Custom Comparator
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π¨βπΌ We've improved things so the
ListItem components don't rerender when there
are unrelated changes, but what if there are changes to the list item state?Hover over one of the list items and notice they all rerender. But we really
only need the hovered item to rerender (as well as the one that's no longer
highlighted).
So let's add a custom comparator to the
memo call in ListItem to only
rerender when the changed props will affect the output.Here's an example of the comparator:
const Avatar = memo(
function Avatar({ user }: { user: User }) {
return <img src={user.avatarUrl} alt={user.name} />
},
(prevProps, nextProps) => {
const avatarUnchanged =
prevProps.user.avatarUrl === nextProps.user.avatarUrl
const nameUnchanged = prevProps.user.name === nextProps.user.name
// return true if we don't want to re-render
return avatarUnchanged && nameUnchanged
},
)
So even if the user object changes, the
Avatar component will only rerender if
the avatarUrl or name properties change.By default, React just checks the reference of the props, so by providing a
custom comparator, we override that default behavior to have a more fine-grained
control over when the component should rerender.
So let's add a custom comparator to the
ListItem component so it only rerenders
when absolutely necessary.Pull up the React Profiler and the DevTools Performance tab to see the impact
of this optimization as you hover over different list items.
