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<markdown>
## SOLID

### Single responsability
Definition: A class should have one and only one reason to change, meaning that a class should have only one job.
Ex: A class that calculates area of a shape also manages authentication, connection to db and rendering. WRONG!
Problems of GOD class:
 - Uses a lot of dependencies
 - Contains different types of logic
 - Nothing is decoupled
 - It's no opened to extension
Solutions: 
 - Use layers to separate logics
 - Proper naming
 - DI
 
### Open-Close

Definition: Objects or entities should be open for extension, but closed for modification.

For PHP:
 - Classes should allow the creation of new properties or methods
 - Classes should not allow modification of current properties or methods.
 - Classes should have rich documented interfaces or prototypes.

```php
public function sum() { 
	foreach ($this->shapes as  $shape) { 
		if(is_a($shape, 'Square')){ 
			$area[] = pow($shape->length, 2); 
		} elseif (is_a($shape, 'Circle')){ 
			$area[] = pi() * pow($shape->radius, 2); 
		}
	 } 
	return array_sum($area); 
}
```
=> Create an interface for all different shapes and make the method "area" compulsory

### Liskov Substitution

Definition: Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.
- If we have a **`Tesla`** is a subtype of a **`Car`**, then I should be able to **`drive`** either a **`Car`** or a **`Tesla`** the same way.
 -  Classes should depend on non-concrete class when possible, like abstract or interfaces
 -  Classes extensions must use the same prototype as their parents.
 - When implementing an Interface there is a contract in the input but should be a contract on the output to. (Also exceptions)
 Problem: If we use the interface we act expecting some output.
 Code Smell: instanceof
 Solution: Doc Blocks and return type hinting
 
### Interface segregation
Definition: A client should never be forced to implement an interface that it doesn't use or clients shouldn't be forced to depend on methods they do not use.
- Many client-specific interfaces are better than one general-purpose interface
Solution:
- Use dependencies
- Split the class into => interfaces + abstractions
-  Implement abstractions in a way that don't knoe about other classses' behavior

```php
interface WorkerInterface {
	public function work(); 
	public function sleep();
}
class Worker implements WorkerInterface { 
	public function work(){} 
	public function sleep(){}
}
class Android implements WorkerInterface {
	public function work(){} 
	public function sleep(){ return null; }
}
class Captain {
        public function manage (WorkerInterface $worker) {
		$worker->work();
		$worker->sleep();
	}
}
```

=> 
```php
interface WorkableInterface {
	public function work(); 
}
interface SleepableInterface {
	public function sleep();
}
class Worker implements WorkableInterface, SleepableInterface{ 
	public function work(){} 
	public function sleep(){}
}
class Android implements WorkableInterface {
	public function work(){} 
}
class Captain {
	 public function manage (¿?¿?¿? $worker) {
		$worker->work();
		$worker->sleep();
	}
}
```
=> 
```php
interface ManageableInterface {
	public function beManaged(); 
}
interface WorkableInterface {
	public function work(); 
}
interface SleepableInterface {
	public function sleep();
}
class Worker implements WorkableInterface, SleepableInterface, ManageableInterface{ 
	public function work(){} 
	public function sleep(){}
	public function beManaged(){
		$this->work();
		$this->sleep(); 
	}	
}
class Android implements WorkableInterface, ManageableInterface {
	public function work(){} 
	public function beManaged(){
		$this->work();
	}
}
class Captain {
	 public function manage (ManageableInterface $worker) {
		$worker->beManaged();
	}
}
``` 
### Dependency inversion
Definition: 
- High-level modules should not depend on low-level modules. Both should depend on abstractions. 
A `Module` object should not depend on a `Child` object, but rather on `ChildInterface` or `AbstractChild`.
- Abstractions should not depend upon details. Details should depend upon abstractions.
And a `Child` object should receive calls not from a `Module`class but from a `ModuleInterface`
Put simply, this means our dependencies should be interfaces/contracts or abstract classes rather than concrete implementations.
```php
class PasswordReminder {
    private $dbConnection;

    public function __construct(MySQLConnection $dbConnection) {
        $this->dbConnection = $dbConnection;
    }
}
```
PasswordReminder => hig level
MySQLConnection => low level
=>

```php

interface DBConnectionInterface {
    public function connect();
}
class MySQLConnection implements DBConnectionInterface {
    public function connect() {
        return "Database connection";
    }
}

class PasswordReminder {
    private $dbConnection;

    public function __construct(DBConnectionInterface $dbConnection) {
        $this->dbConnection = $dbConnection;
    }
}

```

</markdown>
https://medium.com/backticks-tildes/the-s-o-l-i-d-principles-in-pictures-b34ce2f1e898
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