Recently I came across interface from "Laravel 4 From Apprentice to Artisan" book with the example like this:
interface UserRepositoryInterface {
public function all();
}
class DbUserRepository implements UserRepositoryInterface {
public function all()
{
return User::all()->toArray();
}
}
What is interface? Where to put the interface file?
A Interface is a "contract" between itself and any class that implements the interface. The contract states that any class that implements the interface should have all methods defined in the interface. In this case DbUserRepository has to have a method named "all()" or a fatal error will occur when the class is instantiated.
The Interface file can be placed anywhere but the easiest is to put it in the same directory as the class that implements it.
The purpose of the interface is as follows: Say you want to change your app from using a database (and Eloquent) and now instead you are going store data in JSON files and write your own methods for interacting with your JSON files. Now you can create a new repository e.g. JSONRepository and have it implement UserRepositoryInterface and because the interface forces you to define all the same methods that is defined in the interface, you can now be sure that your app will continue to work as it did. All this without you having to modify existing code.
The database example doesn't really make much real world sense to me because it is unlikely that I would change my storage system so drastically and the example always makes it seem like interfaces only have this one very small use case, which cant be further from the truth. Coding to a interface has many benefits for you and your application.
Another example of interfaces in use can be:
Let's say you have a Calculator class and initially it has two operations it can perform (addition and multiplication). But a few weeks later you need to add another operation (e.g. subtraction), now normally this would mean you have to modify the calculator class and thus risk breaking it. But if you are using a interface you can just create the Subtraction class and have it implement the CalculationInterface and now your app has a new operation without you touching existing code.
Example:
Calculator.php
<?php
class Calculator {
protected $result = null;
protected $numbers = [];
protected $calculation;
public function getResult()
{
return $this->result;
}
public function setNumbers()
{
$this->numbers = func_get_args();
}
public function setCalculation(CalculationInterface $calculation)
{
$this->calculation = $calculation;
}
public function calculate()
{
foreach ($this->numbers as $num)
{
$this->result = $this->calculation->run($num, $this->result);
}
return $this->result;
}
}
CalculationInterface.php
<?php
interface CalculationInterface {
public function run($num, $current);
}
Addition.php
<?php
class Addition implements CalculationInterface {
public function run($num, $current)
{
return $current + $num;
}
}
Multiplication.php
<?php
class Multiplication implements CalculationInterface {
public function run($num, $current)
{
/* if this is the first operation just return $num
so that we don't try to multiply $num with null */
if (is_null($current))
return $num;
return $current * $num;
}
}
Then to run the calculate method:
$this->calc = new Calculator;
$this->calc->setNumbers(5, 3, 7, 10);
$this->calc->setCalculation(new Addition);
$result = $this->calc->calculate(); //$result = 25
Now if you want to add a new operation let's say Subtraction you just create the Subtraction class and have it implement the CalculationInterface:
<?php
class Subtraction implements CalculationInterface {
public function run($num, $current)
{
/* if this is the first operation just return $num
so that we don't try to subtract from null */
if (is_null($current))
return $num;
return $current - $num;
}
}
Then to run it:
$this->calc = new Calculator;
$this->calc->setNumbers(30, 3, 7, 10);
$this->calc->setCalculation(new Subtraction);
$result = $this->calc->calculate(); //$result = 10
So in this example you are breaking your functionality up into smaller classes so that you can add, remove or even change them without breaking something else.
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