2-way binding in Vue 2.3 component

Question

I understand the .sync modifier returned in Vue 2.3, and am using it for a simple child component which implements a 'multiple-choice' question and answer. The parent component calls the child like this:

<question
  :stem="What is your favourite colour?"
  :options="['Blue', 'No, wait, aaaaargh!']
  :answer.sync="userChoice"
>

The parent has a string data element userChoice to store the result from the child component. The child presents the question and radio buttons for the options. The essential bits of the child look like this (I'm using Quasar, hence q-radio):

<template>
  <div>
    <h5>{{stem}}</h5>
    <div class="option" v-for="opt in options">
      <label >
        <q-radio v-model="option" :val="opt.val" @input="handleInput"></q-radio>
        {{opt.text}}
      </label>
    </div>
  </div>
</template>

export default {
  props: {
    stem: String,
    options: Array,
    answer: String
  },
  data: () => ({
    option: null
  }),
  methods: {
    handleInput () {
      this.$emit('update:answer', this.option)
    }
  }
}

This is all working fine, apart from the fact that if the parent then changes the value of userChoice due to something else happening in the app, the child doesn't update the radio buttons. I had to include this watch in the child:

  watch: {
    answer () {
      this.option = this.answer
    }
  }

But it feels a little redundant, and I was worried that emitting the event to update the parent's data would in fact cause the child 'watch' event to also fire. In this case it would have no effect other than wasting a few cycles, but if it was logging or counting anything, that would be a false positive...

Maybe that is the correct solution for true 2-way binding (i.e. dynamic Parent → Child, as well as Child → Parent). Did I miss something about how to connect the 'in' and 'out' data on both sides?

In case you're wondering, the most common case of the parent wanting to change 'userChoice' would be in response to a 'Clear Answers' button which would set userChoice back to an empty string. That should have the effect of 'unsetting' all the radio buttons.

Answer 1

Your construction had some oddities that didn't work, but basically answer.sync works if you propagate it down to the q-radio component where the changing happens. Changing the answer in the parent is handled properly, but to clear values, it seems you need to set it to an object rather than null (I think this is because it needs to be assignable).

Update Your setup of options is a notable thing that didn't work.

I use answer in the q-radio to control its checked state (v-model has special behavior in a radio, which is why I use value in conjunction with v-model). From your comment, it looks like q-radio wants to have a value it can set. You ought to be able to do that with a computed based on answer, which you would use instead of your option data item: the get returns answer, and the set does the emit. I have updated my snippet to use the val prop for q-radio plus the computed I describe. The proxyAnswer emits an update event, which is what the .sync modifier wants. I also implemented q-radio using a proxy computed, but that's just to get the behavior that should already be baked-into your q-radio.

(What I describe is effectively what you're doing with a data item and a watcher, but a computed is a nicer way to encapsulate that).

new Vue({
  el: '#app',
  data: {
    userChoice: null,
    options: ['Blue', 'No, wait, aaaaargh!'].map(v => ({
      value: v,
      text: v
    }))
  },
  components: {
    question: {
      props: {
        stem: String,
        options: Array,
        answer: String
      },
      computed: {
        proxyAnswer: {
          get() {
            return this.answer;
          },
          set(newValue) {
            this.$emit('update:answer', newValue);
          }
        }
      },
      components: {
        qRadio: {
          props: ['value', 'val'],
          computed: {
            proxyValue: {
              get() {
                return this.value;
              },
              set(newValue) {
                this.$emit('input', newValue);
              }
            }
          }
        }
      }
    }
  },
  methods: {
    clearSelection() {
      this.userChoice = {};
    }
  }
});

<script src="//cdnjs.cloudflare.com/ajax/libs/vue/2.3.3/vue.min.js"></script>
<div id="app">
  <question stem="What is your favourite colour?" :options="options" :answer.sync="userChoice" inline-template>
    <div>
      <h5>{{stem}}</h5>
      <div class="option" v-for="opt in options">
        <div>Answer={{answer && answer.text}}, option={{opt.text}}</div>
        <label>
        <q-radio :val="opt" v-model="proxyAnswer" inline-template>
          <input type="radio" :value="val" v-model="proxyValue">
        </q-radio>
        {{opt.text}}
      </label>
      </div>
    </div>
  </question>
  <button @click="clearSelection">Clear</button>
</div>