React-native: Confusion about `rowHasChanged` and ListView rendering

[janmonschke]

Hey all,

While working on some performance improvements I found out that rowHasChanged was not called in any of our lists. These lists are infinite-scrolling lists, meaning that when you reach the end, it will fetch more items to display and then dump them into the list.

It seems like all entries in the list are updated whenever we change the DataSource's content. We do have a shouldComponentUpdate in place for all rows, but from how I understood rowHasChanged, these rows would not needed to be rendered again.

I created a simple example that shows how we are appending items to the list: https://gist.github.com/janmonschke/c9c84b6050683da0c64f

What we basically do is:

constructor() {
  // (...)
  this.ds = new ListView.DataSource({rowHasChanged: (r1, r2) => {
    console.log('rowHasChanged');
    return r1 !== r2;
  }});
  // (...)
}

onEndReached = () => {
  // append data to the end
  setData = [...setData, ...genRandomSet()];
  this.setState({
    dataSource: this.ds.cloneWithRows(setData)
  });
  console.log('onEndReached');
};

render() {
  return (
    <ListView
      dataSource={this.state.dataSource}
      renderRow={this.renderRow}
      onEndReached={this.onEndReached}
    />
  );
}

But rowHasChanged is never logged.

Am I using it somehow in a wrong way? Are there better ways to append to a ListView?

Same behaviour in 0.18.0 and 0.19.0

[facebook-github-bot]

Hey janmonschke, thanks for reporting this issue!

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[janmonschke]

Okay, found the solution in: http://stackoverflow.com/a/33871118.

Apparently it's important to clone from the correct DataSource instance ;)

So the code from the gist should have been:

  setData = [...setData, ...genRandomSet()];
  this.setState({
    dataSource: this.state.dataSource.cloneWithRows(setData)
  });

This seems like an error that's easily sneaking into your codebase. I wonder if the correct usage of DataSource should be more emphasised in the documentation.

[ekryski]

Thanks @janmonschke! I agree, I had a bunch of views constantly re-rendering and I couldn't figure out why that was happening and why the rowHasChanged function was never getting called. 👊