How to get updated $refs on dynamically created co

Question

My amount of components depends on array quantity, so when i add new item to array, it should create new component.

When new component is created i want to get reference on it, and that's where i have a misunderstanding. Last added component is undefined when i try to get it.

But, if i trying to get reference on it after some time, it work. I guess it's because of asynchronous, but i'm not sure.

Why this is happening and if there a way to avoid using setTimeout?

<div id="app">
    <button @click="addNewComp">add new component</button>
    <new-comp
        v-for="compId in arr"
        :ref="`components`"
        :index="compId"
        ></new-comp>
    </div>
  <script type="text/x-template " id="compTemplate">
    <h1> I am a component {{index}}</h1>
</script>

---

Vue.component("newComp",{
  template:"#compTemplate",
  props:['index']
})
new Vue({
  el:"#app",
  data:{
    arr:[1,2,3,4]
  },
  methods:{
    addNewComp:function(){
      let arr = this.arr;
      let components = this.$refs.components;
      arr.push(arr.length+1);
      console.log("sync",components.length);
      console.log("sync",components[components.length-1])
      setTimeout(() => {
        console.log("async",components.length);
        console.log("async",components[components.length-1])
      }, 1);
    }
  }
})

codepen link

Answer 1

refs, and $refs, are not reactive.

If you want to pick up the updated value, you should wait until the next render cycle updates the DOM.

Instead of setTimeout you should use Vue.nextTick():

new Vue({
  el:"#app",
  data:{
    arr:[1,2,3,4]
  },
  methods:{
    addNewComp:function(){
      let arr = this.arr;
      let components = this.$refs.components;
      arr.push(arr.length+1);
      console.log("sync",components.length);
      console.log("sync",components[components.length-1])
      Vue.nextTick(() => {                                         // changed here
        console.log("async",components.length);
        console.log("async",components[components.length-1])
      });                                                          // changed here
    }
  }
})

And this is not a "hack", this is the proper way of doing it. From the official API docs:

Vue.nextTick( [callback, context] )

• Arguments:

  • {Function} [callback]
  • {Object} [context]

• Usage:

  Defer the callback to be executed after the next DOM update cycle. Use it immediately after you've changed some data to wait for the DOM update.

  // modify data
  vm.msg = 'Hello'
  // DOM not updated yet
  Vue.nextTick(function () {
    // DOM updated
  })
  
  // usage as a promise (2.1.0+, see note below)
  Vue.nextTick()
    .then(function () {
      // DOM updated
    })