vue.js $watch array of objects

Question

mounted: function() {
  this.$watch(\'things\', function(){console.log(\'a thing changed\')}, true);
}

things is an array of objects

Answer 1

You can watch each element in an array or dictionary for change independently with $watch('arr.0', () => {}) or $watch('dict.keyName', () => {})

from https://vuejs.org/v2/api/#vm-watch:

Note: when mutating (rather than replacing) an Object or an Array, the old value will be the same as new value because they reference the same Object/Array. Vue doesn’t keep a copy of the pre-mutate value.

However, you can iterate the dict/array and $watch each item independently. ie. $watch('foo.bar') - this watches changes in the property 'bar' of the object 'foo'.

In this example, we watch all items in arr_of_numbers, also 'foo' properties of all items in arr_of_objects:

mounted() {
        this.arr_of_numbers.forEach( (index, val) => {
            this.$watch(['arr_of_numbers', index].join('.'), (newVal, oldVal) => {
                console.info("arr_of_numbers", newVal, oldVal);
            });
        });

        for (let index in this.arr_of_objects) {
            this.$watch(['arr_of_objects', index, 'foo'].join('.'), (newVal, oldVal) => {
                console.info("arr_of_objects", this.arr_of_objects[index], newVal, oldVal);
            });
        }

    },
    data() {
        return {
            arr_of_numbers: [0, 1, 2, 3],
            arr_of_objects: [{foo: 'foo'}, {foo:'bar'}]
        }
    }

Answer 2

If someone needs to get an item that was changed inside the array, please, check it:

JSFiddle Example

The post example code:

new Vue({
  ...
  watch: {
    things: {
      handler: function (val, oldVal) {
        var vm = this;
        val.filter( function( p, idx ) {
            return Object.keys(p).some( function( prop ) {
                var diff = p[prop] !== vm.clonethings[idx][prop];
                if(diff) {
                    p.changed = true;                        
                }
            })
        });
      },
      deep: true
    }
  },
  ...
})

Answer 3

You should pass an object instead of boolean as options, so:

mounted: function () {
  this.$watch('things', function () {
    console.log('a thing changed')
  }, {deep:true})
}

Or you could set the watcher into the vue instance like this:

new Vue({
  ...
  watch: {
    things: {
      handler: function (val, oldVal) {
        console.log('a thing changed')
      },
      deep: true
    }
  },
  ...
})

[demo]

Answer 4

There is a more simple way to watch an Array's items without having deep-watch: using computed values

{
  el: "#app",
  data () {
    return {
      list: [{a: 0}],
      calls: 0,
      changes: 0,
    }
  },
  computed: {
    copy () { return this.list.slice() },
  },
  watch: {
    copy (a, b) {
      this.calls ++
      if (a.length !== b.length) return this.onChange()
      for (let i=0; i<a.length; i++) {
        if (a[i] !== b[i]) return this.onChange()
      }
    }
  },
  methods: {
    onChange () {
      console.log('change')
      this.changes ++
    },
    addItem () { this.list.push({a: 0}) },
    incrItem (i) { this.list[i].a ++ },
    removeItem(i) { this.list.splice(i, 1) }
  }
}

https://jsfiddle.net/aurelienlt89/x2kca57e/15/

The idea is to build a computed value copy that has exactly what we want to check. Computed values are magic and only put watchers on the properties that were actually read (here, the items of list read in list.slice()). The checks in the copy watcher are actually almost useless (except weird corner cases maybe) because computed values are already extremely precise.