I am trying to use onViewableItemsChanged event to detect the items of my FlatList that are currently displayed on screen.
In my ViewabilityConfig (Code is provided below), I set the itemVisiblePercentThreshold parameter to 100 which I assumed will require my item to be fully displayed to be considered viewable. However that is not the case for me.
As you can see in the following screenshot:
Screenshot of my App
It is obvious that the top most item is not completely on screen (Which should make the visible items consist only of 3 items). But when I print the length of the array in my onViewableItemsChanged event handler, it returns 4 (and when I inspect the values, including the top most item).
Log Result of Viewable Items Array Length
Is this the problem of FlatList onViewableItemsChanged event? Or did I implemented it incorrectly?
I tried to find solution from the documentation and React-native github but there is no further explanation about how this event works.
Some related snippets of my code are as follow:
FlatList Definition
<FlatList
viewabilityConfig={this.clippingListViewabilityConfig}
inverted={true}
horizontal={false}
data = {this.props.clippingResultArray}
ref={(ref) => this.clippingResultFlatList = ref}
style={{
// flexGrow:0,
// backgroundColor: 'green',
// width:'100%',
// width: Dimensions.get('window').width,
}}
contentContainerStyle={{
// justifyContent:'flex-end',
// flexGrow:0,
// flexDirection:'row',
// alignItems:'flex-end',
}}
renderItem={this.renderClippingListItemRight}
keyExtractor={(item, index) => index.toString()}
onViewableItemsChanged={this.onClippingListViewableChanged}
// removeClippedSubviews={true}
{...this._clippingListItemPanResponder.panHandlers}
/>
onViewableItemsChanged Listener
onClippingListViewableChanged = (info) => {
console.log("***************************NUMBER OF CURRENT VIEWABLE ITEMS:",info.viewableItems.length);
console.log("Item list:",info.viewableItems);
this.setState({
...this.state,
viewableItems: info.viewableItems,
});
};
Viewable Configuration
this.clippingListViewabilityConfig = {
waitForInteraction: false,
itemVisiblePercentThreshold: 100,
minimumViewTime: 500, //In milliseconds
};
Related
Scrolling a SectionList results in unreasonably low JS frame rate dips (~30) for simple cell renders (a single text label) and gets much worse if cell contents are any more complex (several text labels in each cell will result in single digit JS frame rate dips).
The user facing-problem manifested by the low JS frame rate is very slow response times when a user taps anything after scrolling (it can be ~5 seconds delay).
Here's an example repo which creates both a FlatList and a SectionList each with 10,000 items on a single screen (split laterally across the middle of the screen). It's a managed Expo app (for ease of reproduction) and it's written in Typescript (because I'm used to that). The readme describes setup steps if you need them. Run the app on a physical device and turn on React Native's performance monitor to view the JS frame rate. Then scroll each of the lists as fast as you can to view the differing effects on the JS frame rate while scrolling a FlatList vs a SectionList.
Here's the entire source (from that repo's App.tsx file; 61 lines) if you prefer to bootstrap it yourself, or eyeball it here without going to GitHub:
import React from "react"
import { FlatList, SectionList, Text, View } from "react-native"
const listSize = 10_000
const listItems: string[] = Array.from(
Array(listSize).keys(),
).map((key: number) => key.toString())
const alwaysMemoize = () => true
const ListItem = React.memo(
({ title }: { title: string }) => (
<View style={{ height: 80 }}>
<Text style={{ width: "100%", textAlign: "center" }}>{title}</Text>
</View>
),
alwaysMemoize,
)
const flatListColor = "green"
const flatListItems = listItems
const sectionListColor = "blue"
const sectionListItems = listItems.map(listItem => ({
title: listItem,
data: [listItem],
}))
const approximateStatusBarHeight = 40
const App = () => {
const renderItem = React.useCallback(
({ item }: { item: string }) => <ListItem title={item} />,
[],
)
const renderSectionHeader = React.useCallback(
({ section: { title } }: { section: { title: string } }) => (
<ListItem title={title + " section header"} />
),
[],
)
return (
<View style={{ flex: 1, paddingTop: approximateStatusBarHeight }}>
<FlatList
style={{ flex: 1, backgroundColor: flatListColor }}
data={flatListItems}
renderItem={renderItem}
keyExtractor={item => item}
/>
<SectionList
style={{ flex: 1, backgroundColor: sectionListColor }}
sections={sectionListItems}
renderItem={renderItem}
renderSectionHeader={renderSectionHeader}
keyExtractor={item => item}
/>
</View>
)
}
export default App
Have I missed some performance optimization/am I doing something wrong? Or is this React Native's expected performance?
