How to make vuex state update UI when it's updated?

Question

I'm building a shopping cart page using Vuejs and Vuex. Each item in the cart should have a quantity counter to increase or decrease number of pieces ordered for this specific item. When I add products to the cart from the main products page then visit the cart page. I click on the counter and the product quantity is changed but the UI doesn't show this change. It only shows the change after I reload the page and after that it can show any new changes to only this specific counter, and I have to reload after adding every product just to make its counter show UI changes.

This is the cart Vue component simplified:

 <template >
    <tbody>
        <tr v-for="product in cartProducts" :key="product.name">
            <td class="quantity">
                <i class="fa fa-minus" @click="decreaseQuantity(product)"></i>
                <p>{{product.quantity}}</p>
                <i class="fa fa-plus" @click="increaseQuantity(product)"></i>
            </td>
        </tr>
    </tbody>
</template>

<script>
export default {
    computed: {
        cartProducts() {
            return this.$store.state.cartProducts;
        },
    },
    methods: {
        increaseQuantity(drug) {
            const existedDrug = this.cartProducts.find(d => d.id === drug.id);
            existedDrug.quantity += 1;
            this.$store.dispatch("setUserDataCart", this.cartProducts);
            console.log(this.$store.state.cartProducts);
        },
        decreaseQuantity(drug) {
            const existedDrug = this.cartProducts.find(d => d.id === drug.id);
            existedDrug.quantity -= 1;
            this.$store.dispatch("setUserDataCart", this.cartProducts);
            console.log(this.$store.state.cartProducts);
        },
    },
};
</script>

And this is my vuex store:

import Vue from "vue";
import Vuex from "vuex";
import VuexPersist from "vuex-persist";

Vue.use(Vuex);
const vuexPersist = new VuexPersist({
  key: "my-app",
  storage: window.localStorage
});

const store = new Vuex.Store({
  state: {
    cartProducts: []
  },
  mutations: {
    SET_USER_CART(state, data) {
      state.cartProducts = data;
    }
  },
  actions: {
    setUserDataCart({ commit }, data) {
      commit("SET_USER_CART", data);
    }
  },
  plugins: [vuexPersist.plugin]
});

export default store;

I tried multiple approaches like using mapState or using the state as a getter but no luck. Can anyone identify what am I doing wrong?

Answer 1

In Vue.js docs:

Computed properties are by default getter-only

The way you did you're trying to change a computed property without a set method. What could be happening is that changing only the object property and not creating a new one might not be triggering the reaction to update the UI.

I would actually recommend to take another approach and move the logic to store's mutation method, passing only the id of the product you need to increment/decrement. And also use a method that returns a brand new object after the update (like map() method I use to change cartProducts.

store.js:

import Vue from "vue";
import Vuex from "vuex";
import VuexPersist from "vuex-persist";

Vue.use(Vuex);
const vuexPersist = new VuexPersist({
  key: "my-app",
  storage: window.localStorage
});

export default new Vuex.Store({
  state: {
    cartProducts: [
      {
        id: 1,
        name: "Drug 1",
        quantity: 1
      },
      {
        id: 2,
        name: "Drug 2",
        quantity: 1
      }
    ]
  },
  mutations: {
    INCREMENT(state, productId) {
      state.cartProducts = state.cartProducts.map(product => {
        if (product.id === productId) {
          product.quantity += 1;
        }
        return product;
      });
    },
    DECREMENT(state, productId) {
      state.cartProducts = state.cartProducts.map(product => {
        if (product.id === productId) {
          product.quantity -= 1;
        }
        return product;
      });
    }
  },
  actions: {
    incrementQuantity({ commit }, productId) {
      commit("INCREMENT", productId);
    },
    decrementQuantity({ commit }, productId) {
      commit("DECREMENT", productId);
    }
  },
  plugins: [vuexPersist.plugin]
});

cart.js

<template>
  <table>
    <tbody>
      <tr v-for="product in cartProducts" :key="product.name">
        <td class="quantity">
          <button @click="decreaseQuantity(product)">decrease</button>
          <p>{{ product.quantity || 0 }}</p>
          <button @click="increaseQuantity(product)">increase</button>
        </td>
      </tr>
    </tbody>
  </table>
</template>

<script>
export default {
  computed: {
    cartProducts() {
      return this.$store.state.cartProducts;
    }
  },
  methods: {
    increaseQuantity(drug) {
      this.$store.dispatch("incrementQuantity", drug.id);
    },
    decreaseQuantity(drug) {
      this.$store.dispatch("decrementQuantity", drug.id);
    }
  }
};
</script>

Answer 2

Please read vuex documentation carefully. - https://vuex.vuejs.org/guide/ Changing the state should happened in the mutation methods. Here is an simple example from the Docs:

const store = new Vuex.Store({
  state: {
    count: 0
  },
  mutations: {
    increment (state) {
      state.count++
    }
  },
  actions: {
    increment (context) {
      context.commit('increment')
    }
  }
})