反应：第一次点击状态不更新

Question

I am working on a shopping cart sample.

On each click, I am adding an object of the project to the Cart array. When I click the add cart button the first time, it doesn't update the Cart, but it updates on the second time.

Although, when I click the viewCart button in the render's return statement, it shows the accurate number of items in the cart. See my code below:

I need to be able to see the accurate number of items in the cart without clicking the viewCart button. The products come from a local JSON file, so I don't think there is any problem with the loading of the JSON file.

constructor (props) {
    super(props);
    this.state = {
        Cart: []
    };
    this.addItem = this.addItem.bind(this);
}

addItem(productKey) {
    this.setState({ Cart: [...this.state.Cart, ProductsJSON[productKey]]})
    console.log(this.state.Cart);
}

viewCart() {
    console.log(this.state.Cart);
}

render() {
  const AllProducts = ProductsJSON;

  var Products = AllProducts.map((item) => {
    return (
            <div className="col-4" key={item.key} >
                <a onClick={() => this.addItem(item.key)} className="btn btn-primary btn-sm">Add to Cart</a>
            </div> 
        )
})

return (
        <div className="container">
            <div className="row">
                {Products}
                <a onClick={() => this.viewCart()} className="btn btn-primary btn-sm">viewCart</a>
            </div>
        </div>
    )
}

Answer 1

Just because you don't see it in console.log doesn't mean it will not render correctly, the problem here is that setState is asynchronous , thus you will not see it if you console.log it right away ( console.log will execute before state is updated ), but if you still want to do log your cart, you can do:

this.setState(
   { Cart: [...this.state.Cart, ProductsJSON[productKey]] },
   () => console.log(this.state.Cart)
)

setState accepts a function as a callback once the state has been updated.

Answer 2

Arber's solution makes sense but when I tried it, it produced a console warning saying:

State updates from the useState() and useReducer() Hooks don't support the second callback argument. To execute a side effect after rendering, declare it in the component body with useEffect().