React-native FlatList not rerendering row when props change

问题

I'm having an issue with the new FlatList component. Specifically, it does not rerender it's rows, even though props that the row is dependent on changes.

The FlatList docs says that:

This is a PureComponent which means that it will not re-render if props remain shallow- equal. Make sure that everything your renderItem function depends on is passed as a prop that is not === after updates, otherwise your UI may not update on changes. This includes the data prop and parent component state.

THE QUESTION

However, seeing as I change an ID of the selectedCategory item - the prop that should indicate whether the row is 'selected' or not - I believe that the props should rerender. Am I mistaken?

I checked the 'componentWillReceiveProps' methods of both the list and row components, and the list receives the update just fine, but the row's lifecycle method is never called.

If I include a random, useless boolean state value in the list component, and switch it back and forth when the props update, it works - but I don't know why?

state = { updated: false };

componentWillReceiveProps(nextProps) {
  this.setState(oldstate => ({
    updated: !oldstate.updated,
  }));
}

<FlatList
  data={this.props.items.allAnimalCategories.edges}
  renderItem={this._renderRow}
  horizontal={true}
  keyExtractor={(item, index) => item.node.id}
  randomUpdateProp={this.state.updated}
/>

THE CODE

The structure of my code is this: I have a container component with all the logic and state, which contains a FlatList component (presentational, no state), which again contains a custom presentational row.

Container
  Custom list component that includes the FlatList component
  (presentational, stateless) and the renderRow method
    Custom row (presentational, stateless)

The container includes this component:

 <CustomList
   items={this.props.viewer}
   onCategoryChosen={this._onCategoryChosen}
   selectedCategory={this.state.report.selectedCategory}
 />

CustomList:

class CustomList extends Component {
  _renderRow = ({ item }) => {
    return (
      <CustomListRow
        item={item.node}
        selectedCategory={this.props.selectedCategory}
        onPressItem={this.props.onCategoryChosen}
      />
    );
  };

  render() {
    return (
      <View style={_styles.container}>
        <FlatList
          data={this.props.items.categories.edges}
          renderItem={this._renderRow}
          horizontal={true}
          keyExtractor={(item, index) => item.node.id}
          randomUpdateProp={this.state.updated}
        />
      </View>
    );
  }

}

(data comes from Relay)

Finally the row:

render() {
    const idsMatch = this.props.selectedCategory.id == this.props.item.id;
    return (
      <TouchableHighlight onPress={this._onItemPressed}>
        <View style={_styles.root}>
          <View style={[
              _styles.container,
              { backgroundColor: this._getBackgroundColor() },
            ]}>
            {idsMatch &&
              <Image
                style={_styles.icon}
                source={require('./../../res/img/asd.png')}
              />}
            {!idsMatch &&
              <Image
                style={_styles.icon}
                source={require('./../../res/img/dsa.png')}
              />}
            <Text style={_styles.text}>
              {capitalizeFirstLetter(this.props.item.name)}
            </Text>
          </View>
          <View style={_styles.bottomView}>
            <View style={_styles.greyLine} />
          </View>
        </View>
      </TouchableHighlight>
    );
  }

The row is not that interesting, but I included it to show that it is entirely stateless and dependent on it's parents props.

The state is updated like so:

_onCategoryChosen = category => {
    var oldReportCopy = this.state.report;
    oldReportCopy.selectedCategory = category;
    this.setState(Object.assign({}, this.state, { report: oldReportCopy }));
  };

State looks like this:

state = {
    ...
    report: defaultStateReport,
  };

const defaultStateReport = {
  selectedCategory: {
    id: 'some-long-od',
    name: '',
  },
  ...
};

回答1:

The problem here lies within the fact that

You are mutating an existing slice of state instead of creating a mutated copy

_onCategoryChosen = category => {
    var oldReportCopy = this.state.report; // This does not create a copy!
    oldReportCopy.selectedCategory = category;
    this.setState(Object.assign({}, this.state, { report: oldReportCopy }));
};

This should be

_onCategoryChosen = category => {
    var oldReportCopy = Object.assign({}, this.state.report);
    oldReportCopy.selectedCategory = category;
    // setState handles partial updates just fine, no need to create a copy
    this.setState({ report: oldReportCopy });
};

The props of FlatList remain the same, your _renderRow function may rely on the selectedCategory prop which does change (If not for the first mistake), but the FlatList component does not know about that. To solve this, use the extraData prop.
```
<FlatList
  data={this.props.items.categories.edges}
  renderItem={this._renderRow}
  horizontal={true}
  keyExtractor={(item, index) => item.node.id}
  extraData={this.props.selectedCategory}
/>
```

回答2:

Simply you can solve this problem passing props to extraData in flat list component like this,

  <FlatList
    data={this.props.data}
    extraData={this.props}
    keyExtractor={this._keyExtractor}
    renderItem={this._renderItem}
  />

回答3:

I agree with Nimelrian. Also, If your state is an Array you could create an Array Object from the state by doing:

 var oldReportCopy = Object.assign([], this.state.report);

Then use the .push() method to add your new object to it like this:

oldReportCopy.push(selectedCategory);

you can then set this new Array Object back to state:

this.setState({ report: oldReportCopy });

回答4:

Maybe this won't be the case for anyone else but I realized I was only having trouble when the array of items being rendered by the FlatList became empty. In my case I just needed to not render the FlatList at all, and instead render a different View in its place, and that of course fixed my issue with it "not re-rendering".

来源：https://stackoverflow.com/questions/44278526/react-native-flatlist-not-rerendering-row-when-props-change

标签

react-native

react-native-listview