Vuex

To manage the state of an application you should use Vuex.

Note: All of the below is explained in more detail in the official Vuex documentation.

Separation of concerns

Vuex is composed of State, Getters, Mutations, Actions and Modules.

When a user clicks on an action, we need to dispatch it. This action will commit a mutation that will change the state. Note: The action itself will not update the state, only a mutation should update the state.

File structure

When using Vuex at GitLab, separate this concerns into different files to improve readability:

└── store
  ├── index.js          # where we assemble modules and export the store
  ├── actions.js        # actions
  ├── mutations.js      # mutations
  ├── getters.js        # getters
  ├── state.js          # state
  └── mutation_types.js # mutation types

The following example shows an application that lists and adds users to the state. (For a more complex example implementation take a look at the security applications store in here)

`index.js`

This is the entry point for our store. You can use the following as a guide:

import Vue from 'vue';
import Vuex from 'vuex';
import * as actions from './actions';
import * as getters from './getters';
import mutations from './mutations';
import state from './state';

Vue.use(Vuex);

export const createStore = () => new Vuex.Store({
  actions,
  getters,
  mutations,
  state,
});
export default createStore();

`state.js`

The first thing you should do before writing any code is to design the state.

Often we need to provide data from haml to our Vue application. Let's store it in the state for better access.

  export default {
    endpoint: null,

    isLoading: false,
    error: null,

    isAddingUser: false,
    errorAddingUser: false,

    users: [],
  };

Access `state` properties

You can use mapState to access state properties in the components.

`actions.js`

An action is a payload of information to send data from our application to our store.

An action is usually composed by a type and a payload and they describe what happened. Enforcing that every change is described as an action lets us have a clear understanding of what is going on in the app.

In this file, we will write the actions that will call the respective mutations:

  import * as types from './mutation_types';
  import axios from '~/lib/utils/axios_utils';
  import createFlash from '~/flash';

  export const requestUsers = ({ commit }) => commit(types.REQUEST_USERS);
  export const receiveUsersSuccess = ({ commit }, data) => commit(types.RECEIVE_USERS_SUCCESS, data);
  export const receiveUsersError = ({ commit }, error) => commit(types.REQUEST_USERS_ERROR, error);

  export const fetchUsers = ({ state, dispatch }) => {
    dispatch('requestUsers');

    axios.get(state.endpoint)
      .then(({ data }) => dispatch('receiveUsersSuccess', data))
      .catch((error) => {
        dispatch('receiveUsersError', error)
        createFlash('There was an error')
      });
  }

  export const requestAddUser = ({ commit }) => commit(types.REQUEST_ADD_USER);
  export const receiveAddUserSuccess = ({ commit }, data) => commit(types.RECEIVE_ADD_USER_SUCCESS, data);
  export const receiveAddUserError = ({ commit }, error) => commit(types.REQUEST_ADD_USER_ERROR, error);

  export const addUser = ({ state, dispatch }, user) => {
    dispatch('requestAddUser');

    axios.post(state.endpoint, user)
      .then(({ data }) => dispatch('receiveAddUserSuccess', data))
      .catch((error) => dispatch('receiveAddUserError', error));
  }

Actions Pattern: `request` and `receive` namespaces

When a request is made we often want to show a loading state to the user.

Instead of creating an action to toggle the loading state and dispatch it in the component, create:

An action requestSomething, to toggle the loading state
An action receiveSomethingSuccess, to handle the success callback
An action receiveSomethingError, to handle the error callback
An action fetchSomething to make the request.
1. In case your application does more than a GET request you can use these as examples:
  - POST: createSomething
  - PUT: updateSomething
  - DELETE: deleteSomething

The component MUST only dispatch the fetchNamespace action. Actions namespaced with request or receive should not be called from the component The fetch action will be responsible to dispatch requestNamespace, receiveNamespaceSuccess and receiveNamespaceError

By following this pattern we guarantee:

All applications follow the same pattern, making it easier for anyone to maintain the code
All data in the application follows the same lifecycle pattern
Actions are contained and human friendly
Unit tests are easier
Actions are simple and straightforward

Dispatching actions

To dispatch an action from a component, use the mapActions helper:

import { mapActions } from 'vuex';

{
  methods: {
    ...mapActions([
      'addUser',
    ]),
    onClickUser(user) {
      this.addUser(user);
    },
  },
};

`mutations.js`

The mutations specify how the application state changes in response to actions sent to the store. The only way to change state in a Vuex store should be by committing a mutation.

