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Usage

If you are familiar with FlatList, you already know how to use FlashList. You can try out FlashList by changing the component name and adding the estimatedItemSize prop or refer to the example below:

import React from "react";
import { View, Text, StatusBar } from "react-native";
import { FlashList } from "@shopify/flash-list";

const DATA = [
{
title: "First Item",
},
{
title: "Second Item",
},
];

const MyList = () => {
return (
<FlashList
data={DATA}
renderItem={({ item }) => <Text>{item.title}</Text>}
estimatedItemSize={200}
/>
);
};

To avoid common pitfalls, you can also follow these steps for migrating from FlatList, based on our own experiences:

  1. Switch from FlatList to FlashList and render the list once. You should see a warning about missing estimatedItemSize and a suggestion. Set this value as the prop directly.
  2. Important: Scan your renderItem hierarchy for explicit key prop definitions and remove them. If you’re doing a .map() use indices as keys.
  3. Check your renderItem hierarchy for components that make use of useState and verify whether that state would need to be reset if a different item is passed to that component (see Recycling)
  4. If your list has heterogenous views, pass their types to FlashList using getItemType prop to improve performance.
  5. Do not test performance with JS dev mode on. Make sure you’re in release mode. FlashList can appear slower while in dev mode due to a small render buffer.
estimatedItemSize

estimatedItemSize is necessary to achieve optimal performance.

Most of the props from FlatList are available in FlashList, too. This documentation includes both FlatList and additional FlashList props and should be used as a primary reference. But you can also read more about the props available in both FlatList and FlashList here.

Props

FlashList also has a couple of unique props. You already know about estimatedItemSize but there are more props available for minor adjustments.

renderItem​

note

Required

renderItem: ({ item, index, target, extraData }) => void;

Takes an item from data and renders it into the list. Typical usage:

renderItem = ({item}) => (
<Text>{item.title}</Text>
);
...
<FlashList data={[{title: 'Title Text', key: 'item1'}]} renderItem={renderItem} />

Provides additional metadata like index

  • item (Object): The item from data being rendered.
  • index (number): The index corresponding to this item in the data array.
  • target (string) FlashList may render your items for multiple reasons.
    • Cell - This is for your list item.
    • Measurement - Might be invoked for size measurement and won't be visible. You can ignore this in analytics.
    • StickyHeader - This is for your sticky header. Use this to change your item's appearance while it's being used as a sticky header.
  • extraData (Object) - This is the same extraData prop that was passed to FlashList.

data​

note

Required

For simplicity, data is a plain array of items of a given type.

data: ItemT[];

estimatedItemSize​

estimatedItemSize?: number;

estimatedItemSize is a single numeric value that hints FlashList about the approximate size of the items before they're rendered. FlashList can then use this information to decide how many items it needs to draw on the screen before initial load and while scrolling. If most of the items are of different sizes, you can think of an average or median value and if most items are of the same size, just use that number. A quick look at Element Inspector can help you determine this. If you're confused between two values, the smaller value is a better choice. If you don't specify this prop, you will get a warning with a value you can use. We recommend not ignoring that warning and defining estimatedItemSize before the list gets to your users.


CellRendererComponent​

Each cell is rendered using this element. Can be a React Component Class, or a render function. The root component should always be a CellContainer which is also the default component used. Ensure that the original props are passed to the returned CellContainer. The props contain the following properties:

  • onLayout: Method for updating data about the real CellContainer layout
  • index: Index of the cell in the list, you can use this to query data if needed
  • style: Style of CellContainer, including:
    • flexDirection: Depends on whether your list is horizontal or vertical
    • position: Value of this will be absolute as that's how FlashList positions elements
    • left: Determines position of the element on x axis
    • top: Determines position of the element on y axis
    • width: Determines width of the element (present when list is vertical)
    • height: Determines height of the element (present when list is horizontal)

When using with react-native-reanimated, you can wrap CellContainer in Animated.createAnimatedComponent (this is similar to using Animated.View):

note

Changing layout of the cell can conflict with the native layout operations. You may need to set disableAutoLayout to true to prevent this.

const AnimatedCellContainer = Animated.createAnimatedComponent(CellContainer);
return (
<FlashList
CellRendererComponent={(props) => {
return (
<AnimatedCellContainer {...props} style={...}>
);
}}
/>
);
CellRendererComponent?: React.ComponentType<any> | undefined;

ItemSeparatorComponent​

Rendered in between each item, but not at the top or bottom. By default, leadingItem and trailingItem (if available) props are provided.

