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How To Refactor Like a Pro Boss

Michael Cheng / @coderkungfu

Agenda

  1. What is refactoring? Why do we refactor? --

  2. Common refactoring techniques

What is refactoring? Why do we do it?

--

  • Software left alone will decay --

  • A change made to the internal structure of software to make it:

--

1.  Easier to understand

--

2.  Cheaper to make future changes

--

  • This does not change the observable behavior of the software --

  • Improves the design of the code --

  • Tests help ensure that we didn't change behavior

Common refactoring techniques:

--

  1. Change names to communicate intent --

  2. No magic numbers / variables --

  3. One responsibility per function / class --

  4. Do not be obsessed with primitives

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Refactoring Technique 1:

Change names to communicate intent

Technique 1: Change names to communicate intent

Example A - Mystery Variables

--

$a = 'Michael';
$b = 'Cheng';

printf('Hello, my name is %s %s', $a, $b);

--

What does $a and $b stand for? How do you change it to give it meaning?

Technique 1: Change names to communicate intent

Example A - Mystery Variables

*$first_name = 'Michael';
*$last_name = 'Cheng';

printf('Hello, my name is %s %s', $first_name, $last_name);

Technique 1: Change names to communicate intent

Example B - Mystery functions

--

function foo($first_name, $last_name) {
  return sprintf('Hello, my name is %s %s', $first_name, $last_name);
}

echo foo('Luke', 'Skywalker');

--

What does foo stand for? How do you change it to give it meaning?

Technique 1: Change names to communicate intent

Example B - Mystery functions

*function greetings_from($first_name, $last_name) {
  return sprintf('Hello, my name is %s %s', $first_name, $last_name);
}

echo greetings_from('Luke', 'Skywalker');

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Refactoring Technique 2:

No magic numbers / variables

Technique 2: No magic numbers / variables

Example - Mystery number

--

function amount_in_usd($amount) {
  return $amount * 1.4;
}

echo amount_in_usd(100);

--

What is 1.4?

Technique 2: No magic numbers / variables

Example - Mystery number

Bring out the meaning with a variable or a constant.

*define('USD_EXCHANGE_RATE', 1.4);

function amount_in_usd($amount) {
  return $amount * USD_EXCHANGE_RATE;
}

echo amount_in_usd(100);

Technique 2: No magic numbers / variables

Example - Mystery number in a class

class Currency {
* const USD_EXCHANGE_RATE = 1.4;

  public static function in_usd($amount) {
    return $amount * self::USD_EXCHANGE_RATE;
  }
}

echo Currency::in_usd(100);

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Refactoring Technique 3:

One responsibility per function / class

name: technique3_header1

Technique 3: One responsibility per function / class

Example: Club Membership Register

template: technique3_header1

Background

You are a software guy at a country club and you maintain a register of club members in CSV format:

--

Michael,Cheng,[email protected]
Luke,Skywalker,[email protected]

template: technique3_header1

Your manager wants you to make a PHP script to manage the members register.

Feature Story

"I want to add a new member to the end of the file. If the email is already in the list, ignore the new entry."

.center.emoji[ 🤓🤓🤓 ]

Technique 3: One responsibility per function / class

You quickly whip together a proof-of-concept PHP function to meet the above requirement:

function add_member($new_member){
  $fp = fopen('members.csv', 'r+');

  while (($data = fgetcsv($fp)) !== FALSE) {
    if ($data[2] == $new_member[2]) {
      fclose($fp);
      return false;
    }
  }

  fputcsv($fp, $new_member);
  fclose($fp);
}

$new_member = ['Michael', 'Cheng', '[email protected]'];
var_dump(add_member($new_member));

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It works! 😎😎😎

Fuck yeah!

Technique 3: One responsibility per function / class

Questions 🤔

--

  • How many things is add_member doing?

--

1. Opens CSV file

-- 2. Checks for duplicate email(?)

3. Ignores if its already there

-- 4. Adds it in if its not

  • Is it too much? Ideally, each function should only have 1 responsibility. --

  • And this code isn't very readable - you might come back in 2 months and forget what the code means.

