[FlatList] shouldItemUpdate called with incorrect {item,index}

[john-cfng]

Description

symptom:
supply shouldItemUpdate() and data=[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 7, 8, 9, 10, 11, ...] to a FlatList with 3 columns
shouldItemUpdate() is called with incorrect data {index: 6, item: 9}
expect {index: 6, item:7} or {index: 8, item 9}

further investigation:
seems that in FlatList.js, _shouldItemUpdate() does not take numColumns into account when calling shouldItemUpdate()

Reproduction Steps and Sample Code

obverse the values received in shouldItemUpdate()

class AppClass extends React.Component {
 constructor(props) {
 super(props)
 this.data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 7, 8, 9, 10, 11, ] ;
 }
 shouldItemUpdate(props, nextProps) {
 // some code
 }
 render() {
 return (
 <FlatList
 {...(this.modifiedProps)}
 data={this.data}
 numColumns={3}
 shouldItemUpdate={this.shouldItemUpdate}
 renderItem={this.renderItem}>
 </FlatList>
 );
 }
}

Solution

add *numColumns in _shouldItemUpdate function seems to have solved the problem

_shouldItemUpdate = (prev, next) => { 
 const {numColumns, shouldItemUpdate} = this.props; 
 if (numColumns > 1) { 
 return prev.item.length !== next.item.length || 
 prev.item.some((prevItem, ii) => shouldItemUpdate( 
 {item: prevItem, index: prev.index*numColumns + ii}, 
 {item: next.item[ii], index: next.index*numColumns + ii}, 
 )); 
 } else { 
 return shouldItemUpdate(prev, next); 
 } 
 };

Additional Information

• React Native version: 0.43.2
• Platform: Android 4.1.2 & 7.1.1
• Development Operating System: Win10
• Dev tools: Android SDK Tools 26.0.1

[hramos]

Hi there! This issue is being closed because it has been inactive for a while. Maybe the issue has been fixed in a recent release, or perhaps it is not affecting a lot of people. Either way, we're automatically closing issues after a period of inactivity. Please do not take it personally!

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