ItemSeparatorComponent?: React.ComponentType<any>;

ListEmptyComponent​

Rendered when the list is empty. Can be a React Component (e.g. SomeComponent), or a React element (e.g. <SomeComponent />).

ListEmptyComponent?: React.ComponentType<any> | React.ReactElement<any, string | React.JSXElementConstructor<any>>;

ListFooterComponent​

Rendered at the bottom of all the items. Can be a React Component (e.g. SomeComponent), or a React element (e.g. <SomeComponent />).

ListFooterComponent?: React.ComponentType<any> | React.ReactElement<any, string | React.JSXElementConstructor<any>>;

ListFooterComponentStyle​

Styling for internal View for ListFooterComponent.

ListFooterComponentStyle?: StyleProp<ViewStyle>;

ListHeaderComponent​

Rendered at the top of all the items. Can be a React Component (e.g. SomeComponent), or a React element (e.g. <SomeComponent />).

ListHeaderComponent?: React.ComponentType<any> | React.ReactElement<any, string | React.JSXElementConstructor<any>>;

ListHeaderComponentStyle​

Styling for internal View for ListHeaderComponent.

ListHeaderComponentStyle?: StyleProp<ViewStyle>;

contentContainerStyle​

contentContainerStyle?: ContentStyle;

interface ContentStyle {
backgroundColor?: ColorValue;
paddingTop?: string | number;
paddingLeft?: string | number;
paddingRight?: string | number;
paddingBottom?: string | number;
padding?: string | number;
paddingVertical?: string | number;
paddingHorizontal?: string | number;
}

You can use contentContainerStyle to apply padding that will be applied to the whole content itself. For example, you can apply this padding, so that all of your items have leading and trailing space.

note

Horizontal padding is ignored on vertical lists and vertical padding on horizontal ones.

disableAutoLayout​

disableAutoLayout?: boolean;

FlashList applies some fixes to layouts of its children which can conflict with custom CellRendererComponent implementations. You can disable this behavior by setting this to true.

note

Recommendation: Set this to true while you apply special behavior to the CellRendererComponent. Once done set this to false again.

disableHorizontalListHeightMeasurement​

disableHorizontalListHeightMeasurement?: boolean;

FlashList attempts to measure size of horizontal lists by drawing an extra list item in advance. This can sometimes cause issues when used with initialScrollIndex in lists with very little content. You might see some amount of over scroll. When set to true the list's rendered size needs to be deterministic (i.e., height and width greater than 0) as FlashList will skip rendering the extra item for measurement. Default value is false.

drawDistance​

drawDistance?: number;

Draw distance for advanced rendering (in dp/px).

estimatedFirstItemOffset​

estimatedFirstItemOffset?: number;

estimatedFirstItemOffset specifies how far the first item is drawn from start of the list window or offset of the first item of the list (not the header). This prop is necessary if you're using initialScrollIndex prop. Before the initial draw, the list does not know the size of a header or any special margin/padding that might have been applied using header styles etc. If this isn't provided initialScrollIndex might not scroll to the provided index.

estimatedListSize​

estimatedListSize?: { height: number; width: number }

Estimated visible height and width of the list. It is not the scroll content size. Defining this prop will enable the list to be rendered immediately. Without it, the list first needs to measure its size, leading to a small delay during the first render.

extraData​

A marker property for telling the list to re-render (since it implements PureComponent). If any of your renderItem, Header, Footer, etc. functions depend on anything outside of the data prop, stick it here and treat it immutably.

extraData?: any;

horizontal​

If true, renders items next to each other horizontally instead of stacked vertically. Default is false.

horizontal?: boolean;

initialScrollIndex​

Instead of starting at the top with the first item, start at initialScrollIndex.

initialScrollIndex?: number;

inverted​

Reverses the direction of scroll. Uses scale transforms of -1.

inverted?: boolean;

keyExtractor​

keyExtractor?: (item: object, index: number) => string;

Used to extract a unique key for a given item at the specified index. Key is used for optimizing performance. Defining keyExtractor is also necessary when doing layout animations to uniquely identify animated components.

numColumns​

Multiple columns can only be rendered with horizontal={false} and will zig-zag like a flexWrap layout. Items should all be the same height - masonry layouts are not supported.

numColumns?: number;

onBlankArea​

onBlankArea?: (blankAreaEvent: {
offsetStart: number;
offsetEnd: number;
blankArea: number;
}) => void;

FlashList computes blank space that is visible to the user during scrolling or the initial loading of the list.

