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React-Native SectionList item layout calculations
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| /* ----------------------- | |
| * One thing that is not really sepcified in React-Native's documentation | |
| * of SectionList is what should be provided in getItemLayout function. | |
| * In fact, this function is called for **every** items in your list, | |
| * including the sections' header and footer EVEN IF YOU DON'T PROVIDE | |
| * ANY COMPONENTS FOR THESE ITEMS. This is really important, otherwise you | |
| * would have an offset in your items layout and what's displayed! | |
| * This doesn't apply to "ItemSeparatorComponent", however. | |
| * ----------------------- */ | |
| /* ----------------------- | |
| * Layout calculation is made only once, when the data changes. | |
| * This helps improving performance by doing the calculations only once | |
| * ----------------------- */ | |
| const itemsLayout = useMemo(() => { | |
| if (!data) return []; | |
| const layouts: Array<ListItemLayout> = []; | |
| let index = 0; | |
| let offset = 0; | |
| data.forEach(section => { | |
| // Section header | |
| const TOTAL_HEADER_HEIGHT = SECTION_HEADER_HEIGHT + ITEM_SEPARATOR_HEIGHT * 2; | |
| layouts[index] = { length: TOTAL_HEADER_HEIGHT, offset, index }; | |
| index++; | |
| offset += TOTAL_HEADER_HEIGHT; | |
| // Section items | |
| section.data.forEach((_match, i) => { | |
| /* ----------------------- | |
| * We need to remove the last ItemSeparator from our calculations otherwise | |
| * we might offset our layout a little. As per the documentation, a ItemSeparatorComponent is | |
| * "Rendered in between adjacent Items within each section." | |
| * ----------------------- */ | |
| const MATCH_HEIGHT = | |
| COMPACT_MATCH_CARD_HEIGHT + (i === section.data.length - 1 ? 0 : ITEM_SEPARATOR_HEIGHT); | |
| layouts[index] = { | |
| length: MATCH_HEIGHT, | |
| offset, | |
| index, | |
| }; | |
| index++; | |
| offset += MATCH_HEIGHT; | |
| }); | |
| // Section footer | |
| layouts[index] = { length: 0, offset, index }; | |
| index++; | |
| }); | |
| return layouts; | |
| }, [data]); | |
| // And then my list looks something like this | |
| <SectionList | |
| sections={data} | |
| stickySectionHeadersEnabled | |
| initialScrollIndex={initialScrollIndex} | |
| renderItem={({ item }) => <Match match={item} compact />} | |
| getItemLayout={(d, index) => itemsLayout[index] ?? { length: 0, offset: 0, index }} | |
| renderSectionHeader={({ section: { title } }) => <Header title={title} style={{ marginVertical: ITEM_SEPARATOR_HEIGHT }} />} | |
| ItemSeparatorComponent={() => <View height={ITEM_SEPARATOR_HEIGHT} />} | |
| /> |
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