_{Cover photo by Silvio Kundt on Unsplash}

The Decorator pattern is a structural design pattern that is used when we need to enhance the functionality of a given object without changing it's interface.

In this pattern, we usually have a base/bare-bones class that implements a certain interface. Along with this, we have multiple "wrapper" classes that allow users to enhance the methods of the base class without changing the interface.

This means that even the "Wrapper" classes implement the same interface

Let's consider an example to understand this better. We will discuss about a usecase that requires storing user information to a storage system and what happens when we need to encrypt some data before storing. 🗃

Storage system

Assume that you have a storage system for your websites audience that stores the users' data: email, in our case. Being a good developer, you had designed this module to be super generic and it is now being used by multiple teams in your organization. 😎

Everything sounds great? Here's the problem. You have a new requirement - Only your team needs to encrypt the users' data before storing it. 🤯

You obviously cannot modify the existing module. Too many teams are using it and they'll kill you if you do 🙃. You also cannot re-write everything from scratch. You don't have enough time for that and it also isn't the best way of solving this.

You remember a colleague telling you about the decorator pattern and you decide to put it into action 💡. Here's how it goes:

1. You have an interface, StorageSystem that contains two methods, writeInfo and readInfo.
2. The struct you had previously created by implementing StorageSystem is GenericStorageSystem - Assume that this just stores the information in memory.
3. To fulfill the requirement, you still need some struct to implement StorageSystem while also encrypting the data. Yes, I repeat, you can't just create a new struct from scratch. You need to use GenericStorageSystem.
4. You decide to create EncryptedStorageSystem. It wraps over the GenericStorageSystem and implements the writeInfo and readInfo which call writeInfo and readInfo of GenericStorageSystem respectively after encrypting/decrypting the data.

Voila! You've saved a bunch time and also have a scalable solution. Here, have a cookie for your efforts 🍪

Let's see how this looks in Go code:

type StorageSystem interface {
    writeInfo(info string)
    readInfo() string
}

type GenericStorageSystem struct {
    info string
}

func (g *GenericStorageSystem) writeInfo(info string) {
    // write information to file system as is.
    fmt.Println("Writing info to the file system: ", info)
    g.info = info
}

func (g *GenericStorageSystem) readInfo() string {
    // Return info from file system as is.
    fmt.Println("Reading info from the file system: ", g.info)
    return g.info
}

type EncryptedStorageSystem struct {
    storageSystem StorageSystem
}

func (e *EncryptedStorageSystem) writeInfo(info string) {
    fmt.Println("Encrypting the data")
    encryptedInfo := encrypt(info)
    e.storageSystem.writeInfo(encryptedInfo)
}

func (e *EncryptedStorageSystem) readInfo() string {
    info := e.storageSystem.readInfo()
    decryptedInfo := decrypt(info)
    fmt.Println("Decrypting info from the file system: ", decryptedInfo)
    return decryptedInfo
}

Let's also create dummy encrypt and decrypt functions. These just rotate the string in different directions

func encrypt (info string) string {
    return info[1:len(info)] + string(info[0])
}


func decrypt (info string) string {
    return string(info[len(info) - 1]) + info[0:len(info)-1]
}

Aaaand we're done 💃🏻. Let's check it out in action:

func main() {
    info := "kingslayer@gmail.com"
    genericStorage := GenericStorageSystem{}

    genericStorage.writeInfo(info)
    genericStorage.readInfo()

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

    encryptedStorage = EncryptedStorage{
        storageSystem: genericStorage,
    }

    encryptedStorage.writeInfo(info)
    encryptedStorage.readInfo()
}

You should see this in the output:

Writing info to the file system:  kingslayer@gmail.com
Reading info from the file system:  kingslayer@gmail.com
------------
Encrypting the data
Writing info to the file system:  ingslayer@gmail.comk
Reading info from the file system:  ingslayer@gmail.comk
Decrypting info from the file system:  kingslayer@gmail.com

Notice how the end result in both the cases are similar. You wrote a string, and got back the same string when you requested it. However, in the second case, the data was encrypted while storing and decrypted while reading - The most important part is that all of this is abstracted from the user.

There you have it - The decorator pattern 🎉

Now that you have a good understanding of the decorator pattern, let's discuss why it's a good thing:

• Notice how you did not have to create a new storage from scratch. This saved a bunch of time and also didn't change the API for your storage system. Hence, no unhappy users.
• Also notice that the EncryptedStorageSystem implements the StorageSystem interface and wraps around any StorageSystem; not just GenericStorageSystem. This is pretty powerful because you will be able to create multiple recursive wrappers. You can now, even compress your data after encrypting, if you're into that 💪🏽😁

Note: The Decorator pattern is different from the Adapter pattern because the adapter pattern converts one struct into another. Hence, there is a change in API. The decorator pattern, on the other hand, simply wraps around the existing struct without changing the API.

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

Cheers ☕️