Creating a better getItemLayout for SectionList in React Native

TLDR: Today, I published a new npm package called react-native-get-item-layout-section-list. This package is a helper for the getItemLayout prop in a SectionList in React Native.

Motivation #

The getItemLayout prop in a SectionList is an optimization prop that improves performance of the list by helping it to quickly calculate the size and position of its items.

When you provide the getItemLayout prop, React Native can:

• Jump directly to any list item without sequentially rendering all previous items.
• Maintain scroll position accurately during layout changes or content updates.
• Reduce the need for dynamic measurement as users scroll, leading to smoother experiences.
• Access other props, such as initialScrollIndex and scrollToLocation

FlatList #

In a FlatList, it's actually pretty trivial (especially with fixed item heights); you need to return an object with the length, offset, and index properties:

getItemLayout={(data, index) => ({
  length: ITEM_HEIGHT,
  offset: ITEM_HEIGHT * index,
  index,
})}

However, in a SectionList, it's a bit more complicated. The offset is calculated on a number of different things:

• list header
• section headers
• items
• item separators
• section footers
• section separators

Additionally, the only real documentation for getItemLayout is on the FlatList page, and that doesn't help much when you're trying to figure out how to calculate the offset for a SectionList.

This is where the react-native-get-item-layout-section-list package comes in. It provides a helper function that allows you to pass all of the possible options that effect offset and performs the calculations for you.

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