Vue2 watch Set Not Working

问题

I have a simple Vue app that is supposed to add a number to a Set when you click the "Add to Set" button --

https://codepen.io/jerryji/pen/mKqNvm?editors=1011

<div id="app">
  <input type="number" placeholder="enter a number" v-model="n">
  <button type="button" @click.prevent="addToSet()">Add to Set</button>
</div>

new Vue({
  el: "#app",
  data: {
    n: null,
    nSet: new Set()
  },
  methods: {
    addToSet: function() {
      this.nSet.add(this.n);
      console.log(this.n + ' added to Set');
    }
  },
  watch: {
    nSet: function (newVal, oldVal) {
      console.log(newVal);
    }
  }
});

Why is nothing logged in the console by the watch?

回答1:

Vue adds special handling for Arrays, but not for Sets. As a result, Vue doesn't automatically detect changes to Set membership. You can force an update, though

  this.nSet.add(this.n);
  this.$forceUpdate();

回答2:

It's because Vue doesn't support Set, Map, WeakSet and WeakMap. And it's because browsers didn't support these structures well. Especially WeakMap. But... They decided to support these structures. Maybe in version 3 - when they decide to drop support for older browsers. So, for now use an object, add properties with Vue.$set() and watch for changes with deep: true.

回答3:

Saving and re Setting the Set using the .values() method on Set worked for me and i didn't have to use $forceUpdate

Using $forceUpdate might be the more sensible way to go though. In some use cases in the past i have found forcing components to update to be problematic.

new Vue({
  el: "#app",
  data: {
    n: null,
    nSet: new Set()
  },
  methods: {
    addToSet: function() {
      let set = this.nSet;
      let newSet = set.add(this.n)
      this.nSet = new Set(newSet.values())
    }
  },
  watch: {
    nSet: function (newVal, oldVal) {
      console.log('newVal', ...newVal);
    }
  }
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.16/vue.min.js"></script>


<div id="app">
  <input type="number" placeholder="enter a number" v-model="n">
  <button type="button" @click.prevent="addToSet()">Add to Set</button>
  <p>n = {{ n }}</p>
</div>

来源：https://stackoverflow.com/questions/50911030/vue2-watch-set-not-working

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