Discovering Espresso for Android: creating custom Matchers.

Hi! This time I'll talk about custom Matchers that can be used with Espresso for Android (and not only). We'll go through steps how to create them and I'll provide you examples of already existing and very useful ones.

First of all a couple of words about Hamcrest library, which provides us with common matchers and possibility to create custom matchers, to pay tribute to it's authors.

From Humcrest project main page - Hamcrest provides a library of matcher objects (also known as constraints or predicates) allowing 'match' rules to be defined declaratively, to be used in other frameworks. Typical scenarios include testing frameworks, mocking libraries and UI validation rules.

In one of my previous post I've described already how to use some custom Hamcrest matchers. The base idea is that matcher is initialized with the expected values, which are compared against the actual object we are matching when invoking it.

Among of the common matchers you can create your custom matchers using abstract class TypeSaveMatcher.class , with three methods to override:

public boolean matchesSafely(Fruit fruit) - matcher's logic,
public void describeTo(Description description) - description of the expected result,
protected void describeMismatchSafely(Fruit item, Description mismatchDescription) - description of actual value.

Example:

public static Matcher<Object> withItemText(String itemText) {
  // use preconditions to fail fast when a test is creating an invalid matcher.
  checkArgument(!(itemText.equals(null)));
  return withItemText(equalTo(itemText));
}

public static Matcher<Object> withItemText(final Matcher<String> matcherText) { 
  // use preconditions to fail fast when a test is creating an invalid matcher.
  checkNotNull(matcherText);
  return new TypeSafeMatcher<Object>() {
    
    @Override
    public void describeTo(Description description) {
      description.appendText("expected text: " + matcherText);
    }

    @Override
    public void describeMismatchSafely(Object item description, Description mismatchDescription) {
      mismatchDescription.appendText("actual text: " + item.toString());
    }

    @Override
    public boolean matchesSafely(Object item) {
      return matcherText.equals(item);
    }
  };
} 

Here TypeSafeMatcher does the cast to a Object for us. The matchesSafely() method checks if the Object contains a text equal to itemText - and the describeTo() method produces a failure message when a test fails. Sometimes describeMismatchSafely() method is omitted.

Usage:

import static com.your.package.test.Matchers.withItemText;
... 
onData(withItemText("someText")).inAdapterView(withId(R.id.someId)).perform(click()); 

Esspresso introduced one more matcher type - BoundedMatcher<T,S extends T> - some matcher sugar that lets you create a matcher for a given type but only process items of a specific subtype of that matcher.

Example:

//Matcher for checking if EditText fields contain text hints
public static Matcher<View> withItemHint(String itemHintText) {
  // use preconditions to fail fast when a test is creating an invalid matcher.
  checkArgument(!(itemHintText.equals(null)));
  return withItemHint(is(itemHintText));
}

public static Matcher<View> withItemHint(final Matcher<String> matcherText) {
  // use preconditions to fail fast when a test is creating an invalid matcher.
  checkNotNull(matcherText);
  return new BoundedMatcher<View, EditText>(EditText.class) {

    @Override
    public void describeTo(Description description) {
      description.appendText("with item hint: " + matcherText);
    }

    @Override
    protected boolean matchesSafely(EditText editTextField) {
      return matcherText.matches(editTextField.getHint().toString());
    }
  };
}

Usage:

import static com.your.package.test.Matchers.withItemHint;
... 
onView(withItemHint(editText.getHint().toString())).check(matches(isDisplayed())); 

How does it works - we go through all elements on the screen which are the subtypes of View.class, casting them to EditText.class, getting hint text with editTextField.getHint().toString() and matching to itemHintText.

And at the end some useful links:
https://github.com/yandex-qatools/matchers-java - Java library which implements hamcrest matchers for collections and webdriver webelements.
https://gist.github.com/frankiesardo/7490059 - custom matchers from Frankie Sardo for matching drawables.