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Yannick PEREIRA-REIS

Full stack web developer Symfony2, VueJs and AngularJs in Lyon (France), I also play with DevOps tools like Docker, Vagrant, Elasticsearch, Logstash, Kibana, Supervisor, ...

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In the following code samples we are using ES6/ES2015/Babel.

Two way data binding with VueJS and Vuex

If you don’t really know what two way data binding is, let’s see a few definitions:

Two way binding means that any data-related changes affecting the model are immediately propagated to the matching view(s), and that any changes made in the view(s) (say, by the user) are immediately reflected in the underlying model. When app data changes, so does the UI, and conversely.

Two way binding just means that, when properties in the model get updated, so does the UI. When UI elements get updated, the changes get propagated back to the model.

It’s possible to use two way binding with javascript frameworks like AngularJs, ReactJs, VueJs….

Setup state/store with Vuex

Of course, to setup Vuex with Vuejs you need two things:

Let’s see a very simple VueJS/Vuex configuration

Click on the Result tab to see “VueJs two way binding” in action.

The external resources are important:

In this first example, if you open the debug tool of your browser (console), you should see this error message.

vue.min.js:6 Error: [vuex] Do not mutate vuex store state outside mutation handlers.
    at r (vuex.min.js:6)
    at nt.t._vm.$watch.deep (vuex.min.js:6)
    at ho.run (vue.min.js:7)
    at ho.update (vue.min.js:7)
    at qi.notify (vue.min.js:7)
    at Object.set [as lastname] (vue.min.js:6)
    at input (eval at li (vue.min.js:1), <anonymous>:2:416)
    at HTMLInputElement.t (vue.min.js:6)

This error is throw by Vuex, because we have enabled the Vuex strict mode.

This ensures that all state mutations can be explicitly tracked by debugging tools.

We’ll talk about this later in the article…

Is two way data binding a best practice or not ?

I see one big advantage to use two way binding:

  • Real time view updates thanks to Virtual DOM (very fast and not (re)rendering sub components if not needed).

It’s perfectly suitable for small applications with not too much real time state updates.

But I see many drawbacks too:

  • Watchers needed to update view when model is updated.
  • No way to track model updates in a centralized place.
  • Many places where state can be updated.
  • No way (not really true because we can rely on component watchers) to debounce or filter (or whatever) updates on state.

Vuex strict mode

const store = new Vuex.Store({
  // ...
  strict: true
})
  • This ensures that all state mutations can be explicitly tracked by debugging tools.
  • Whenever Vuex state is mutated outside of mutation handlers, an error will be thrown.
  • Strict mode runs a synchronous deep watcher on the state tree for detecting inappropriate mutations, and it can be quite expensive when you make large amount of mutations to the state. Make sure to turn it off in production to avoid the performance cost.

Solutions to track state changes and improve performance with Vuex

To remove the Vuex error and update the state in a mutation without adding or updating a lot of code, an option can be to deep clone the object before updating it with two way binding in our form.

Deep clone and watch

The following few lines show how to deep clone the object and how to see updates on the object and on the deep copy. We are using lodash to clone the object.

What are we doing here ?

  • We simply bypass the Vuex error.
  • We are still using two way binding, but with the cloned object and not the original one.
  • We must add a watcher/handler on this cloned object to track updates.
  • We add a mutation to update the state at the end of the process.
  • In the watcher/handler we can call the mutation to update the source object.

The computed property and the debounced mutation call, allows to track the effective state updates

computed: {
  userState() {
    return this.$store.state.user
  }
}

handler: _.debounce(function (user) {
  this.$store.commit('updateUser', user);
}, 500), deep: true

Be careful ! You cannot debounce the function in the mutation itself. Indeed, the code would be executed within the next event loop cycle, not really in the mutation function. The Vuex error will appear again.

  mutations: {
    updateUser: _.debounce(function (state, user) {
      Object.assign(state.user, user);
    },500)
  }

The “deep clone” solution is not really a perfect solution because we are still using two way binding, and we are adding a watcher manually which has performance cost. So… how can we improve the situation ?

One way data binding and explicit data update

Actually, two way binding is not something really needed in most cases. What about removing it and relying on one way binding and explicit data updates ?

What are we doing here ?

  • We are no more cloning the object.
  • We are using one way binding and explicitly updating object properties.
<input :value="user.lastname" v-on:keyup.stop="updateLastname($event.target.value)" />
<input :value="user.firstname" v-on:keyup.stop="updateFirstname($event.target.value)"/>
  • We need methods to update object properties.
methods: {
 updateFirstname(firstname) {
   this.$store.commit('updateUser', {firstname});
 },
 updateLastname(lastname) {
   this.$store.commit('updateUser', {lastname});
 }
}

And… that’s it !

After a small refactoring

It’s possible to refactor the code to have a single method to update object properties.

The interesting parts here are:

<input :value="user.lastname" v-on:keyup.stop="updateField('lastname', $event.target.value)" />
<input :value="user.firstname" v-on:keyup.stop="updateField('firstname', $event.target.value)"/>
methods: {
 updateField(field, value) {
   this.$store.commit('updateUser', {
    [field]: value
   });
 },
}

Refactor again and again

If we are updating a user in another VueJs component we need to duplicate this part of the code:

methods: {
 updateField(field, value) {
   this.$store.commit('updateUser', {
    [field]: value
   });
 },
}

What about moving the computed property key of the object literal in the mutation itself ?

methods: {
 updateField(field, value) {
   this.$store.commit('updateUser', {field, value});
 },
}

mutations: {
  updateUser: function (state, {field, value}) {
    Object.assign(state.user, {
    [field]: value
   });
  }
}

[EDIT] Two-way Computed Property

You could use a built-in VueJs functionality describred at the end of this page. But once again be careful because it works only for simple computed properties, not for object (with many depth levels potentially).

So if you have an object with 10 properties to update, you will need to set 10 two way computed properties (each with a get and a set method). It’s gonna be a lot more verbose than the previous solution.

This kinda configuration

computed: {
  prop1: {
    get () {
      return this.$store.state.obj.prop1
    },
    set (value) {
      this.$store.commit('updateprop1', value)
    }
  },
  prop2: {
    get () {
      return this.$store.state.obj.prop2
    },
    set (value) {
      this.$store.commit('updateprop2', value)
    }
  },
  ...
  ,
  prop10: {
    get () {
      return this.$store.state.obj.prop10
    },
    set (value) {
      this.$store.commit('updateprop10', value)
    }
  }
}

Conclusion

Two way data binding is really easy to setup with all major javascript frameworks (it’s often the default behavior). It’s a very good option for small applications or POC. But for very complex UI you should consider using one way data binding and explicit state updates/mutations.