State 列表更改不会导致重新渲染 Next.js/React

Question

I've got the following state:

const [places, setPlaces] = useState(false)
const [selectedPlaces, setSelectedPlaces] = useState([])

I asynchronously populate places by calling an API that returns an array of objects that looks something like:

[
    {name: "Shop #1", id: 1},
    {name: "Shop #2", id: 2}
]

My goal is to render these objects, and have their ID to be added/removed from the selectedPlaces state.

Render:

return (
  <div>
    <div>
      You have selected {selectedPlaces.length} total places
    </div>
    (places === false)
      ? <div>Loading...</div>
      : places.map(place => { // render places from the places state when loaded
          let [name, id] = [place.name, place.id]
          return <div onClick={() => {
            setSelectedPlaces(selected => {
              selected.push("dummy data to simplify")
              return selected
            })
          }}>{name}</div>
        })

  </div>
)

I've removed the key and added dummy data to make the movements simpler.

The problem arises when clicking on a div, the "You have selected... total places" doesn't refresh until I force a re-render using fast refresh or through other methods (using browser/NextJS). Is this correct behaviour? It's as-if the state isn't being changed, but a console.log on the setSelectedPlaces displays fresh data symbolizing it is being changed.

I've tried:

• Creating a useEffect handler for the selectedPlaces state which would setAmtPlaces using the length of the selected places. The same issue arises.
• Searched/read-through multiple posts/GitHub issues like this and this
• Replacing the list state with true/false in previous times I've encountered this issue, but I cannot use that approach with this problem since it's a dynamic amount of data being loaded.

Answer 1

• Add a {} wrapper for the ternary operator:

   { places === false? (...): (....) }

• push mutates the state. Use spread or concat in setSelectedPlaces

   setSelectedPlaces(selected => selected.concat("abc") )

• let [name, id] = [place.name, place.id] can be change to let { name, id } = place

Here's a snippet:

 const { useState } = React; function App() { const [places, setPlaces] = useState([ { name: "Shop #1", id: 1 }, { name: "Shop #2", id: 2 } ]) const [selectedPlaces, setSelectedPlaces] = useState([]) const onPlaceClick = id => setSelectedPlaces(state => state.concat(id)) return ( <div> <div> You have selected {selectedPlaces.length} total places </div> { (places === false)? <div>Loading...</div>: places.map(({ id, name }) => <div onClick={_ => onPlaceClick(id)}>{name}</div> ) } <span>{selectedPlaces.join()}</span> </div> ) } ReactDOM.render( <App />, document.getElementById("react") );

 <script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.1/umd/react.production.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.1/umd/react-dom.production.min.js"></script> <div id="react"></div>