It's a good idea to think of the state before writing any code.

Remember that actions only describe that something happened, they don't describe how the application state changes.

Never commit a mutation directly from a component

  import * as types from './mutation_types';

  export default {
    [types.REQUEST_USERS](state) {
      state.isLoading = true;
    },
    [types.RECEIVE_USERS_SUCCESS](state, data) {
      // Do any needed data transformation to the received payload here
      state.users = data;
      state.isLoading = false;
    },
    [types.REQUEST_USERS_ERROR](state, error) {
      state.isLoading = false;
    },
    [types.REQUEST_ADD_USER](state, user) {
      state.isAddingUser = true;
    },
    [types.RECEIVE_ADD_USER_SUCCESS](state, user) {
      state.isAddingUser = false;
      state.users.push(user);
    },
    [types.REQUEST_ADD_USER_ERROR](state, error) {
      state.isAddingUser = true;
      state.errorAddingUser = error;
    },
  };

`getters.js`

Sometimes we may need to get derived state based on store state, like filtering for a specific prop. Using a getter will also cache the result based on dependencies due to how computed props work This can be done through the getters:

// get all the users with pets
export const getUsersWithPets = (state, getters) => {
  return state.users.filter(user => user.pet !== undefined);
};

To access a getter from a component, use the mapGetters helper:

import { mapGetters } from 'vuex';

{
  computed: {
    ...mapGetters([
      'getUsersWithPets',
    ]),
  },
};

`mutation_types.js`

From vuex mutations docs:

It is a commonly seen pattern to use constants for mutation types in various Flux implementations. This allows the code to take advantage of tooling like linters, and putting all constants in a single file allows your collaborators to get an at-a-glance view of what mutations are possible in the entire application.

export const ADD_USER = 'ADD_USER';

How to include the store in your application

The store should be included in the main component of your application:

  // app.vue
  import store from 'store'; // it will include the index.js file

  export default {
    name: 'application',
    store,
    ...
  };

Communicating with the Store

<script>
import { mapActions, mapState, mapGetters } from 'vuex';
import store from './store';

export default {
  store,
  computed: {
    ...mapGetters([
      'getUsersWithPets'
    ]),
    ...mapState([
      'isLoading',
      'users',
      'error',
    ]),
  },
  methods: {
    ...mapActions([
      'fetchUsers',
      'addUser',
    ]),

    onClickAddUser(data) {
      this.addUser(data);
    }
  },

  created() {
    this.fetchUsers()
  }
}
</script>
<template>
  <ul>
    <li v-if="isLoading">
      Loading...
    </li>
    <li v-else-if="error">
      {{ error }}
    </li>
    <template v-else>
      <li
        v-for="user in users"
        :key="user.id"
      >
        {{ user }}
      </li>
    </template>
  </ul>
</template>

Vuex Gotchas

Do not call a mutation directly. Always use an action to commit a mutation. Doing so will keep consistency throughout the application. From Vuex docs:

why don't we just call store.commit('action') directly? Well, remember that mutations must be synchronous? Actions aren't. We can perform asynchronous operations inside an action.
```
  // component.vue

  // bad
  created() {
    this.$store.commit('mutation');
  }

  // good
  created() {
    this.$store.dispatch('action');
  }
```
Use mutation types instead of hardcoding strings. It will be less error prone.
The State will be accessible in all components descending from the use where the store is instantiated.

Testing Vuex

Testing Vuex concerns

Refer to vuex docs regarding testing Actions, Getters and Mutations.

Testing components that need a store

Smaller components might use store properties to access the data. In order to write unit tests for those components, we need to include the store and provide the correct state:

//component_spec.js
import Vue from 'vue';
import { createStore } from './store';
import component from './component.vue'

describe('component', () => {
  let store;
  let vm;
  let Component;

  beforeEach(() => {
    Component = Vue.extend(issueActions);
  });

  afterEach(() => {
    vm.$destroy();
  });

  it('should show a user', () => {
    const user = {
      name: 'Foo',
      age: '30',
    };

    store = createStore();

    // populate the store
    store.dispatch('addUser', user);

    vm = new Component({
      store,
      propsData: props,
    }).$mount();
  });
});

Testing Vuex actions and getters

Because we're currently using babel-plugin-rewire, you may encounter the following error when testing your Vuex actions and getters: [vuex] actions should be function or object with "handler" function

To prevent this error from happening, you need to export an empty function as default:

// getters.js or actions.js

// prevent babel-plugin-rewire from generating an invalid default during karma tests
export default () => {};