Technique 3: One responsibility per function / class

What should you do?

--

  • Abstract away implementation details into separate functions. --

  • Ask your self: "What if..." questions.

Technique 3: One responsibility per function / class

Step 1: Identify the different behavior

--

  1. Opens CSV file Get List of members --

  2. Checks for duplicate email if member is already on the list --

  3. Ignores if its already there (this is result of 2) --

  4. Adds it in if its not new member to the list

name: refactor_separate_functions

Technique 3: One responsibility per function / class

Step 2: Break into separate functions

template: refactor_separate_functions

a. Pull out the code (extract) that "gets list of members" into a function

function get_members() {
  $fp = fopen('members.csv', 'r');

  $members = [];
  while (($values = fgetcsv($fp)) !== FALSE) {
    array_push($members, $values);
  }

  fclose($fp);
  return $members;
}

template: refactor_separate_functions

b. Extract "Checks if member is already on the list" into a function

function is_member_in_list($members, $new_member){
  foreach($members as $member) {
    if ($member[2] == $new_member[2]) // Email field
      return true;
  }
  return false;
}

template: refactor_separate_functions

c. Extract "Adds new member to the list" into a function

function add_member_to_file($user) {
  $fp = fopen('members.csv', 'a');
  fputcsv($fp, $user);
  fclose($fp);
}

Technique 3: One responsibility per function / class

Step 3: Refactor add_member to use the new functions

function add_member($new_member){
  $current_members = get_members();
  if (is_member_in_list($current_members, $new_member))
    return false;

  add_member_to_file($new_member);
  return true;
}

$new_member = ['Michael', 'Cheng', '[email protected]']
var_dump(add_member($new_member));

--

.center.emoji[ 😁😁😁 ]

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After 2 months...

Technique 3: One responsibility per function / class

Feature Change

--

"I want this to also check that the name of the user is not in the list and ignore that too."

.center.emoji[ 😣😣😣 ]

--

People with same name but different email addresses are also being added to the list. You have to help him prevent this in the future.

Technique 3: One responsibility per function / class

The Code Change

--

Functions list:

  • get_members()
  • is_member_in_list($members, $new_member)
  • add_member_to_file($user)
  • add_member($new_member)

--

Where do you think the code change should happen?

Technique 3: One responsibility per function / class

The Code Change

Functions list:

  • get_members()
  • is_member_in_list($members, $new_member)
  • add_member_to_file($user)
  • add_member($new_member)

Where do you think the code change should happen?

Technique 3: One responsibility per function / class

Main function

function add_member($new_member){
  $current_members = get_members();
* if (is_member_in_list($current_members, $new_member))
    return false;

  add_member_to_file($new_member);
  return true;
}

Update is_member_in_list function so that it checks for full name.

Technique 3: One responsibility per function / class

The original is_member_in_list function

function is_member_in_list($members, $new_member){
  foreach($members as $member) {
    if ($member[2] == $new_member[2]) // Email field
      return true;
  }
  return false;
}

Technique 3: One responsibility per function / class

Updated is_member_in_list function that checks for full name.

function is_member_in_list($members, $new_member){
  foreach($members as $member) {
    if ($member[2] == $new_member[2]) // Email field
      return true;
*   if (
*       $member[0] == $new_member[0] && // First name
*       $member[1] == $new_member[1]    // Last name
*   ) {
*   	return true;
*   }
  }
  return false;
}

Technique 3: One responsibility per function / class

What have we learnt?

--

  • Readable code FTW! --

  • How to break up a long function into separate functions - each with 1 responsibility --

  • Future code changes can be easily made by changing one of the functions without affecting the main function's API

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Refactoring Technique 4:

Do not be obsessed with primitives

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To be continued...

(Next month?)

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Meanwhile...

Check out the sample codes.

https://github.com/CoderKungfu/refactoring_php_demo

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Questions?

Twitter: @CoderKungfu

https://github.com/CoderKungfu/refactoring_php_demo