Difficulty: intermediate
Estimated Time: 30 minutes

I got this well written explanation from kamranahmedse

I only converted the examples to Go. It is assumed that you know how to use vi and to compile and run Go code.

Design Patterns For Humans


🎉 Ultra-simplified explanation to design patterns! 🎉

A topic that can easily make anyone's mind wobble. Here I try to make them stick in to your mind (and maybe mine) by explaining them in the simplest way possible.


Check out my blog and say "hi" on Twitter.

Introduction

Design patterns are solutions to recurring problems; guidelines on how to tackle certain problems. They are not classes, packages or libraries that you can plug into your application and wait for the magic to happen. These are, rather, guidelines on how to tackle certain problems in certain situations.

Design patterns are solutions to recurring problems; guidelines on how to tackle certain problems

Wikipedia describes them as

In software engineering, a software design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. It is not a finished design that can be transformed directly into source or machine code. It is a description or template for how to solve a problem that can be used in many different situations.

⚠️ Be Careful

  • Design patterns are not a silver bullet to all your problems.
  • Do not try to force them; bad things are supposed to happen, if done so.
  • Keep in mind that design patterns are solutions to problems, not solutions finding problems; so don't overthink.
  • If used in a correct place in a correct manner, they can prove to be a savior; or else they can result in a horrible mess of a code.

Also note that the code samples below are in Go, however this shouldn't stop you because the concepts are same anyways.

Types of Design Patterns

  • Creational
  • Structural
  • Behavioral

Congratulations!

You have learned about design pattern for humans in Go

🚦 Wrap Up Folks

And that about wraps it up. I will continue to improve this, so you might want to watch/star this repository to revisit. Also, I have plans on writing the same about the architectural patterns, stay tuned for it.

👬 Contribution

  • Report issues
  • Open pull request with improvements
  • Spread the word
  • Reach out with any feedback Twitter URL

License

License: CC BY 4.0

Design Patterns for Humans

Step 1 of 3

Step 1 - Introduction

Creational Design Patterns

In plain words

Creational patterns are focused towards how to instantiate an object or group of related objects.

Wikipedia says

In software engineering, creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. The basic form of object creation could result in design problems or added complexity to the design. Creational design patterns solve this problem by somehow controlling this object creation.

  • Simple Factory
  • Factory Method
  • Abstract Factory
  • Builder
  • Prototype
  • Singleton

🏠 Simple Factory

Real world example

Consider, you are building a house and you need doors. You can either put on your carpenter clothes, bring some wood, glue, nails and all the tools required to build the door and start building it in your house or you can simply call the factory and get the built door delivered to you so that you do not need to learn anything about the door making or to deal with the mess that comes with making it.

In plain words

Simple factory simply generates an instance for client without exposing any instantiation logic to the client

Wikipedia says

In object-oriented programming (OOP), a factory is an object for creating other objects – formally a factory is a function or method that returns objects of a varying prototype or class from some method call, which is assumed to be "new".

Programmatic Example

First of all we have a door interface and the implementation

vi simple.go

package main
import "fmt"

type Door interface {
    getWidth() float64
    getHeight() float64
}

type door struct {
    width float64
    height float64
}

func (d door) getWidth() float64 {
    return d.width
}

func (d door) getHeight() float64 {
    return d.height
}

Then we have our door factory that makes the door and returns it

func WoodenDoor(width float64, height float64) Door {
    return door{
    width: width,
    height: height}
}

And then it can be used as

func main() {
    var door1 Door
    door1 = WoodenDoor(100,200)
    fmt.Printf("Width is %f\n", door1.getWidth())
    fmt.Printf("Height is %f\n", door1.getHeight())
}

Go ahead save and quit vi. Then, build and run.

go build simple.go

./simple

When to Use?

When creating an object is not just a few assignments and involves some logic, it makes sense to put it in a dedicated factory instead of repeating the same code everywhere.

🏭 Factory Method

Real world example

Consider the case of a hiring manager. It is impossible for one person to interview for each of the positions. Based on the job opening, she has to decide and delegate the interview steps to different people.

In plain words

It provides a way to delegate the instantiation logic to child classes.

Wikipedia says

In class-based programming, the factory method pattern is a creational pattern that uses factory methods to deal with the problem of creating objects without having to specify the exact class of the object that will be created. This is done by creating objects by calling a factory method—either specified in an interface and implemented by child classes, or implemented in a base class and optionally overridden by derived classes—rather than by calling a constructor.

