How to Safely move Logic out of Activities

In my last post, I argued that we should stop putting our app logic in Activitys and Fragments because it makes both unit testing and functional testing our apps more difficult. In this post, I’ll try to suggest a method of safely removing app logic from our Activitys and Fragments, drawing on a central idea discussed in Michael Feathers’ Working Effectively with Legacy Code: characterization tests.

In the first section, I briefly introduce the idea of characterization tests. In the second section, I walk through some of the complications that arise when you’re trying to write characterization tests for Activity/Fragments. Source code examples are excerpts from my fork of the Google IO app.

Brief Intro Characterization Tests

Changes in a system can be made in two primary ways. I like to call them Edit and Pray and Cover and Modify…When you use Edit and Pray, you carefully plan the changes you are going to make, you make sure that you understand the code you are going to modify, and then you start to make the changes. When you’re done, you run the system to see if the change was enabled, and then you poke around further to make sure that you didn’t break anything…Cover and Modify is a different way of making changes. The idea behind it is that it is possible to work with a safety net when we change software.

Michael Feathers, Working Effectively with Legacy Code, pg. 32

Characterization tests are tests that form the “safety net” that Feathers refers to in the above quotation. The purpose of characterization tests is, as the name implies, to characterize the current behavior of the system. These tests help us ensure that when we refactor our code, we are preserving the current behavior of the system. For every change we make, the result of running our characterization tests will tell us whether we’ve accidentally changed the behavior of our app.

Usually, writing a characterization tests is a pretty simple process. According to Feathers:

  1. Write tests for the area where you will make your changes. Write as many cases as you feel you need to understand the behavior of the code.
  2. After doing this, take a look at the specific things you are going to change, and attempt to write tests for those.
  3. If you are attempting to extract or move some functionality, write tests that verify the existence and connection of those behaviors on a case-by-case basis. Verify that you are exercising the code that you are going to move and that it is connected properly. Exercise conversions.

Pg. 218

safety net

Image Credit: Cea on Flickr Creative Commons

Characterization Tests for Activities and Fragments

Unfortunately, writing characterization tests for Activity/Fragments is not as simple as writing characterization tests in the general case. In fact, writing characterization tests for any object instantiated by the Android OS is not easy (think Activity, Fragment, Service, BroadcastReceiver, etc.). In this section, I explore two things that make writing characterization tests for Activitys and Fragments challenging. I also suggest ways to work around these challenges.

Law of Demeter Violations

Activitys and Fragments have dependencies. Some of those dependencies are only accessible if we violate the law of demeter. For example, the PresenterFragmentImpl needs to get a hold of a ContentResolver:

public void onAttach(Activity activity) {

    // Register content observers with the content resolver.
    if (mContentObservers != null) {
        Iterator<Map.Entry<Uri, ThrottledContentObserver>> observers =
        while (observers.hasNext()) {
            Map.Entry<Uri, ThrottledContentObserver> entry =;
                    entry.getKey(), true, entry.getValue());

Characterizing the behavior of this method is challenging because law of demeter violations force us to make our mocks return mocks so that we can verify the interactions with the Fragment’s dependencies:

private void characterizeOnAttach(Actor actor, Asserter asserter) {
    // Arrange
    mPresenterFragSpy.configure(mFragmentManager, 0, mModel,
      new QueryEnum[]{}, new UserActionEnum[]{});

    final Activity activity = mock(Activity.class);
    final ContentResolver contentResolver = mock(ContentResolver.class);

    // Act

    // Assert

public void characterizeOnAttachIfRegisteredContentObservers() throws Exception {
    characterizeOnAttach(new Actor() {
        public void act(Activity activity) {
            mPresenterFragSpy.registerContentObserverOnUri(Uri.EMPTY, queriesToRun);
    }, new Asserter() {
        public void doAssert(ContentResolver contentResolver) {
            // ...
              eq(true), notNull(ThrottledContentObserver.class));

Typically, we don’t want to write tests with mocks returning mocks since this suggests that our design may smell. In this case, we are merely trying to characterize the already existing behavior of our Activity or Fragment. Once we refactor, we should have a design that doesn’t require this, but in the mean-time, we need this characterization test to make sure that we are preserving app behavior during our refactor.

(Nearly) Impossible DI

Fragments and Activitys get their dependencies via an accessor method. getLoaderManager, getContentResolver, or getFragmentManager are a few examples of such accessor methods. Here’s a method that uses one of these accessors:

 2public void onActivityCreated(Bundle savedInstanceState) {
 3    super.onActivityCreated(savedInstanceState);
 5    mLoaderIdlingResource =
 6            new LoaderIdlingResource(getClass().getName() + "/" + getId(), getLoaderManager());
 8    // Load data queries if any.
 9    if (mInitialQueriesToLoad != null && mInitialQueriesToLoad.length > 0) {
10        LoaderManager manager = getLoaderManager();
11        for (int i = 0; i < mInitialQueriesToLoad.length; i++) {
12            manager.initLoader(mInitialQueriesToLoad[i].getId(), null, this);
13        }
14    } else {
15        // No data query to load, update the view.
16        mUpdatableView.displayData(mModel, null);
17    }

If we’re characterizing a method like this one, we need to verify that the Fragment calls the correct methods on the dependencies returned by these accessor methods. Verifying the interaction of an object with its dependencies means that we need to be able to swap out those dependencies with mocks.

One way of doing this would be to have a special subclass of the Activity or Fragment you’d like to test. This sub-class could override the accessors to return mocks. Our characterization tests could then instantiate these special subclasses and use the mocks returned by the accessors to verify the interactions.

This isn’t a great solution. For reasons that I hope are clear, using this solution would force us to repeatedly subclass every Activity or Fragment in our app to return mock accessors.

A better solution is to use mockito spies. Like mocks returning mocks, we typically don’t use spies in our tests because it suggests a smelly design. However, in this case, spies help us write characterization tests without having make a non-trivial changes to the code we are trying to characterize or without having to repeatedly subclass all of our Activitys and Fragments to return mocks. Here’s an example of how spies help us do this:

public void characterizeOnActivityCreatedIfInitialQueriesToLoad() throws Exception {

    final ExploreModel.ExploreQueryEnum sessions = ExploreModel.ExploreQueryEnum.SESSIONS;
    mPresenterFragSpy.configure(mFragmentManager, 0, mModel, new QueryEnum[]{sessions}, new UserActionEnum[]{});

    final LoaderManager loaderManager = mock(LoaderManager.class);


    verify(loaderManager).initLoader(eq(sessions.getId()), isNull(Bundle.class), notNull(LoaderManager.LoaderCallbacks.class));

Using a spy, we’re able to stub out the getLoaderManager method to return a mock LoaderManager. We can then verify that onActivityCreated calls the right methods on the LoaderManager.


Safely moving logic out of large Activitys and Fragments can be a lot of work, but its less work than manually verifying that we haven’t broken anything every time we make a change to the Activity or Fragment we are refactoring.

Even if we recognize that writing these characterization tests is better than manual verification, writing them can be pretty soul-sucking. Keep in mind that we don’t need to move all of the logic out at once, so we don’t need to write all characterization tests at once.

Instead, we can identify various responsibilities that are carried out by our Activitys and Fragments and plan to extract classes to handle those responsibilities. Once we have a clear picture of the class we want to extract from the Activity or Fragment, we can write characterization tests that will serve as a safety net for that single extraction, rather than attempting to write characterization tests for all the non-trivial methods in the Activity/Fragment we are refactoring.