Kotlin Clean Code For Android — Part 3 — Presenter

Test Driven App Development in Android

Having understood how to test the Android Activity and Interactor in earlier blogs of the series, let’s look at the Presenter and its testing.

What is the work of the Presenter?

A) To change the data retrieved from Interactor according to the UI needs. Typically you may want to decorate the data or derive a value from the set of data fetched by Interactor ( For example, the total number of future trips in the list of the trips that contains both future and past trips).

B) Filter data, when needed.

C) Sort the data (default sorting), when needed.

In this example, we use the Presenter for decoration.

Date of travel is in the YYYY/MM/DD format when passed from Interactor, we need to show a message showing the days to departure if the trip is in future, similar to the below picture.

If it is a past trip, show a message with the days since the departure.

Let’s look at the Presenter

Presenter implements the PresenterInput interface. In this class, method presentHomeDatatakes care of data decoration. It creates the view model from the model that is passed by the Interactor and passes it to the Activity to display the data. Most of the data is copied without any change to the view model from the model, except noOfDaysToFly, where the private method setDaysFlyDecorationTextdoes the calculation of the days to fly, based on the current date.

Once the view model is created, the presenter calls the Activity using the ActivityInput Interface.

The class member output of type ActivityInput is a WeakReference so that we don’t create circular references. These members were wired by Configurator which will be explained in a future post, if you use the Kotlin-CleanCode4Android-code-generator, these classes will be autogenerated for you.

Fragment being a WeakReference

If you look at the code again, you will be able to understand the logic, it is straightforward.

Unit Testing

Let’s look at the unit testing the code, we will start with the test to verify whether the Activity is called with right inputs.

Should we need a Fragment class to test the presenter?

No, we should use the mock of Fragment. Let us create the Mock first

Let us create a test method to verify if the displayHomeMetaData is called with right inputs

Now knowing that the next class in the unidirectional data flow is called as expected, let us test the decoration logic — present the number of days to fly instead of the date of travel.

One of the FIRST principles in writing successful test cases is that the test should be repeatable.

The value for viewModel.noOfDaysToFly is calculated based on the current date, the value for viewModel.noOfDaysToFly can change depending upon the time of unit test execution. We overcome that by setting the current time as (2018-Nov-30) in the test code and make the test performing consistently.

We use the class FragmentInputSpy to verify the values of the view model.

Like the above example, we can test each and every public method of the presenter. By this time, you may have realized how powerful is Android Clean Code design pattern — it helps you unit test the app piece by piece, without hardwiring dependencies. Check the complete Presenter test class here.

I suggest you clone the example project, if not done already and before we close this exercise, run the test cases with code coverage(Android Studio → Menu →Run →Run with coverage). Check the code coverage of the Presenter class.

What’s next? We’ll talk about Router in the next post.

Links to the series

Kotlin Clean Code For Android — Part 2 — Interactor ⬅ PREVIOUS

NEXT ➡ Kotlin Clean Code For Android — Part 4 — Router

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Acknowledgments

Logo by Suresh C R

Article Inspired by Clean Swift