What is the Principle of React's Synthetic Events?

React's SyntheticEvent is an event wrapper implemented by React to ensure cross-browser compatibility. The principle of SyntheticEvent can be summarized into the following key points:

1. Event Wrapping

React implemented its own event system to address compatibility issues with native events across different browsers. This system mimics the native event system but provides a consistent interface and behavior. When an event occurs (e.g., a user clicks a button), React creates an instance of SyntheticEvent, which contains all event information regardless of the browser.

2. Event Bubbling

In React, all events automatically bubble, meaning the event propagates from the deepest node where it was triggered up to the outermost node. SyntheticEvent follows this mechanism, so you only need to listen for events on a higher-level node to handle corresponding events from lower-level nodes.

3. Event Delegation

React does not bind event handlers directly to the actual DOM elements but uses a technique called event delegation. React adds a single event listener on the top-level document node (typically document) to listen for all supported event types. When an event occurs, React determines which registered event handlers need to be called based on the event's target and bubbling path.

4. Synthetic Event Object Pool

For performance considerations, React implements a pool for SyntheticEvent objects. When an event occurs and its handler is invoked, React allocates a SyntheticEvent object from the pool and populates it with relevant event information. Once the handler is called, the object is cleared and returned to the pool for reuse in subsequent events. This process reduces the pressure on garbage collection and the cost of creating SyntheticEvent objects.

5. Relationship with Native Events

Although React uses SyntheticEvent, it is still based on native events. When a native event is triggered, React's event delegation mechanism processes it and creates a SyntheticEvent to pass to the corresponding event handler. Developers, when writing event handlers, operate on the SyntheticEvent provided by React rather than directly on native DOM events.

Example

Suppose we have a button click event, and we want to log the event object when clicked:

jsx
class MyComponent extends React.Component {
  handleClick = (event) => {
    console.log(event); // Here, event is an instance of SyntheticEvent
    console.log(event.nativeEvent); // Here, we can access the native DOM event object
  };

  render() {
    return <button onClick={this.handleClick}>Click me</button>;
  }
}

In this code, the handleClick method receives an instance of SyntheticEvent. We can access all properties and methods of this object as if handling a native event. However, since it is synthetic, its behavior is consistent across all browsers.

In summary, React's SyntheticEvent system provides an efficient and consistent way to handle event differences across browsers, while optimizing performance and simplifying the complexity of event handling.