Additional lifecycle methods (getDerivedStateFromProps, getSnapshotBeforeUpdate)
Class components in React have additional lifecycle methods. Two of them are getDerivedStateFromProps
and getSnapshotBeforeUpdate
. Let’s understand how and when to use them.
getDerivedStateFromProps
getDerivedStateFromProps
is a static method. It is called before rendering the component. Its purpose is to synchronize the internal state with changes in the props. This method returns an object that updates the state or null
if there are no updates.
Example:
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.state = { value: '' };
}
static getDerivedStateFromProps(nextProps, prevState) {
if (nextProps.value !== prevState.value) {
return { value: nextProps.value };
}
return null;
}
render() {
return <div>{this.state.value}</div>;
}
}
In this example, getDerivedStateFromProps
checks if the value
prop has changed. If it has, the state is updated.
getSnapshotBeforeUpdate
getSnapshotBeforeUpdate
is called before the DOM is updated. It allows you to capture information from the DOM before it is changed. This method returns a value that is passed as the third argument to componentDidUpdate
.
Example:
class ScrollList extends React.Component {
constructor(props) {
super(props);
this.listRef = React.createRef();
}
getSnapshotBeforeUpdate(prevProps, prevState) {
if (prevProps.list.length < this.props.list.length) {
return this.listRef.current.scrollHeight;
}
return null;
}
componentDidUpdate(prevProps, prevState, snapshot) {
if (snapshot !== null) {
this.listRef.current.scrollTop += this.listRef.current.scrollHeight - snapshot;
}
}
render() {
return (
<div ref={this.listRef}>
{this.props.list.map(item => (
<div key={item.id}>{item.text}</div>
))}
</div>
);
}
}
Here, getSnapshotBeforeUpdate
captures the scroll height before the update. componentDidUpdate
uses this information to adjust the scroll after the update.
Conclusion
The getDerivedStateFromProps
and getSnapshotBeforeUpdate
methods are useful for efficiently managing state and DOM updates.
Use getDerivedStateFromProps
to synchronize state and props. Use getSnapshotBeforeUpdate
to capture information from the DOM before updates. With this, you have more control over the behavior of class components.