Split Phase

Divergent Change is a code smell that describes when the same section of code needs to change because of several different reasons. It would be better to divide up the responsibilities. This learning hour is similar to the one on Split Loop but is intended to show how to combine Split Phase with Split Loop.

Learning Goals

• Use “Split Phase” to separate concerns into different sequential steps

Session Outline

• 10 min connect: Classify the code
• 10 min concept: Make the Change Easy
• 5 min demo: Split Phase
• 35 min do: Split Phase in Theatrical Players and Office Cleaning Robot
• 5 min reflect: Explain the main idea

Connect: Classify the code

Give people copies of the code for Office Cleaning Robot version 1 in a format they can sketch on, like paper or an online activity board. Give them three colours of pen to mark sections of code that have these concerns:

• Parsing input commands
• Moving the Robot Cleaner
• Unclear or mixed

Also ask them to think about what design principles this code might break, or if there are any obvious code smells.

When everyone has had a chance to read the code and colour it, gather together and talk about what you found. The code should be stripy - mixed concerns all over the place. People might talk about problems with the Single Responsibility Principle or with Divergent Change. If they don’t mention these things, don’t worry, you’ll explain in the next part.

Concept: Divergent Change

Explain that the reason you wanted them to classify the code like this is because the next features that will be developed are

• Input commands in JSON format
• Display the track of the robot’s movement

Looking at the top level loop in ‘parseInput’, this code suffers from Divergent Change. This loop does two things that don’t need to be together. We’d like to refactor the code before we begin developing new features. As Kent Beck says ‘make the change easy (warning: this may be hard), then make the easy change’.

Demo: Split Phase

In either Theatrical Players or Office Cleaning Robot (version 1) show that you need to do several refactorings in order to separate the different kinds of logic. Show how to do that safely, including Split Phase and also Split Loop.

Concrete Practice: Split Phase in Theatrical Players and Office Cleaning Robot

In pairs, do the same split phase and split loop refactorings in both Office Cleaning Robot (version 1) and Theatrical Players.

Conclusions: Explain the main idea

Explain the main idea If someone asked you when and how to do ‘split phase’, what would you tell them? Everyone note down their ideas individually.