Top Down TDD

Example of top-down style (London school) TDD

About project

Goal

Implement a simple Tic Tac Toe game

Components

All components except main.go implement the corresponding interface (see ./abstractions) folder.

main.go

Controls game.

1. start a game
2. while the game is not over, ask user inputs and update the board
3. when game is over, show a message.

Game

This component uses all other components

functionalities:

• ask player names
• ask user inputs (set mark to arbitrary position)
• check if the game is over
• show board

Board

functionalities:

• register user input (mark and position)
• check if game is over
• get a winner ("o", "x", or "" when there is not winner)

Players

functionalities:

• register player (accepts player name)
• get current player
• alternate current player when turn switches
• get player by mark ("o" or "x")

Presenter

functionalities:

• shows message (in the console, in this implementation)

UserInput

functionalities:

• ask user input (from the console, in this implementation)

What is top down TDD?

Starting from the highest level of the components (main.go in this case), you test interactions between components by using mocks. Unlike traditional TDD, where you have a red - green - refactor cycle, you have red - red - ... - red - green cycle.

For example of main.go,

// if game is over, show message

gameMock := mocks.NewMockGame(mockCtrl)
gameMock.EXPECT().InitGame()
gameMock.EXPECT().IsOver().Return(true)
gameMock.EXPECT().ShowResultMessage()

subject(gameMock)

You write a test above first, then implement Game.

1. (red) you get an error something like mocks is not defined
2. (red) you create a mock for Game
3. (red) you get an error like IsOver method is not implemented
4. (red) you create a method IsOver in the mock
5. ...repeat...
6. (red) you create a method ShowResultMessage in the mock
7. (red) now you get an error InitGame is not called
8. (red) you call InitGame in the main.go file
9. (red) you get error like IsOver is not called
10. ...repeat...
11. (green) implementation is correctly done

Why top down?

When designing an application, it's natural for me to start from high level since I don't know yet how the lowest level behaves. Top down TDD would match how I design.

How to play the game

run go run ./main.go

How to run the tests

run go test ./...