Another question as a side note: If I don't set the height of each of the cells to something reasonably tall (eg 80 in the example above) and just let the height adjust to the height of the contained text (14pt I believe), the JS frame rate dip for both types of lists becomes quite bad; 20 JS frames or less.
I'll also note here that the performance tests shown in the screen shots were done on an iPhone 13 mini.
I'm trying to have a normal info view and a sectionList inside a scrollView. Though the view gets scrolled, when I try to use scrollToLocation for scrolling to specific index selected it is not scrolling. Also I have tried using other props onMomentumScrollEnd in sectionList which is also not working. When I remove scrollView it works perfectly.
<ScrollView>
<View style={{ height: 300, backgroundColor: 'rgba(0,0,0,0.2)' }} />
<View>
<SectionList
ref={(ref) => (this.contentRef = ref)}
stickySectionHeadersEnabled={false}
showsVerticalScrollIndicator={false}
sections={sectionListData}
keyExtractor={(item) => item.id}
onMomentumScrollEnd={() => {
this.setState({ onScrollFinished: true });
this.setViewableItem();
}}
onScrollEndDrag={() => {
this.setViewableItem();
}}
onViewableItemsChanged={this.onViewableItemsChanged}
renderItem={this.renderSectionItem}
renderSectionHeader={!this.props.isMenuLoading && this.renderSectionHeader}
initialNumToRender={500}
onScrollToIndexFailed={(info) => console.log('info', info)}
/>
</View>
</ScrollView>
setActiveIndex(key) {
this.setState({ activeIndex: key, updatedAt: Date.now() });
if (isValidElement(this.headerRef)) {
this.headerRef.scrollToIndex({ index: key, animated: true, viewPosition: 0.5 });
}
if (isValidElement(this.contentRef)) {
this.contentRef.scrollToLocation({
sectionIndex: key,
itemIndex: 0,
animated: false,
viewPosition: 0
});
}
}
It appears this is a long-standing issue which no one seems to have an answer for. Issue #25295 on the react-native repository from June 2019 reported this behavior and was automatically closed as stale after 3 months. Issue #31136 reports the same problem, and there are presumably other references to it as well.
The behavior appears to be related to the underlying implementation of the scroll* imperative functions. Specifically, the call stack looks something like this:
scrollToIndex in VirtualizedList
scrollToLocation in VirtualizedSectionList
scrollToLocation in SectionList
The bit that fails is when VirtualizedList attempts to call this._scrollRef.scrollTo, which is apparently not defined when the SectionList is nested in a ScrollView or other scrollable component like FlatList.
My suggestion would be to refactor the layout such that there are not scrollable items nested under other scrollable items on the same axis.
I'm trying to make buttons out of react native FlatList items, that means when I click on them they change color.
This is the function that gets rendered by the renderItem prop:
renderRows(item, index) {
return (
<TouchableOpacity
key={index}
style={[styles.item, this.calculatedSize()]}
onPress={() => this.onPressImage(index)}
>
<Image
style={[styles.picture]}
source={{ uri: item.uri }}
/>
<View
style={[
styles.imageTextContainer,
{
backgroundColor:
_.indexOf(this.active, index) === -1
? 'rgba(0,0,0,0.2)'
: 'rgba(26, 211, 132, 0.7)',
},
]}
>
<Text style={styles.imageText}>{item.title}</Text>
</View>
</TouchableOpacity>
)
}
oh yeah and im using loadash to get the index.
The function "onPressImage(index)" works fine, in "this.active" (array) I always have the positions(integer) of the elements I would like to change color,
however nothing happens, the only thing you can see is the response by the touchableOpacity. What am I doing wrong ?
Like Andrew said, you need to trigger a re-render, usually by updating state.
However, if the items don't depend on anything outside the FlatList, I would recommend creating a component (preferably a PureComponent if possible) for the items themselves, and updating their state upon press. This way it will only re-render each list item individually if there is a change instead of the parent component.
