In this article written by Dr. Dominik Hauser, author of the book Test–Driven iOS Development with Swift 3.0, you will learn that when starting TDD, writing unit tests would be easy for most people. The hard part is to transfer the knowledge from writing the test to driving the development. What can be assumed? What should be done before one writes the first test? What should be tested to end up with a complete app?

(For more resources related to this topic, see here.)

As a developer, you are used to thinking in terms of code. When you see a feature on the requirement list for an app, your brain already starts to layout the code for this feature. And for recurring problems in iOS development (such as building table views), you most probably have already developed your own best practices.

In TDD, you should not think about the code while working on the test. The tests have to describe what the unit under test should do and not how it should do it. It should be possible to change the implementation without breaking the tests.

To practice this approach of development, we will develop a simple to-do list app in the remainder of this book. It is, on purpose, a boring and easy app. We want to concentrate on the TDD workflow, not complex implementations. An interesting app would distract from what is important in this book—how to do TDD.

This article introduces the app that we are going to build, and it shows the views that the finished app will have.

We will cover the following topics in this article:

• The task list view
• The task detail view
• The task input view
• The structure of an app
• Getting started with Xcode
• Setting up useful Xcode behaviors for testing

The task list view

When starting the app, the user sees a list of to-do items. The items in the list consist of a title, an optional location, and the due date. New items can be added to the list by an add (+) button, which is shown in the navigation bar of the view. The task list view will look like this:

User stories:

• As a user, I want to see the list of to-do items when I open the app
• As a user, I want to add to-do items to the list

In a to-do list app, the user will obviously need to be able to check items when they are finished. The checked items are shown below the unchecked items, and it is possible to uncheck them again. The app uses the delete button in the UI of UITableView to check and uncheck items. Checked items will be put at the end of the list in a section with the Finished header. The user can also delete all the items from the list by tapping the trash button. The UI for the to-do item list will look like this:

User stories:

• As a user, I want to check a to-do item to mark it as finished
• As a user, I want to see all the checked items below the unchecked items
• As a user, I want to uncheck a to-do item
• As a user, I want to delete all the to-do items

When the user taps an entry, the details of this entry is shown in the task detail view.

The task detail view

The task detail view shows all the information that's stored for a to-do item. The information consists of a title, due date, location (name and address), and a description. If an address is given, a map with an address is shown. The detail view also allows checking the item as finished. The detail view looks like this:

User stories:

• As a user, given that I have tapped a to-do item in the list, I want to see its details
• As a user, I want to check a to-do item from its details view

The task input view

When the user selects the add (+) button in the list view, the task input view is shown. The user can add information for the task. Only the title is required. The Save button can only be selected when a title is given. It is not possible to add a task that is already in the list. The Cancel button dismisses the view. The task input view will look like this:

User stories:

• As a user, given that I have tapped the add (+) button in the item list, I want to see a form to put in the details (title, optional date, optional location name, optional address, and optional description) of a to-do item
• As a user, I want to add a to-do item to the list of to-do items by tapping on the Save button

We will not implement the editing and deletion of tasks. But when you have worked through this book completely, it will be easy for you to add this feature yourself by writing the tests first.

Keep in mind that we will not test the look and design of the app. Unit tests cannot figure out whether an app looks like it was intended. Unit tests can test features, and these are independent of their presentation. In principle, it would be possible to write unit tests for the position and color of UI elements. But such things are very likely to change a lot in the early stages of development. We do not want to have failing tests only because a button has moved 10 points.

However, we will test whether the UI elements are present on the view. If your user cannot see the information for the tasks, or if it is not possible to add all the information of a task, then the app does not meet the requirements.

The structure of the app

The following diagram shows the structure of the app:

The Table View Controller, the delegate, and the data source

In iOS apps, data is often presented using a table view. Table views are highly optimized for performance; they are easy to use and to implement. We will use a table view for the list of to-do items.

A table view is usually represented by UITableViewController, which is also the data source and delegate for the table view. This often leads to a massive Table View Controller, because it is doing too much: presenting the view, navigating to other view controllers, and managing the presentation of the data in the table view.

It is a good practice to split up the responsibility into several classes. Therefore, we will use a helper class to act as the data source and delegate for the table view. The communication between the Table View Controller and the helper class will be defined using a protocol. Protocols define what the interface of a class looks like. This has a great benefit: if we need to replace an implementation with a better version (maybe because we have learned how to implement the feature in a better way), we only need to develop against the clear interface. The inner workings of other classes do not matter.

Table view cells

As you can see in the preceding screenshots, the to-do list items have a title and, optionally, they can have a due date and a location name. The table view cells should only show the set data. We will accomplish this by implementing our own custom table view cell.

The model

The model of the application consists of the to-do item, the location, and an item manager, which allows the addition and removal of items and is also responsible for managing the items. Therefore, the controller will ask the item manager for the items to present. The item manager will also be responsible for storing the items on disc.

