Introduction to ReactJS

Before we dive into the specifics of invoking methods from a child class in ReactJS, let's first understand what ReactJS is. ReactJS is a JavaScript library that is extensively used to develop highly interactive user interfaces for web applications. It was developed by Facebook and is now maintained by Facebook and a community of individual developers and corporations.

In ReactJS, we encapsulate components. These components are independent and reusable bits of code. They serve the same purpose as JavaScript functions, but they work in isolation and return HTML via a render function.

Understanding the Component Hierarchy

To understand how to call a method from a child class in ReactJS, you first need to understand the component hierarchy. In ReactJS, components are structured in a parent and child relationship, similar to a family tree. The parent component can have multiple child components, and each child component can further have its own child components.

You can think of this hierarchy as a tree where each node represents a component. The root of the tree is the parent component and the nodes connected to it are its child components. This tree-like structure is often referred to as the "component tree".

The Challenge

Now, let's talk about our main challenge - calling a method from a child class. In a typical parent-child relationship in programming, it's straightforward to pass data from the parent to the child. This is often done through props, which is short for properties. Props flow downwards from parent to child.

However, it is tricky to pass data or invoke methods in the opposite direction, from the child back up to the parent. Why? Because in ReactJS (and in many other programming paradigms), data flows in a single direction, from top (parent) to bottom (child). This is known as "one-way data flow".

Imagine a river flowing from a mountain top (the parent component) to the sea (the child component). The water (data) naturally flows downstream, but getting it back up to the top is no mean feat. This is the challenge we are facing.

The Solution: Using Callback Functions

To call a method from a child class, we can use a technique called "passing a callback". A callback function is a function that is passed as a parameter to another function, and the callback function is called (or executed) inside the other function.

Let's use an analogy to understand this. Imagine you're at home (parent component) and your friend (child component) borrows your book. But you want the book back after your friend is done reading. To ensure you get your book back, you ask your friend to call you (callback function) when they're done reading the book. Your friend agrees to call you back (execute the callback function) when they're done.

Similarly, in React, the parent component passes a function (the callback) to the child component via props. The child component then calls (executes) this function when it needs to communicate something back to the parent.

Code Example

Let's look at an example. We'll create a parent component, Parent, and a child component, Child. The parent has a method called parentMethod that we want to call from the child component.

class Parent extends React.Component {
  parentMethod = () => {
    console.log("Called from the child component");
  }

  render() {
    return <Child parentMethod={this.parentMethod} />
  }
}

In the Parent component, we pass parentMethod as a prop to Child. Now, in the Child component, we can call parentMethod as follows:

class Child extends React.Component {
  callParentMethod = () => {
    this.props.parentMethod();
  }

  render() {
    return <button onClick={this.callParentMethod}>Call Parent Method</button>;
  }
}

In the Child component, we create a button. When the button is clicked, it triggers callParentMethod, which in turn calls parentMethod from the Parent component via this.props.parentMethod().

Wrapping Up

Calling a method from a child class in ReactJS may seem a little complicated at first, especially due to the one-way data flow. However, by using callback functions and understanding the flow of props, it becomes fairly straightforward.

Always remember, in ReactJS, parent and child components communicate through props. Parents send props down to their children, and children communicate back to their parents via callbacks.

By mastering this technique, you'll be able to create more interactive and complex components and take your ReactJS skills to the next level. Happy coding!