Values reported:

  • offsetStart: visible blank space on top of the screen (while going up). If value is greater than 0, it's visible to user.
  • offsetEnd: visible blank space at the end of the screen (while going down). If value is greater than 0, it's visible to user.
  • blankArea: maximum of offsetStart and offsetEnd. You might see negative values indicating that items are rendered outside the list's visible area.
warning

This callback will be triggered even if the blanks are excepted - for example, when the list does not have enough items to fill the screen.

note

This event isn't synced with onScroll event from the JS layer but works with native methods onDraw (Android) and layoutSubviews (iOS).

onEndReached​

onEndReached?: () => void;

Called once when the scroll position gets within onEndReachedThreshold of the rendered content.

onEndReachedThreshold​

onEndReachedThreshold?: number;

How far from the end (in units of visible length of the list) the bottom edge of the list must be from the end of the content to trigger the onEndReached callback. Thus a value of 0.5 will trigger onEndReached when the end of the content is within half the visible length of the list.

onLoad​

onLoad: (info: { elapsedTimeInMs: number }) => void;

This event is raised once the list has drawn items on the screen. It also reports elapsedTimeInMs which is the time it took to draw the items. This is required because FlashList doesn't render items in the first cycle. Items are drawn after it measures itself at the end of first render. If you're using ListEmptyComponent, this event is raised as soon as ListEmptyComponent is rendered.

onViewableItemsChanged​

interface ViewToken {
index: number;
isViewable: boolean;
item: string;
key: string;
timestamp: number;
}

onViewableItemsChanged?: ((info: {
viewableItems: ViewToken[];
changed: ViewToken[];
}) => void) | null | undefined

Called when the viewability of rows changes, as defined by the viewabilityConfig prop. Array of changed includes ViewTokens that both visible and non-visible items. You can use the isViewable flag to filter the items.

note

If you are tracking the time a view becomes (non-)visible, use the timestamp property. We make no guarantees that in the future viewability callbacks will be invoked as soon as they happen - for example, they might be deferred until JS thread is less busy.

onRefresh​

onRefresh?: () => void;

If provided, a standard RefreshControl will be added for "Pull to Refresh" functionality. Make sure to also set the refreshing prop correctly.

getItemType​

getItemType?: (
item: T,
index: number,
extraData?: any
) => string | number | undefined;

Allows developers to specify item types. This will improve recycling if you have different types of items in the list. Right type will be used for the right item.Default type is 0. If you don't want to change for an indexes just return undefined. You can see example of how to use this prop here.

Performance

This method is called very frequently. Keep it fast.

overrideItemLayout​

overrideItemLayout?: (
layout: { span?: number; size?: number },
item: T,
index: number,
maxColumns: number,
extraData?: any
) => void;

This method can be used to provide explicit size estimates or change column span of an item.

Providing specific estimates is a good idea when you can calculate sizes reliably. FlashList will prefer this value over estimatedItemSize for that specific item. Precise estimates will also improve precision of scrollToIndex method and initialScrollIndex prop. If you have a separator below your items, you can include its size in the estimate.

Changing item span is useful when you have grid layouts (numColumns > 1) and you want few items to be bigger than the rest.

Modify the given layout. Do not return any value from the method. FlashList will fallback to default values if this is ignored.

