Plugins

Vuex stores accept the plugins option that exposes hooks for each mutation. A Vuex plugin is simply a function that receives the store as the only argument:

const myPlugin = store => {
  // called when the store is initialized
  store.subscribe((mutation, state) => {
    // called after every mutation.
    // The mutation comes in the format of `{ type, payload }`.
  })
}

And can be used like this:

const store = new Vuex.Store({
  // ...
  plugins: [myPlugin]
})

Committing Mutations Inside Plugins

Plugins are not allowed to directly mutate state - similar to your components, they can only trigger changes by committing mutations.

By committing mutations, a plugin can be used to sync a data source to the store. For example, to sync a websocket data source to the store (this is just a contrived example, in reality the createPlugin function can take some additional options for more complex tasks):

export default function createWebSocketPlugin (socket) {
  return store => {
    socket.on('data', data => {
      store.commit('receiveData', data)
    })
    store.subscribe(mutation => {
      if (mutation.type === 'UPDATE_DATA') {
        socket.emit('update', mutation.payload)
      }
    })
  }
}
const plugin = createWebSocketPlugin(socket)

const store = new Vuex.Store({
  state,
  mutations,
  plugins: [plugin]
})

Taking State Snapshots

Sometimes a plugin may want to receive "snapshots" of the state, and also compare the post-mutation state with pre-mutation state. To achieve that, you will need to perform a deep-copy on the state object:

const myPluginWithSnapshot = store => {
  let prevState = _.cloneDeep(store.state)
  store.subscribe((mutation, state) => {
    let nextState = _.cloneDeep(state)

    // compare `prevState` and `nextState`...

    // save state for next mutation
    prevState = nextState
  })
}

Plugins that take state snapshots should be used only during development. When using webpack or Browserify, we can let our build tools handle that for us:

const store = new Vuex.Store({
  // ...
  plugins: process.env.NODE_ENV !== 'production'
    ? [myPluginWithSnapshot]
    : []
})

The plugin will be used by default. For production, you will need DefinePlugin for webpack or envify for Browserify to convert the value of process.env.NODE_ENV !== 'production' to false for the final build.

Built-in Logger Plugin

If you are using vue-devtools you probably don't need this.

Vuex comes with a logger plugin for common debugging usage:

import createLogger from 'vuex/dist/logger'

const store = new Vuex.Store({
  plugins: [createLogger()]
})

The createLogger function takes a few options:

const logger = createLogger({
  collapsed: false, // auto-expand logged mutations
  filter (mutation, stateBefore, stateAfter) {
    // returns `true` if a mutation should be logged
    // `mutation` is a `{ type, payload }`
    return mutation.type !== "aBlacklistedMutation"
  },
  transformer (state) {
    // transform the state before logging it.
    // for example return only a specific sub-tree
    return state.subTree
  },
  mutationTransformer (mutation) {
    // mutations are logged in the format of `{ type, payload }`
    // we can format it any way we want.
    return mutation.type
  }
})

The logger file can also be included directly via a <script> tag, and will expose the createVuexLogger function globally.

Note the logger plugin takes state snapshots, so use it only during development.

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