onView

 /**
   * Creates a {@link ViewInteraction} for a given view. Note: the view has to be part of the view
   * hierarchy. This may not be the case if it is rendered as part of an AdapterView (e.g.
   * ListView). If this is the case, use Espresso.onData to load the view first.
   *
   * <p>This method builds a ViewInteraction object - it does not interact with the application
   * under test at all. It is expected that the caller use the ViewInteraction object to perform an
   * action or assertion.
   *
   * @param viewMatcher used to select the view.
   * @see #onData(org.hamcrest.Matcher)
   */
  // TODO change parameter to type to Matcher<? extends View> which currently causes Dagger issues
  @CheckReturnValue
  @CheckResult
  public static ViewInteraction onView(final Matcher<View> viewMatcher) {
    return BASE.plus(new ViewInteractionModule(viewMatcher)).viewInteraction();
  }

onView可以通过指定的Matcher来定位到要测试的目标元素，它适用于简单的UI，不适用于AdapterView（指的是ListView，Spinner，GridView等），它有以下限制：

• 要测试的目标元素必须在可见视图内，比如说Listview当前可见item为0-10条，那么就不能用onView来指定11以上（不在屏幕范围内）的元素，不然会报错。
• 它在满足上述条件下可以用来测试Listview，但并不保险。

onData

  /**
   * Creates an {@link DataInteraction} for a data object displayed by the application. Use this
   * method to load (into the view hierarchy) items from AdapterView widgets (e.g. ListView).
   *
   * <p>This method builds a DataInteraction object - it does not interact with the application
   * under test at all. It is expected that the caller use the ViewInteraction object to perform an
   * action or assertion.
   *
   * @param dataMatcher a matcher used to find the data object.
   * @return a DataInteraction that will perform an action or assertion.
   */
  @CheckReturnValue
  @CheckResult
  public static DataInteraction onData(Matcher<? extends Object> dataMatcher) {
    return new DataInteraction(dataMatcher);
  }

onData是通过绑定数据来获取到指定元素，它适用于AdapterView，但不适用于RecyclerView。

• 可以指定屏幕外的元素
• 如果指定屏幕外的元素，它将滑动到指定的元素，然后才会进行后续的操作。

onView使用

@RunWith(AndroidJUnit4.class)
public class EspressoTest {
    @Test
    public void test(){
        onData(withId(R.id.btn)).perform(click());
    }
}

onData使用

• 如果List的数据源是String类型

List<String>list;

onData(allOf(instanceOf(String.class),is("item0"))).perform(click());

这里的item0是ListView的第0项的item数据，该数据类型是String类型，allOf将两条匹配规则整合成一条。

• 如果List的数据源是HashMap类型

List<Map<String,String>>list;

onData(hasEntry(is("title"),is("content"))).perform(click());

hasEntry的泛型类型是一个Map类型，如下源码所示：

  /**
   * Creates a matcher for {@link java.util.Map}s matching when the examined {@link java.util.Map} contains
   * at least one entry whose key satisfies the specified <code>keyMatcher</code> <b>and</b> whose
   * value satisfies the specified <code>valueMatcher</code>.
   * <p/>
   * For example:
   * <pre>assertThat(myMap, hasEntry(equalTo("bar"), equalTo("foo")))</pre>
   * 
   * @param keyMatcher
   *     the key matcher that, in combination with the valueMatcher, must be satisfied by at least one entry
   * @param valueMatcher
   *     the value matcher that, in combination with the keyMatcher, must be satisfied by at least one entry
   */
  public static <K, V> org.hamcrest.Matcher<java.util.Map<? extends K, ? extends V>> hasEntry(org.hamcrest.Matcher<? super K> keyMatcher, org.hamcrest.Matcher<? super V> valueMatcher) {
    return org.hamcrest.collection.IsMapContaining.<K,V>hasEntry(keyMatcher, valueMatcher);
  }

它的两个参数分别对应HashMap的key和value。

• 如果是Object类型

List<Book>list;

onData(allOf(withBookTitle("item20"),withBookContent("content20"))).perform(click());

public static Matcher<Object> withBookTitle(final String bookTitle){
        return new BoundedMatcher<Object, Book>(Book.class) {

            @Override
            public void describeTo(Description description) {
                description.appendText(bookTitle);
            }

            @Override
            protected boolean matchesSafely(Book item) {
                return bookTitle.equals(item.getTitle());
            }
        };
    }
    public static Matcher<Object> withBookContent(final String bookContent){
        return new BoundedMatcher<Object, Book>(Book.class) {
            @Override
            public void describeTo(Description description) {
                description.appendText(bookContent);
            }

            @Override
            protected boolean matchesSafely(Book item) {
                return bookContent.equals(item.getContent());
            }
        };
    }

该例子的ListView的每个Item项有两个元素，一个表示title的TextView,一个表示content的TextView，item20表示ListView的第20个item的title。上述Matcher是自定义的Matcher。