Updating in React Lifecycle

Expert-Level Explanation

The updating phase in React occurs when a component's props or state change. Key methods in this phase are:

1. shouldComponentUpdate: This method is called before rendering when new props or states are being received. It returnstrue (default) to continue with the rendering process or false to prevent the render. It's used for performance optimisation.

2. getSnapshotBeforeUpdate: Invoked right before the rendered output is committed to the DOM. It captures some information (e.g., scroll position) from the DOM, which can be passed tocomponentDidUpdate.

3. componentDidUpdate: Called immediately after updating occurs. This method is not called for the initial render. It's used for operations like network requests or DOM updates based on the changed state or props.

Creative Explanation

Imagine updating a React component like renovating a room:

• shouldComponentUpdate: Deciding whether the renovation is necessary based on the new requirements.

• getSnapshotBeforeUpdate: Take pictures of the room before renovation to remember the positions of furniture.

• componentDidUpdate: applying the renovations and then rearranging the furniture back to its original or a new position based on the pictures taken.

Practical Explanation with Code

An example of these methods in a class component:

import React, { Component } from 'react';

class MyComponent extends Component {
  shouldComponentUpdate(nextProps, nextState) {
    return nextProps.id !== this.props.id;
  }

  getSnapshotBeforeUpdate(prevProps, prevState) {
    if (prevState.data.length > this.state.data.length) {
      return this.container.scrollHeight;
    }
    return null;
  }

  componentDidUpdate(prevProps, prevState, snapshot) {
    if (snapshot !== null) {
      this.container.scrollTop = snapshot;
    }
  }

  render() {
    return <div ref={el => this.container = el}>{/* content */}</div>;
  }
}

Real-World Example

In an e-commerce website, a ProductDetails component displays information about a product. When a user selects a different product, the component updates. shouldComponentUpdate checks if the product ID changed, getSnapshotBeforeUpdate captures the current scroll position, andcomponentDidUpdate might fetch new details and restore the scroll position.