How to watch only after the initial load from API in VueJS?-CodePudding

I am getting data from API with which I am populating the form in my component. I need to trigger watchers only after the initial populating of data. Like in async way. But the watcher is getting triggered immediately. I need to disable the Update button only if any value is changed after the initial populating of data.

<template>
  <div id="app">
    <input type="text" v-model="user.userId" /> <br />
    <br />
    <input type="text" v-model="user.title" /> <br />
    <br />
    <button :disabled="isDisabled">Update</button>
  </div>
</template>

<script>
export default {
  name: "App",
  watch: {
    user: {
      handler(oldVal, newVal) {
        if (oldVal != newVal) {
          this.isLoaded = false;
        }
      },
      deep: true,
    },
  },
  computed: {
    isDisabled() {
      return this.isLoaded;
    },
  },
  async created() {
    await fetch("https://jsonplaceholder.typicode.com/todos/1")
      .then((response) => response.json())
      .then((json) => {
        this.user = json;
        this.isLoaded = true;
      });
  },
  data() {
    return {
      user: {
        userId: 0,
        id: 0,
        title: "",
        completed: false,
      },
      isLoaded: true,
    };
  },
};
</script>

I have referred Vue, await for Watch and Are watches asynchronous? and Vue.js How to watcher before mounted() , can't get data from watch but I am unable to follow.

Here's a preview : https://codesandbox.io/embed/great-euler-skd3v?fontsize=14&hidenavigation=1&theme=dark

CodePudding user response：

This needs to be determined with some condition.

isLoaded already serves the purpose of determining the state of initial loading, but the name is confusing, because it determines that data is not loaded.

It can be:

  watch: {
    user: {
      if (this.isLoading && oldVal != newVal) {
        this.isLoading = false;
      }
      ...

The watcher doesn't need to be deep and could be unwatched when it's not needed:

async created() {
  let unwatchUser = this.$watch('user', (oldVal, newVal) => {
    if (this.isLoading && oldVal != newVal) {
      this.isLoading = false;
      unwatchUser();
    }
  })
  ...

A common way to designate that data hasn't been loaded yet is to set it to null, i.e. no value. This doesn't need isLoading flag or a watcher. If null is undesirable because of referred object properties, this can be overcome with optional chaining and conditional rendering:

  <div v-if="user">
      <input type="text" v-model="user.userId" />
      ...
  <div v-else class="spinner"/>

CodePudding user response：

The simplest answer for the question:

Q: How to watch only after the initial load from API in VueJS?

A: Add flag inside your watch (e.g. isLoaded).

Also there is couple things wrong with your code:

async/await in created does nothing,
isDisabled is not needed cause is basing only on 1 value from data. You can just use this value instead (isLoading).
If you api calls fail, isLoading flag will not change, better approach is to move it to finally.

Solution of your problem (codesandbox) :

<template>
  <div id="app">
    <div v-if="!isFetching">
      <input type="text" v-model="user.userId" /> <br />
      <br />
      <input type="text" v-model="user.title" /> <br />
      <br />
      <button :disabled="!isLoaded">Update</button>
    </div>
    <div v-else>Loading...</div>
  </div>
</template>

<script>
export default {
  name: "App",
  data() {
    return {
      user: {
        userId: 0,
        id: 0,
        title: "",
        completed: false,
      },
      isFetching: false,
      isLoaded: false
    };
  },
  watch: {
    user: {
      handler(oldVal, newVal) {
        if (!this.isFetching) {
          // this comparision doesn't work (cause oldVal/newVal is an object)
          if (oldVal != newVal) {
            this.isLoaded = false;
          }
        }
      },
      deep: true
    },
  },
  created() {
    this.isFetching = true;
    fetch("https://jsonplaceholder.typicode.com/todos/1")
      .then((response) => response.json())
      .then((json) => {
        this.user = json;
        this.isLoaded = true;
      })
      .finally(() => this.isFetching = false)
  },
};
</script>