Outside-in TDD (London Style)

Kata from Sandro Mancuso

Objectives

• Learn and practice the double loop of TDD

• Test application from outside, according to side effect

Connection - 10'

• What does it bring to design classes as a consumer of the class ?

• What could it bring to do it in pair ?

• [Which kind of tool can help you design better software ?] - Optional

Concepts - 10'

In outside-in TDD we only implement what is needed to serve the outside.

• Write an acceptance test that fails

  • Something that can be accepted by a stakeholder

  • The feature test

• Keep it red

  • It will indicate what to test

  • Drive to the next step

• This double loop continues till the feature test turns green

By using this practice :

• Think about what you want to test in isolation (mocking)

• Create unit tests

• What are your test boundaries ?

Use mocks to design the collaborations

The test is used to design the system, and part of that design is deciding how the different components interact / how they are bounds to each others

Concrete Practice - 2h minimum

Here are your instructions :

Create a simple bank application with the following features :

• Deposit money

• Withdraw money

• Print a bank statement to the console

Acceptance criteria

Statement should have the following the format :

DATE       | AMOUNT  | BALANCE
10/04/2014 | 500.00  | 1400.00
02/04/2014 | -100.00 | 900.00
01/04/2014 | 1000.00 | 1000.00

How to start ?

In pair :

• Start with an acceptance test

• Use int for money and String for dates

• Ignore the formatting of the statement for now

Start by designing Service contract

You cannot change the public interface of this class.

public class AccountService {
    public void Deposit(int amount);
    public void Withdraw(int amount);
    public void PrintStatement();
}

As a coach use the Double Loop diagram to demo where the people are.

Sequence Diagram

Introduce the use of Sequence diagram after some time spend on the kata

Use sequence diagram to :

• Think about how the different objects will collaborate

• Identify what should be mock and when

Test Driven Development != Test Driven Design

Good design is driven by experience & principles, not by any TDD approach in and of itself.

Take times to reflect on the design outside of the test : great to use diagrams for it

Conclusion - 10'

• In pair, list all the pros / cons of this Double Loop Approach

• Share it all together

Resources

• Bank kata explained
• Sandro Mancuso's implementing the solution
• London vs Chicago (video series)

  • Sandro Mancuso and Uncle Bob

• Mocking as a design tool article
• Clean architecture through outside-in TDD