New Requirements
Does the code change with every new requirement? If so, then its recommended that this code is pulled out and encapsulated somewhere else. Essentially separating code which is fixed and code which is constantly changing.
But why? Later we can alter or extend the code without affecting the parts which don’t vary.
Tips
- Always look for code thats changes with every new requirement.
- Alter or extend code that varies without affecting code which doesn’t.
- When using a design pattern, pay attention to this principle and how the pattern implements it.
Example
Here’s a system for ordering pancakes.
Over time requests are going to be made which will force this code to change, i.e some pancakes might be taken off the menu or new ones may need to be added.
If we changed this and accidentally made a mistake, we’re going to bring down the entire pancake ordering system.
class Pancake
{
public function order($type)
{
if ($type === 'classic') {
$pancake = new ClassicPancake();
}
if ($type === 'blueberry') {
$pancake = new BlueBerryPancake();
}
if ($type === 'chocolate_chip') {
$pancake = new ChocolateChipPancake();
}
if ($type === 'banana') {
$pancake = new BananaPancake();
}
$pancake->cook();
$pancake->plate();
$pancake->addButter();
return $pancake;
}
}
You might come across similar blocks of code which probably make sense to be shifted into their own (similar group of) classes (imagine if you will, blocks of 5k lines of code in a controller).
Identify which parts of the code which vary
The code below will need to be updated with bits being added or removed.
if ($type === 'classic') {
$pancake = new ClassicPancake();
}
if ($type === 'blueberry') {
$pancake = new BlueBerryPancake();
}
if ($type === 'chocolate_chip') {
$pancake = new ChocolateChipPancake();
}
if ($type === 'banana') {
$pancake = new BananaPancake();
}
Encapsulate code somewhere else
By using a PancakeFactory (Static Factory pattern) here, the creation of the concrete pancake class is separated from the pancake->order method.
class Pancake
{
public function order($type)
{
$pancake = PancakeFactory::make($type);
$pancake->cook();
$pancake->plate();
$pancake->addButter();
return $pancake;
}
}
Now any changes will need to make to the pancake menu can be carried out on the PancakeFactory class without affecting the rest of the code.
class PancakeFactory
{
public static function make($type)
{
if ($type === 'classic') {
return new ClassicPancake();
}
if ($type === 'blueberry') {
return new BlueBerryPancake();
}
if ($type === 'chocolate_chip') {
return new ChocolateChipPancake();
}
if ($type === 'banana') {
return new BananaPancake();
}
return null; // Maybe throw an exception here.
}
}