Beginners often tend to manage the model objects within the controller. Then, the controller has a reference to a collection of items, and the addition and removal of items is directly done by the controller. This is not recommended, because if we decide to change the storage of the items (for example, by using Core Data), their addition and removal would have to be changed within the controller. It is difficult to keep an overview of such a class; and because of this reason, it is a source of bugs.

It is much easier to have a clear interface between the controller and the model objects, because if we need to change how the model objects are managed, the controller can stay the same. We could even replace the complete model layer if we just keep the interface the same. Later in the article, we will see that this decoupling also helps to make testing easier.

Other view controllers

The application will have two more view controllers: a task detail View Controller and a View Controller for the input of the task.

When the user taps a to-do item in the list, the details of the item are presented in the task detail View Controller. From the Details screen, the user will be able to check an item.

New to-do items will be added to the list of items using the view presented by the input View Controller.

The development strategy

In this book, we will build the app from inside out. We will start with the model, and then build the controllers and networking. At the end of the book, we will put everything together.

Of course, this is not the only way to build apps. But by separating on the basis of layers instead of features, it is easier to follow and keep an overview of what is happening. When you later need to refresh your memory, the relevant information you need is easier to find.

Getting started with Xcode

Now, let's start our journey by creating a project that we will implement using TDD.

Open Xcode and create a new iOS project using the Single View Application template. In the options window, add ToDo as the product name, select Swift as language, choose iPhone in the Devices option, and check the box next to Include Unit Tests. Let the Use Core Data and Include UI Tests boxes stay unchecked.

Xcode creates a small iOS project with two targets: one for the implementation code and the other for the unit tests. The template contains code that presents a single view on screen. We could have chosen to start with the master-detail application template, because the app will show a master and a detail view. However, we have chosen the Single View Application template because it comes with hardly any code. And in TDD, we want to have all the implementation code demanded by failing tests.

To take a look at how the application target and test target fit together, select the project in Project Navigator, and then select the ToDoTests target. In the General tab, you'll find a setting for the Host Application that the test target should be able to test. It will look like this:

Xcode has already set up the test target correctly to allow the testing of the implementations that we will write in the application target.

Xcode has also set up a scheme to build the app and run the tests. Click on the Scheme selector next to the stop button in the toolbar, and select Edit Scheme.... In the Test action, all the test bundles of the project will be listed. In our case, only one test bundle is shown—ToDoTests. On the right-hand side of the shown window is a column named Test, with a checked checkbox. This means that if we run the tests while this scheme is selected in Xcode, all the tests in the selected test suite will be run.

Setting up useful Xcode behaviors for testing

Xcode has a feature called behaviors. With the use of behaviors and tabs, Xcode can show useful information depending on its state.

Open the Behaviors window by going to Xcode | Behaviors | Edit Behaviors. On the left-hand side are the different stages for which you can add behaviors (Build, Testing, Running, and so on). The following behaviors are useful when doing TDD.

The behaviors shown here are those that I find useful. Play around with the settings to find the ones most useful for you. Overall, I recommend using behaviors because I think they speed up development.

Useful build behaviors

When building starts, Xcode compiles the files and links them together. To see what is going on, you can activate the build log when building starts. It is recommended that you open the build log in a new tab because this allows switching back to the code editor when no error occurs during the build. Select the Starts stage and check Show tab named. Put in the Log name and select in active window. Check the Show navigator setting and select Issue Navigator. At the bottom of the window, check Navigate to and select current log. After you have made these changes, the settings window will look like this:

Build and run to see what the behavior looks like.

Testing behaviors

To write some code, I have an Xcode tab called Coding. Usually, in this tab, the test is open on the left-hand side, and in the Assistant Editor which is on the right-hand side, there is the code to be tested (or in the case of TDD, the code to be written). It looks like this:

When the test starts, we want to see the code editor again. So, we add a behavior to show the Coding tab. In addition to this, we want to see the Test Navigator and debugger with the console view.

When the test succeeds, Xcode should show a bezel to notify us that all tests have passed. Go to the Testing | Succeeds stage, and check the Notify using bezel or system notification setting. In addition to this, it should hide the navigator and the debugger, because we want to concentrate on refactoring or writing the next test.

In case the testing fails (which happens a lot in TDD), Xcode will show a bezel again. I like to hide the debugger, because usually, it is not the best place to figure out what is going on in the case of a failing test. And in most of the cases in TDD, we already know what the problem is. But we want to see the failing test. Therefore, check Show navigator and select Issue navigator. At the bottom of the window, check Navigate to and select first new issue.

You can even make your Mac speak the announcements. Check Speak announcements using and select the voice you like. But be careful not to annoy your coworkers. You might need their help in the future.

Now, the project and Xcode are set up, and we can start our TDD journey.

Summary

In this article, we took a look at the app that we are going to build throughout the course of this book. We took a look at how the screens of the app will look when we are finished with it. We created the project that we will use later on and learned about Xcode behaviors.

Resources for Article:

Further resources on this subject:

• Thinking Functionally [article]
• Hosting on Google App Engine [article]
• Cloud and Async Communication [article]