mounted method is fired before data loaded - VueJS

Question

I'm using Vue Resource to retrieve an images collection from a REST API. The request is sent in the created hook of my Vue component.

The problem is, I'm trying to access the retrieved data in the mounted hook, but the data isn't loaded.

I get this error in the console:

[Vue warn]: Error in mounted hook: "TypeError: Cannot read property 'forEach' of undefined"

Here is my component:

<script>
export default {
  data() {
    return { imgs : '' };
  },
  created() {
    // the full url is declare in my main.js
    this.imgs = this.$resource('acf/v3/pages/4');

    this.imgs.query().then((response) => {
      console.log('success', response);
      this.imgs = response.data.acf.gallery;
    }, (response) => {
      console.log('erreur', response);
    });
  },
  mounted() {
    // get the ref="image" in my dom template
    let imgs = this.$refs.image;

    imgs.forEach((img) => {
      // I do some stuff with imgs
    });
  }
}
</script>

If I wrap a setTimeout around the content of mounted, everything works fine.

So, I don't understand how I can wait for my data to load before the mounted hook is executed. Isn't this the role of the Vue lifecycle hooks?

Answer 1

Since the this.imgs.query() call is async, your mounted hook is being called before the then handler is setting this.imgs (which I'm assuming is being bound with v-for to an element in your template with an attribute ref="image"). So, even though the component has been mounted to the DOM, the $refs have not been set up yet.

I would make a method to "do some stuff with imgs" and then call that method in a $nextTick callback in the then handler of the async call. The callback passed to $nextTick will be "executed after the next DOM update cycle", meaning the $refs will be set up at that point.

<script>
export default {
  data() {
    return { imgs: '' };
  },
  created() {
    // the full url is declare in my main.js
    this.imgs = this.$resource('acf/v3/pages/4');

    this.imgs.query().then((response) => {
      console.log('success', response);
      this.imgs = response.data.acf.gallery;
      this.$nextTick(() => this.doStuffWithImgs());
    }, (response) => {
      console.log('erreur', response);
    });
  },
  methods: {
    doStuffWithImgs() {
      // get the ref="image" in my dom template
      let imgs = this.$refs.image;

      imgs.forEach((img) => {
        // I do some stuff with imgs
      });
    }
  }
}
</script>