Programmatic Example

Taking our hiring manager example above. First of all we have an interviewer interface and some implementations for it

Name this factory.go

package main
import "fmt"

type Interviewer struct {
    job string
    question string
}

func Developer() Interviewer {
    return Interviewer{
        job: "Developer",
        question: "Asking about design patterns!"}
}

func CommunityExecutive() Interviewer {
    return Interviewer{
        job: "Community Executive",
        question: "Asking about community building"}
}

Now let us create our HiringManager

type HiringManager struct {
    job string
    title string
}

func (i Interviewer) NewManager(title string) {
    return HiringManager{
    job: i.job,
    title: title}
}

Now any child can extend it and provide the required interviewer and then it can be used as

func main() {
    var i Interviewer
    i = Developer()
    fmt.Printf("%s\n",i.job)
    fmt.Printf("%s\n",i.question)
    ifactory := Interviewer{
        job: "Developer",
        question: "Asking about design patterns!"}
}

When to use?

Useful when there is some generic processing in a class but the required sub-class is dynamically decided at runtime. Or putting it in other words, when the client doesn't know what exact sub-class it might need.

🔨 Abstract Factory

Real world example

Extending our door example from Simple Factory. Based on your needs you might get a wooden door from a wooden door shop, iron door from an iron shop or a PVC door from the relevant shop. Plus you might need a guy with different kind of specialities to fit the door, for example a carpenter for wooden door, welder for iron door etc. As you can see there is a dependency between the doors now, wooden door needs carpenter, iron door needs a welder etc.

In plain words

A factory of factories; a factory that groups the individual but related/dependent factories together without specifying their concrete classes.

Wikipedia says

The abstract factory pattern provides a way to encapsulate a group of individual factories that have a common theme without specifying their concrete classes

Programmatic Example

Translating the door example above. First of all we have our Door interface and some implementation for it

interface Door
{
    public function getDescription();
}

class WoodenDoor implements Door
{
    public function getDescription()
    {
        echo 'I am a wooden door';
    }
}

class IronDoor implements Door
{
    public function getDescription()
    {
        echo 'I am an iron door';
    }
}

Then we have some fitting experts for each door type

interface DoorFittingExpert
{
    public function getDescription();
}

class Welder implements DoorFittingExpert
{
    public function getDescription()
    {
        echo 'I can only fit iron doors';
    }
}

class Carpenter implements DoorFittingExpert
{
    public function getDescription()
    {
        echo 'I can only fit wooden doors';
    }
}

Now we have our abstract factory that would let us make family of related objects i.e. wooden door factory would create a wooden door and wooden door fitting expert and iron door factory would create an iron door and iron door fitting expert

interface DoorFactory
{
    public function makeDoor(): Door;
    public function makeFittingExpert(): DoorFittingExpert;
}

// Wooden factory to return carpenter and wooden door
class WoodenDoorFactory implements DoorFactory
{
    public function makeDoor(): Door
    {
        return new WoodenDoor();
    }

    public function makeFittingExpert(): DoorFittingExpert
    {
        return new Carpenter();
    }
}

// Iron door factory to get iron door and the relevant fitting expert
class IronDoorFactory implements DoorFactory
{
    public function makeDoor(): Door
    {
        return new IronDoor();
    }

    public function makeFittingExpert(): DoorFittingExpert
    {
        return new Welder();
    }
}

And then it can be used as

$woodenFactory = new WoodenDoorFactory();

$door = $woodenFactory->makeDoor();
$expert = $woodenFactory->makeFittingExpert();

$door->getDescription();  // Output: I am a wooden door
$expert->getDescription(); // Output: I can only fit wooden doors

// Same for Iron Factory
$ironFactory = new IronDoorFactory();

$door = $ironFactory->makeDoor();
$expert = $ironFactory->makeFittingExpert();

$door->getDescription();  // Output: I am an iron door
$expert->getDescription(); // Output: I can only fit iron doors

As you can see the wooden door factory has encapsulated the carpenter and the wooden door also iron door factory has encapsulated the iron door and welder. And thus it had helped us make sure that for each of the created door, we do not get a wrong fitting expert.

When to use?

When there are interrelated dependencies with not-that-simple creation logic involved

👷 Builder

Real world example

Imagine you are at Hardee's and you order a specific deal, lets say, "Big Hardee" and they hand it over to you without any questions; this is the example of simple factory. But there are cases when the creation logic might involve more steps. For example you want a customized Subway deal, you have several options in how your burger is made e.g what bread do you want? what types of sauces would you like? What cheese would you want? etc. In such cases builder pattern comes to the rescue.

In plain words

Allows you to create different flavors of an object while avoiding constructor pollution. Useful when there could be several flavors of an object. Or when there are a lot of steps involved in creation of an object.

Wikipedia says

The builder pattern is an object creation software design pattern with the intentions of finding a solution to the telescoping constructor anti-pattern.

