Factory pattern is a commonly used creational design pattern. It is normally used when the user is expected to choose between multiple options.

Let's take an example to understand.

Pet store

Let's take a scenario of how this would work in a pet store. To understand this completely, we'll look at the implementation from both, the view of the shop-owner(developer creating the factory) and the customer (user using the interface)

Owner's perspective

Assume that you're the owner of a dog store (you only put little doggos up for adoption). Since you're in the software world, every dog is an instance of a Dog class that you have. Now, when a customer arrives, you simply create a new instance of Dog and let them adopt that.🐶

Lately, however, the customers have started requesting for variety. They're looking for options to adopt cats too.😼

Being a clever shop-owner, you've identified that this demand can only keep getting more varied. People will continue expecting more variety.😨😤

You need a robust, scalable system to generate new pets for customers Enter, the factory pattern

You make a list of all the common traits (features) of your pets. They allow you to get the name, get the sound they make and get their age. This list allows you to create an interface with the following functions:

type Pet interface {
    GetName() string
    GetAge() string
    GetSound() string
}

Now, you can create any number of pets that have the same features (implement the same interface). You could have cats, dogs, fish, parrots, anything - as long as the implement the Pet interface!😯 As of now, let's create the Dog and the Cat:

// pet is a struct that implements Pet interface and
// would be used in any animal struct that we create.
// See `Dog` and `Cat` below
type pet struct {
    name  string
    age   int
    sound string
}

func (p *pet) GetName() string {
    return p.name
}

func (p *pet) GetSound() string {
    return p.sound
}

func (p *pet) GetAge() int {
    return p.age
}

type Dog struct {
    pet
}

type Cat struct {
    pet
}

An additional thing you would require is a factory that would return a different pet (dog/cat) based on the user's request. Simply put, if the user asks for a dog, give them a cute doggo, duh.🙄🦮

func GetPet(type string) Pet {
    if type == "dog" {
        return &Dog{
            pet{
                name:  "Chester",
                age:   2,
                sound: "bark",
            },
        }
    }
    if type === "cat" {
        return &Cat{
            pet{
                name:  "Mr. Buttons",
                age:   3,
                sound: "meow",
            },
        }
    }
}

Notice how the GetPet function only tells that it returns a Pet - Not a Dog or a Cat explicitly. Hence, this function is open to extension (by writing more structs that implement the Pet interface). Adding more Pet types will not affect the existing users who just want Dogs anyway.

Congratulations! you've created a Pet shop using the factory pattern🎉❤️

Customer's perspective

Let's look at it from the perspective of the user. All they would need to do is call the GetPet function with whatever config (in this case, type) they want. In return, all they know is that they are getting a Pet.🤔 This may sound bizarre in the real world sense of things, but when it comes to code, it's better to maintain abstraction.😌

The users can freely go about "using" the Pet as they like. This "usage" would remain the same irrespective of what type of pet they got back (because all pets implement the common interface!!)

Let's test it out

func describePet(pet Pet) string {
    return fmt.Sprintf("%s is %d years old. It's sound is %s", pet.GetName(), pet.GetAge(), pet.GetSound())
}

func main() {
    petType := "dog"

    dog := GetPet(petType)
    petDescription := describePet(dog)

    fmt.Println(petDescription)
    fmt.Println("-------------")

    petType = "cat"
    cat := GetPet(petType)
    petDescription = describePet(cat)

    fmt.Println(petDescription)
}

The output should look like:

Chester is 2 years old. It's sound is bark
-------------
Mr. Buttons is 3 years old. It's sound is meow

You can find all the code for this tutorial on this github repo

Hope this made understanding the factory pattern easier 🚀

Cheers ☕️