Immer data not updating in Vue

Question

I'm trying to use Immer with Vue. It appears that the state is updating, but Vue isn't updating the UI

// immutable.js
import produce, { applyPatches } from "immer"

let undo_buffer = []
export var state = { items: [] }
const handle_add_patch = (patch, inverse_patches) => {
  console.log("Inverse Patches: ", inverse_patches)
  undo_buffer.push(inverse_patches)
}
export const add_item = (item_name) => {
  console.log("Starting add_item call")
  const next_state = produce(
    state,
    draft => {
      draft.items.push({ name: item_name })
    },
    handle_add_patch
  )
  console.log("next state: ", next_state)
  state = next_state
}
export const undo = () => {
  const undo_patch = undo_buffer.pop()
  if (!undo_patch) return
  let new_state = applyPatches(state, undo_patch)
  console.log("New State: ", new_state)
  state = new_state
}

<!-- item_list.Vue -->
<template>
  <div>
    <button @click.prevent="add_item()">Add Item</button>
      {{ items }}
    <button @click.prevent="undo()">Undo</button>
  </div>
</template>
<script>
import * as immutable from './immutable.js'
export default {
  computed: {
    items: function(){ return immutable.state.items }
  },
  methods: {
    add_item(){
      console.log("State Before: ", immutable.state)
      immutable.add_item("Hello")
      console.log("State After: ", immutable.state)
    },
    undo(){
      console.log("State Before: ", immutable.state)
      immutable.undo()
      console.log("State After: ", immutable.state)
    }
  }
}
</script>

The console.log shows that the items array is changing, but the items in the Vue template just shows an empty array. How can I make this visible within Vue?

Answer 1

Computed properties are cached and they aren't recomputed until associated component data is changed. Since Immer object isn't a part of the component, recomputation never occurs.

Caching can be prevented by using getter method instead of computed property:

{{ getItems() }}

...

methods: {
  getItems: () => immutable.state.items
  addItem() {
    immutable.add_item("Hello");
    this.$forceUpdate();
  }
}

A more direct approach is to force a property to be recomputed:

data() {
  return { _itemsDirtyFlag: 0 }
},
computed: {
  items: {
    get() {
      this._itemsDirtyFlag; // associate it with this computed property
      return immutable.state.items;
    }
},
methods: {
  updateItems() {
    this._itemsDirtyFlag++;
  },
  addItem() {
    immutable.add_item("Hello");
    this.updateItems();
  }
}

A similar approach that uses Vue.util.defineReactive internal is used by vue-recomputed.

Answer 2

This issue is not related with Immer. vuejs caches the computed properties hence they do not update when your data updates. This is mentioned clearly in vuejs docs at https://v2.vuejs.org/v2/guide/computed.html#Computed-Caching-vs-Methods

Instead of a computed property, we can define the same function as a method. For the end result, the two approaches are indeed exactly the same. However, the difference is that computed properties are cached based on their reactive dependencies. A computed property will only re-evaluate when some of its reactive dependencies have changed.

Since Immer is not a reactive dependency, the computed property never runs again.

Solution for this would be to change the computed property to a method. This way it will not be cached and the method will run each time with updated value.