即使将更新的值从父级传递给子级，也不会呈现子级

Question

In the parent component, I receive data from the server and then map this data into a jsx format. Inside this mapping I have a child component and try to pass a value from state of parent to child as a property, however when I update state of this value, the render function for child is not executed.

Expected behavior: As a user I see a list of items. If I click on an item it should become as checked.

export class ReactSample extends React.Component {
  constructor(props){
    super(props);
    this.state = {
      items: [],
      mappedItems: [],
      selectedIds: [],
      isSelected: false,
      clickedTripId: null
    };

    this.toggleSelection = this.toggleSelection.bind(this);
  }

  componentWillMount(){
    console.log("Component mounting")
  }

  toggleSelection (id, e) {
    if(!_.includes(this.state.selectedIds, id)) {
      this.setState((state) => ({selectedIds: 
state.selectedIds.concat(id)}));
      this.setState(() => ({clickedTripId: id}));
      this.mapItems(this.state.items);
    }
  }

  componentDidMount() {
    const self = this;
    MyService.getItems()
      .then(res => {
        self.setState(() => ({ items: res.allItems }));
        self.setState(() => ({ mappedItems: 
this.mapItems(res.allItems) }));
      }
    )
  }

  mapItems (items) {
return items.map(trip => {
  return (
    <li key={trip.id} onClick={(e) => (this.toggleSelection(trip.id, 
e))}>
      <span>{trip.title}</span>
      <Tick ticked={this.state.clickedTripId}/>
      <span className="close-item"></span>
    </li>
  );
});
  }

  getItems() {

  }

  render() {
    return (
      <div>
        <a className="title">This is a react component!</a>
        <Spinner showSpinner={this.state.items.length <= 0}/>
        <div className="items-container">
          <ul id="itemsList">
            {this.state.mappedItems}
          </ul>
        </div>
      </div>
    );
  }
}


export class Tick extends React.Component {
  constructor(props) {
    super(props);
  }

  render() {
    console.log('RENDER');
    return (<span className={this.props.ticked ? 'tick display' : 
'tick hide' }></span>);
  }
}

Answer 1

I see a couple issues.

In toggleSelection you aren't doing anything with the result of mapItems. This kind of bug would be much easier to avoid if you just remove mappedItems from state and instead just call mapItems within your render method.

The other issue is you are passing this.state.clickedTripId as the ticked property. I assume you meant to pass something more like this.state.clickedTripId === trip.id .

Answer 2

As Ryan already said, the problem was that mappedItems where not updated when toggleSelection was clicked. As it is obvious from the code mapItems returns data in jsx format. To update it I had to call this.setState({mappedItems: this.mapItems(this.state.items)}) which means that I call mapItems and then I assign the result to the state. In this case my list will be updated and Tick component will receive this.state.clickedItemId as a tick property. There is one more issue that needs to be done to make this code working: this mapped list needs to be updated after this.state.clickedItemId is updated. The method setState is asynchronous which means that this.setState({mappedItems: this.mapItems(this.state.items)}) has to be called only after this.state.clickedItemId is updated. To achieve this, the setState method can receive a callback function as a second parameter. The code snippet is the following:

toggleSelection (id, e) {
  if(!_.includes(this.state.selectedIds, id)) {
    this.setState((state) => ({
      clickedItemId: id,
      selectedIds: state.selectedIds.concat(id)
    }), () => this.setState({mappedItems: this.mapItems(this.state.items)}));
  }
}

In this case, at the time the mapItems function is executed all data from the state that is needed here will be already updated:

mapItems (items) {
  return items.map(item => {
    return (
      <li key={item.id} onClick={(e) => (this.toggleSelection(item.id, e))}>
        <span>{item.title}</span>
        <span>{this.state.clickedItemId}</span>
        <Tick ticked={this.state.clickedItemId === item.id}/>
        <span className="close-item"></span>
      </li>
    );
  });
}