Introduction

The composite pattern is a structural pattern which allows you to create tree structures of objects and treat them as a single object. The tree can contain individual or groups of objects which can be treated in the exact same way. This pattern is useful to represent a hierarchy of objects.

Applicability

• your model can be represented as a tree structure
• you want to simplify the interaction of the client code with your model

Implementation

You can find the full example source code here. As an example of the implementation I'm going to create an application that represents a Text Editor.

1. The first step would be to define the objects of the model of your application. Initially, in the example we are going to have:

• the SimpleText object, which will be used to render text on the UI.
• the SimpleImage object, which will be used to render images on the UI.
• the Block component, which represents a group of objects. I will use this object to create paragraphs.

2. Once we have defined our objects we have to create a common interface for them. All of my objects will have the function render.

interface Component {
  render: () => void;
}

3. Now we have to create the classes of the different objects defined in the first step. These classes have to implement the interface we just defined.

class SimpleText implements Component {
  private text: string;

  constructor(text: string) {
    this.text = text;
  }

  public render() {
    console.log("Rendering text:", this.text);
  }
}

class SimpleImage implements Component {
  private src: string;

  constructor(src: string) {
    this.src = src;
  }

  public render() {
    console.log("Rendering image:", this.src);
  }
}

class Block implements Component {
  private children: Component[] = [];

  public add(child: Component): void {
    this.children.push(child);
  }

  public render() {
    this.children.forEach((child) => child.render());
  }
}

As you can see, if we render the simple objects like SimpleText or SimpleImage they will execute some logic implemented in those classes. But if we render the Block component it will call the render function of all of it's children. We can interact with the tree of objects as it was a single object if we interact with the root object.

4. And that's it! Like this we have implemented the Composite pattern. A simple example of how the client would use this code creating just one block containing a line of text and an image:

const block = new Block();
block.add(new SimpleText("Hello"));
block.add(new SimpleImage("https://example.com/image.png"));

block.render();

/*  
  Output:

  Rendering text: Hello
  Rendering image: https://example.com/image.png
*/

A more complex example of nested blocks would be:

const nestedBlock = new Block();
nestedBlock.add(new SimpleText("Hello, check the following image:"));
nestedBlock.add(new SimpleImage("https://example.com/image.png"));

const mainBlock = new Block();
mainBlock.add(nestedBlock);
mainBlock.add(new SimpleText("Thanks for reading!"));

mainBlock.render();

/*  
  Output:

  Rendering text: Hello, check the following image:
  Rendering image: https://example.com/image.png
  Rendering text: Thanks for reading!
*/

Advantages

• Work with complex object structures in a simpler way, making it easier to manipulate and interact with the tree of objects.
• Open/Closed Principle. You can introduce new types of objects to the application without modifying the existing code.

Resources

• Example source code
• Composite - refactoring.guru
• Open/Closed Principle