Having said that let me add a bit about what telescoping constructor anti-pattern is. At one point or the other we have all seen a constructor like below:

public function __construct($size, $cheese = true, $pepperoni = true, $tomato = false, $lettuce = true)
{
}

As you can see; the number of constructor parameters can quickly get out of hand and it might become difficult to understand the arrangement of parameters. Plus this parameter list could keep on growing if you would want to add more options in future. This is called telescoping constructor anti-pattern.

Programmatic Example

The sane alternative is to use the builder pattern. First of all we have our burger that we want to make

class Burger
{
    protected $size;

    protected $cheese = false;
    protected $pepperoni = false;
    protected $lettuce = false;
    protected $tomato = false;

    public function __construct(BurgerBuilder $builder)
    {
        $this->size = $builder->size;
        $this->cheese = $builder->cheese;
        $this->pepperoni = $builder->pepperoni;
        $this->lettuce = $builder->lettuce;
        $this->tomato = $builder->tomato;
    }
}

And then we have the builder

class BurgerBuilder
{
    public $size;

    public $cheese = false;
    public $pepperoni = false;
    public $lettuce = false;
    public $tomato = false;

    public function __construct(int $size)
    {
        $this->size = $size;
    }

    public function addPepperoni()
    {
        $this->pepperoni = true;
        return $this;
    }

    public function addLettuce()
    {
        $this->lettuce = true;
        return $this;
    }

    public function addCheese()
    {
        $this->cheese = true;
        return $this;
    }

    public function addTomato()
    {
        $this->tomato = true;
        return $this;
    }

    public function build(): Burger
    {
        return new Burger($this);
    }
}

And then it can be used as:

$burger = (new BurgerBuilder(14))
                    ->addPepperoni()
                    ->addLettuce()
                    ->addTomato()
                    ->build();

When to use?

When there could be several flavors of an object and to avoid the constructor telescoping. The key difference from the factory pattern is that; factory pattern is to be used when the creation is a one step process while builder pattern is to be used when the creation is a multi step process.

🐑 Prototype

Real world example

Remember dolly? The sheep that was cloned! Lets not get into the details but the key point here is that it is all about cloning

In plain words

Create object based on an existing object through cloning.

Wikipedia says

The prototype pattern is a creational design pattern in software development. It is used when the type of objects to create is determined by a prototypical instance, which is cloned to produce new objects.

In short, it allows you to create a copy of an existing object and modify it to your needs, instead of going through the trouble of creating an object from scratch and setting it up.

Programmatic Example

In PHP, it can be easily done using clone

class Sheep
{
    protected $name;
    protected $category;

    public function __construct(string $name, string $category = 'Mountain Sheep')
    {
        $this->name = $name;
        $this->category = $category;
    }

    public function setName(string $name)
    {
        $this->name = $name;
    }

    public function getName()
    {
        return $this->name;
    }

    public function setCategory(string $category)
    {
        $this->category = $category;
    }

    public function getCategory()
    {
        return $this->category;
    }
}

Then it can be cloned like below

$original = new Sheep('Jolly');
echo $original->getName(); // Jolly
echo $original->getCategory(); // Mountain Sheep

// Clone and modify what is required
$cloned = clone $original;
$cloned->setName('Dolly');
echo $cloned->getName(); // Dolly
echo $cloned->getCategory(); // Mountain sheep

Also you could use the magic method __clone to modify the cloning behavior.

When to use?

When an object is required that is similar to existing object or when the creation would be expensive as compared to cloning.

💍 Singleton

Real world example

There can only be one president of a country at a time. The same president has to be brought to action, whenever duty calls. President here is singleton.

In plain words

Ensures that only one object of a particular class is ever created.

Wikipedia says

In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system.

Singleton pattern is actually considered an anti-pattern and overuse of it should be avoided. It is not necessarily bad and could have some valid use-cases but should be used with caution because it introduces a global state in your application and change to it in one place could affect in the other areas and it could become pretty difficult to debug. The other bad thing about them is it makes your code tightly coupled plus mocking the singleton could be difficult.

Programmatic Example

To create a singleton, make the constructor private, disable cloning, disable extension and create a static variable to house the instance

final class President
{
    private static $instance;

    private function __construct()
    {
        // Hide the constructor
    }

    public static function getInstance(): President
    {
        if (!self::$instance) {
            self::$instance = new self();
        }

        return self::$instance;
    }

    private function __clone()
    {
        // Disable cloning
    }

    private function __wakeup()
    {
        // Disable unserialize
    }
}

Then in order to use

$president1 = President::getInstance();
$president2 = President::getInstance();

var_dump($president1 === $president2); // true