The Elusive getSnapshotBeforeUpdate: Can We Replicate Its Magic with Hooks?
It’s been over a year since React introduced Hooks, revolutionizing the way we build components. While Hooks have been widely adopted, one lingering question remains: can we replicate the functionality of all React class lifecycle methods using Hooks? Specifically, can we implement getSnapshotBeforeUpdate, a lesser-used but still valuable method?
The Demo App: A Chat Pane with Auto-Scroll
To explore this question, let’s create a demo app with a simple chat pane that auto-scrolls to the latest message when new messages are added. This common requirement is found in popular chat apps like WhatsApp, Skype, and iMessage.
How getSnapshotBeforeUpdate Works Its Magic
In a nutshell, getSnapshotBeforeUpdate checks for new chat messages and returns the dimension to be scrolled within the chat pane. This value is then passed to the componentDidUpdate method, which updates the scroll position of the chat pane. But how can we achieve this with Hooks?
Attempting to Implement getSnapshotBeforeUpdate with Hooks
We’ll create a custom Hook called useGetSnapshotBeforeUpdate, which takes a function argument. This function will be invoked with prevProps and prevState, mimicking the behavior of getSnapshotBeforeUpdate. To achieve this, we’ll use the useLayoutEffect Hook, which is fired before the browser paints the changes to the screen.
The Internals of useGetSnapshotBeforeUpdate
Our custom Hook will keep track of prevProps and prevState, and invoke the user’s callback function with these values. We’ll also prevent useLayoutEffect from running on mount by using a ref value componentJustMounted.
Accommodating for componentDidUpdate
To complete the solution, we’ll create a custom useComponentDidUpdate Hook, which will be returned from useGetSnapshotBeforeUpdate. This Hook will take a callback function, which will be invoked with prevProps, prevState, and the snapshot returned from getSnapshotBeforeUpdate.
Testing Our Implemented Solution
Let’s refactor our demo app to use Hooks and test our implementation. While our solution seems promising, it ultimately fails to capture the snapshot before the DOM is updated. Why?
The Reason Behind Our Failure
The issue lies in the fact that all Hooks are invoked in the React “commit phase,” after React has updated the DOM and refs internally. Since getSnapshotBeforeUpdate is invoked before the commit phase, it’s impossible to replicate its behavior using only useEffect and useLayoutEffect.
The Takeaway
While we may not be able to fully replicate getSnapshotBeforeUpdate with Hooks, this exercise has taught us valuable lessons about React fundamentals and the limitations of Hooks. By exploring the boundaries of what’s possible, we can deepen our understanding of React and continue to push the limits of what we can achieve.