Performance

This method is called very frequently. Keep it fast.

overrideProps​

overrideProps?: object;

We do not recommend using this prop for anything else than debugging. Internal props of the list will be overriden with the provided values.

progressViewOffset​

progressViewOffset?: number;

Set this when offset is needed for the loading indicator to show correctly.

refreshControl​

refreshControl?: React.ReactElement<any, string | React.JSXElementConstructor<any>>;

A custom refresh control element. When set, it overrides the default <RefreshControl> component built internally. The onRefresh and refreshing props are also ignored. Only works for vertical VirtualizedList.

refreshing​

refreshing?: boolean;

Set this true while waiting for new data from a refresh.

renderScrollComponent​

import type { ScrollViewProps } from "react-native";

renderScrollComponent?:
| React.ComponentType<ScrollViewProps>
| React.FC<ScrollViewProps>;

Rendered as the main scrollview.

viewabilityConfig​

interface ViewabilityConfig: {
minimumViewTime: number;
viewAreaCoveragePercentThreshold: number;
itemVisiblePercentThreshold: number;
waitForInteraction: boolean;
}

viewabilityConfig?: ViewabilityConfig;

viewabilityConfig is a default configuration for determining whether items are viewable.

warning

Changing viewabilityConfig on the fly is not supported

Example:

<FlashList
viewabilityConfig={{
waitForInteraction: true,
itemVisiblePercentThreshold: 50,
minimumViewTime: 1000,
}}
...

minimumViewTime​

Minimum amount of time (in milliseconds) that an item must be physically viewable before the viewability callback will be fired. A high number means that scrolling through content without stopping will not mark the content as viewable. The default value is 250. We do not recommend setting much lower values to preserve performance when quickly scrolling.

viewAreaCoveragePercentThreshold​

Percent of viewport that must be covered for a partially occluded item to count as "viewable", 0-100. Fully visible items are always considered viewable. A value of 0 means that a single pixel in the viewport makes the item viewable, and a value of 100 means that an item must be either entirely visible or cover the entire viewport to count as viewable.

itemVisiblePercentThreshold​

Similar to viewAreaCoveragePercentThreshold, but considers the percent of the item that is visible, rather than the fraction of the viewable area it covers.

waitForInteraction​

Nothing is considered viewable until the user scrolls or recordInteraction is called after render.

viewabilityConfigCallbackPairs​

type ViewabilityConfigCallbackPairs = ViewabilityConfigCallbackPair[];

interface ViewabilityConfigCallbackPair {
viewabilityConfig: ViewabilityConfig;
onViewableItemsChanged:
| ((info: { viewableItems: ViewToken[]; changed: ViewToken[] }) => void)
| null;
}

viewabilityConfigCallbackPairs: ViewabilityConfigCallbackPairs | undefined;

List of ViewabilityConfig/onViewableItemsChanged pairs. A specific onViewableItemsChanged will be called when its corresponding ViewabilityConfig's conditions are met.

FlashList methods

prepareForLayoutAnimationRender()​

prepareForLayoutAnimationRender(): void;

Run this method before running layout animations, such as when animating an element when deleting it. This method disables recycling for the next frame so that layout animations run well.

warning

Avoid using this when making large changes to the data as the list might draw too much to run animations since the method disables recycling temporarily. Single item insertions or deletions should animate smoothly. The render after animation will enable recycling again and you can stop avoiding making large data changes.

recordInteraction()​

recordInteraction();

Tells the list an interaction has occurred, which should trigger viewability calculations, e.g. if waitForInteractions is true and the user has not scrolled. You should typically call recordInteraction() when user for example taps on an item or invokes a navigation action.

scrollToEnd()​

scrollToEnd?: (params?: { animated?: boolean | null | undefined });

Scrolls to the end of the content.

scrollToIndex()​

scrollToIndex(params: {
animated?: boolean | null | undefined;
index: number;
viewOffset?: number | undefined;
viewPosition?: number | undefined;
});

Scroll to a given index.

scrollToItem()​

scrollToItem(params: {
animated?: boolean | null | undefined;
item: any;
viewPosition?: number | undefined;
});

Scroll to a given item.

scrollToOffset()​

scrollToOffset(params: {
animated?: boolean | null | undefined;
offset: number;
});

Scroll to a specific content pixel offset in the list.

Param offset expects the offset to scroll to. In case of horizontal is true, the offset is the x-value, in any other case the offset is the y-value.

Param animated (false by default) defines whether the list should do an animation while scrolling.

ScrollView props

FlashList, as FlatList, uses ScrollView under the hood. You can take a look into the React Native documentation for ScrollView to see the exhaustive list of props.

Unsupported FlatList props

The following props from FlatList are currently not implemented:

Unsupported methods:

There are also FlatList props that would bring no value if ported to FlashList due to the differences in their underlying implementation:

We don't currently plan to implement these props.