Thanks, updating the state was a little bit difficult, the items array I've used has been declared outside the class component, thus only using an "active" array with indices that should change color in the state wasn't enough, I had to map the items array in the state like so:
state = {
items: items.map(e => ({
...e,
active: false,
}))
}
then I could manipulate the active state of the element I wanted to change color like this:
onPressItem(index) {
const { items } = this.state
const newItems = [...this.state.items]
if (items[index].active) {
newItems[index].active = false
} else {
newItems[index].active = true
}
this.setState({ items: newItems })
}
and change the color like so:
style={{
backgroundColor: item.active
? 'rgba(26, 211, 132, 0.7)'
: 'rgba(0,0,0,0.2)',
}}
<View style={{ flex: 1 }}>
<FlatList style={styles.container}
refreshing={this.state.refreshing}
data={this.props.recieveLetter}
renderItem={this.renderItem}
keyExtractor={extractKey}
ListFooterComponent={this.renderFooter}
onRefresh={this.handleRefresh}
onEndReached={this.onEndReached}
onEndReachedThreshold={0}
/>
</View>
onEndReached is not called when I scrolling to the end and I can't get more data from the API.
Working for me, add below line to FlatList:
onEndReached={this.handleLoadMore.bind(this)} //bind is important
onEndReachedThreshold needs to be a number between 0 and 1 to work correctly, so try setting it to 0.1 if you need a small threshold, or even 0.01, and it should work in most cases.
However, from my testing in react native v0.57.4, onEndReached has an erratic behavior even then, sometimes it's not called when you scroll too quickly in Android, and if you are on iOS and the list does the bounce effect when reaching the end, it may be called several times. The most consistent way of triggering my end of list function was to make an end of list check myself, made possible using the props from ScrollView (which FlatLists accept). I did it using onScroll prop like this:
//Outside of the component
const isCloseToBottom = ({layoutMeasurement, contentOffset, contentSize}) => {
const paddingToBottom = 90; //Distance from the bottom you want it to trigger.
return layoutMeasurement.height + contentOffset.y >=
contentSize.height - paddingToBottom;
};
//later, In my screen render
<FlatList
//other flatlist props, then...
onScroll={({nativeEvent}) => {
if (isCloseToBottom(nativeEvent)) {
if (!this.state.gettingMoreList) {
this.setState({
gettingMoreList: true
}, () => {
this.loadMoreList(); //Set gettingMoreList false after finishing.
});
}
}
}}
scrollEventThrottle={1000}
/>
I am trying to create a chat in React native using a <Flatlist />
Like WhatsApp and other chat apps, the messages start at the bottom.
After fetching the messages from my API, I call
this.myFlatList.scrollToEnd({animated: false});
But it scrolls somewhere in the middle and sometimes with fewer items to the bottom and sometimes it does nothing.
How can I scroll initially to the bottom?
My chat messages have different heights, so I can't calculate the height.
I had similar issue. If you want to have you chat messages start at the bottom, you could set "inverted" to true and display your messages and time tag in an opposite direction.
Check here for "inverted" property for FlatList. https://facebook.github.io/react-native/docs/flatlist#inverted
If you want to have you chat messages start at the top, which is what I am trying to achieve. I could not find a solution in FlatList, because as you said, the heights are different, I could not use getItemLayout which make "scrollToEnd" behave in a strange way.
I follow the approach that #My Mai mentioned, using ScrollView instead and do scrollToEnd({animated: false}) in a setTimeout function. Besides, I added a state to hide the content until scrollToEnd is done, so user would not be seeing any scrolling.
I solved this issue with inverted property and reverse function
https://facebook.github.io/react-native/docs/flatlist#inverted
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reverse
<FlatList
inverted
data={[...data].reverse()}
renderItem={renderItem}
keyExtractor={(item) => item.id}
/>
You can use this solution in chat component.
I faced the same issue with you and then I moved to use ScrollView.
It is fixed:
componentDidMount() {
setTimeout(() => {
this.scrollView.scrollToEnd();
});
}
<ScrollView ref={(ref) => { this.scrollView = ref; }} style={styles.messages}>
{
messages.map((item, i) => (
<Message
key={i}
direction={item.userType === 'banker' ? 'right' : 'left'}
text={item.message}
name={item.name}
time={item.createdAt}
/>
))
}
</ScrollView>`
Set initialScrollIndex to your data set's length - 1.
I.e.
<Flatlist
data={dataSet}
initialScrollIndex={dataSet.length - 1}
/>
There are two types of 'good' solutions as of 2021.
