInteraction appCompatEditText2 = onView(
      allOf(withId(R.id.etRightNumber), isDisplayed()));
    appCompatEditText2.perform(replaceText("30"), closeSoftKeyboard());

    ViewInteraction appCompatButton = onView(
      allOf(withId(R.id.btnCalculate), withText("Calculate"),
        withParent(allOf(withId(R.id.activity_main),
          withParent(withId(android.R.id.content)))),
        isDisplayed()));
    appCompatButton.perform(click());

    ViewInteraction textView = onView(
      allOf(withId(R.id.tvResult), withText("50"),
        childAtPosition(
          childAtPosition(
            withId(R.id.activity_main),
            0),
          3),
        isDisplayed()));
    textView.check(matches(withText("50")));

  }

  private static Matcher<View> childAtPosition(
    final Matcher<View> parentMatcher, final int position) {

    return new TypeSafeMatcher<View>() {
      @Override
      public void describeTo(Description description) {
        description.appendText("Child at position " + position + " in parent ");
        parentMatcher.describeTo(description);
      }

      @Override
      public boolean matchesSafely(View view) {
        ViewParent parent = view.getParent();
        return parent instanceof ViewGroup && parentMatcher.matches(parent)
          && view.equals(((ViewGroup) parent).getChildAt(position));
      }
    };
  }
}