ComponentwillMount is called twice

Question

I'm my componentWillMount() is called everytime I switch routes.

Is there other way how to handle changes in store's state?

When I use the two functions for the first time it's ok but, when I switch routes and go back and try to use them again I get this message

warning.js:45 Warning: setState(...): Can only update a mounted or mounting component. This usually means you called setState() on an unmounted component. This is a no-op. Please check the code for the undefined component.

InventoryList.js

import React from "react";
import InventoryItem from "../components/InventoryItem";
import InventoryItemStore from "../stores/InventoryItemStore";
import { Link } from "react-router";

export default class InventoryList extends React.Component {
    constructor() {
        super();
        this.state = {
            items: InventoryItemStore.getAll(),
        }
    }

    componentWillMount() {
        InventoryItemStore.on("change", () => {
            this.setState({
                items: InventoryItemStore.getAll()
            });
        });
    }

    render(...);
}

InventoryStore.js

import { EventEmitter } from "events";

import dispatcher from "../dispatcher";

class InventoryItemStore extends EventEmitter {

    constructor() {
        super()
        this.items = [
            {
                id: 1,
                title: "first item",
                stockQuantity: 10
            },
            {
                id: 2,
                title: "second item",
                stockQuantity: 5
            }
        ];
    }

    getAll() {
        return this.items;
    }

    // Adds new item to the inventory store
    addItem( title, stockQuantity ) {
        const id = Date.now();
        this.items.push({
            id,
            title, // We don't have to do title: title because of ES6... Thx ES6
            stockQuantity
        });

        this.emit("change");
    }


    /**
     * Lower the stock quantity of certain item
     * @param  {integer} id
     * @param  {integer} stockQuantity
     */
    lowerQuantity( id, orderQuantity ) {

        this.items.map((item) => {

            if ( item.id == id ) {
                item.stockQuantity = item.stockQuantity - orderQuantity;
            }

        });

        this.emit("change");

    }


    handleActions( action ) {

        switch( action.type ) {
            case "ADD_ITEM": {
                const { title, stockQuantity } = action;
                this.addItem( title, stockQuantity );
            }
            case "LOWER_QUANTITY": {
                const { id, orderQuantity } = action;
                this.lowerQuantity( id, orderQuantity );
            }
        }

    }

}

const inventoryItemStore = new InventoryItemStore;

dispatcher.register(inventoryItemStore.handleActions.bind(inventoryItemStore));

export default inventoryItemStore;

Answer 1

Your component will get unmounted every time you change route, and a new one will be mounted when you change back.

Since you're registering a eventhandler with InventoryItemStore.on but never unregistering it you're left with two components listening to change and the one that isn't mounted throws the error.

Use componentWillUnmount to un-register your component so it doesn't hang around like a ghost and haunt you when you navigate back.

See React lifecycle for more life-cycle hooks.