First one is with timeout, references and useEffect. Here's the full example using Functional Components and Typescript:
// Set the height of every item of the list, to improve perfomance and later use in the getItemLayout
const ITEM_HEIGHT = 100;
// Data that will be displayed in the FlatList
const [data, setData] = React.useState<DataType>();
// The variable that will hold the reference of the FlatList
const flatListRef = React.useRef<FlatList>(null);
// The effect that will always run whenever there's a change to the data
React.useLayoutEffect(() => {
const timeout = setTimeout(() => {
if (flatListRef.current && data && data.length > 0) {
flatListRef.current.scrollToEnd({ animated: true });
}
}, 1000);
return () => {
clearTimeout(timeout);
};
}, [data]);
// Your FlatList component that will receive ref, data and other properties as needed, you also have to use getItemLayout
<FlatList
data={data}
ref={flatListRef}
getItemLayout={(data, index) => {
return { length: ITEM_HEIGHT, offset: ITEM_HEIGHT * index, index };
}}
{ ...otherProperties }
/>
With the example above you can have a fluid and animated scroll to bottom. Recommended for when you receive a new message and has to scroll to the bottom, for example.
Apart from this, the second and easier way is by implementing the initialScrollIndex property that will instantly loads the list at the bottom, like that chat apps you mentioned. It will work fine when opening the chat screen for the first time.
Like this:
// No need to use useEffect, timeout and references...
// Just use getItemLayout and initialScrollIndex.
// Set the height of every item of the list, to improve perfomance and later use in the getItemLayout
const ITEM_HEIGHT = 100;
<FlatList
data={data}
getItemLayout={(data, index) => {
return { length: ITEM_HEIGHT, offset: ITEM_HEIGHT * index, index };
}}
{ ...otherProperties }
/>
I found a solution that worked for me 100%
Added the ref flatListRef to my flatlist:
<Flatlist
reference={(ref) => this.flatListRef = ref}
data={data}
keyExtractor={keyExtractor}
renderItem={renderItem}
/>
Then whenever you want to automatically scroll to bottom of the list use:
this.flatListRef._listRef._scrollRef.scrollToEnd({ animating: true });
yes you should access the element _listRef then _scrollRef then call the scrollToEnd 🙄
react-native 0.64.1
react 17.0.2
I've struggled on this as well and found the best possible solution for me that renders without a glitch is:
Use inverted={-1} props
Reverse the order of messages objects inside my array with data={MyArrayofMessages.reverse()} in my case data={this.state.messages.reverse()} using reverse() javascript function.
Stupidly easy and renders instantaneously !
Use inverted={1} and reverse your data by using the JS reverse function. It worked for me
<FlatList contentContainerStyle={{ flex: 1, justifyContent: 'flex-end' }} />
I am guessing that RN cannot guess your layout so it cannot know how much it needs to "move". According to the scroll methods in the docs you might need to implement a getItemLayout function, so RN can tell how much it needs to scroll.
https://facebook.github.io/react-native/docs/flatlist.html#scrolltoend
Guys if you want FlatList scroll to bottom at initial render. Just added inverted={-1} to your FlatList. I have struggle with scroll to bottom for couple of hours but it ends up with inverted={-1}. Don't need to think to much about measure the height of FlatList items dynamically using getItemLayout and initialScrollIndex or whats so ever.
I found a solution that worked for me 100%
let scrollRef = React.useRef(null)
and
<FlatList
ref={(it) => (scrollRef.current = it)}
onContentSizeChange={() =>
scrollRef.current?.scrollToEnd({animated: false})
}
data={data}/>
If you want to display the message inverted, set "inverted" to true in the flat list.
<Flatlist
data={messageData}
inverted={true}
horizontal={false}
/>
If you just want to scroll to the last message, you can use initialScrollIndex
<Flatlist
data={messageData}
initialScrollIndex={messageArray.length - 1}
horizontal={false}
/>
I spent couple of hours struggling with showing the first message on top without being able to calculate the item's height as it contains links and messages. But finally i've been able to...
What i've done is that i wrapped the FlatList in a View, set FlatList as inverted, made it to take all available space and then justified content. So now, conversations with few messages starts at top but when there are multiple messages, they will end on bottom. Something like this:
<View style={ConversationStyle.container}>
<FlatList
data={conversations}
initialNumToRender={10}
renderItem={({ item }) => (
<SmsConversationItem
item={item}
onDelete={onDelete}
/>
)}
keyExtractor={(item) => item.id}
getItemCount={getItemCount}
getItem={getItem}
contentContainerStyle={ConversationStyle.virtualizedListContainer}
inverted // This will make items in reversed order but will make all of them start from bottom
/>
</View>
And my style looks like this:
const ConversationStyle = StyleSheet.create({
container: {
flex: 1
},
virtualizedListContainer: {
flexGrow: 1,
justifyContent: 'flex-end'
}
};