How to listen for 'props' changes

You can watch props to execute some code upon props changes:

new Vue({
  el: '#app',
  data: {
    text: 'Hello'
  },
  components: {
    'child' : {
      template: `<p>{{ myprop }}</p>`,
      props: ['myprop'],
      watch: { 
      	myprop: function(newVal, oldVal) { // watch it
          console.log('Prop changed: ', newVal, ' | was: ', oldVal)
        }
      }
    }
  }
});

<script src="https://unpkg.com/vue/dist/vue.js"></script>

<div id="app">
  <child :myprop="text"></child>
  <button @click="text = 'Another text'">Change text</button>
</div>

Have you tried this ?

watch: {
  myProp: {
    // the callback will be called immediately after the start of the observation
    immediate: true, 
    handler (val, oldVal) {
      // do your stuff
    }
  }
}

https://v2.vuejs.org/v2/api/#watch

In my case I needed a solution where anytime any props would change, I needed to parse my data again. I was tired of making separated watcher for all my props, so I used this:

  watch: {
    $props: {
      handler() {
        this.parseData();
      },
      deep: true,
      immediate: true,
    },
  },

Key point to take away from this example is to use deep: true so it not only watches $props but also it's nested values like e.g. props.myProp

You can learn more about this extended watch options here: https://v2.vuejs.org/v2/api/#vm-watch

You need to understand, the component hierarchy you are having and how you are passing props, definitely your case is special and not usually encountered by the devs.

Parent Component -myProp-> Child Component -myProp-> Grandchild Component

If myProp is changed in parent component it will be reflected in the child component too.

And if myProp is changed in child component it will be reflected in grandchild component too.

So if myProp is changed in parent component then it will be reflected in grandchild component. (so far so good).

Therefore down the hierarchy you don't have to do anything props will be inherently reactive.

Now talking about going up in hierarchy

If myProp is changed in grandChild component it won't be reflected in the child component. You have to use .sync modifier in child and emit event from the grandChild component.

If myProp is changed in child component it won't be reflected in the parent component. You have to use .sync modifier in parent and emit event from the child component.

If myProp is changed in grandChild component it won't be reflected in the parent component (obviously). You have to use .sync modifier child and emit event from the grandchild component, then watch the prop in child component and emit an event on change which is being listened by parent component using .sync modifier.

Let's see some code to avoid confusion

Parent.vue

<template>
    <div>
    <child :myProp.sync="myProp"></child>
    <input v-model="myProp"/>
    <p>{{myProp}}</p>
</div>
</template>

<script>

    import child from './Child.vue'

    export default{
        data(){
            return{
                myProp:"hello"
            }
        },
        components:{
            child
        }
    }
</script>

<style scoped>
</style>

Child.vue

<template>
<div>   <grand-child :myProp.sync="myProp"></grand-child>
    <p>{{myProp}}</p>
</div>

</template>

<script>
    import grandChild from './Grandchild.vue'

    export default{
        components:{
            grandChild
        },
        props:['myProp'],
        watch:{
            'myProp'(){
                this.$emit('update:myProp',this.myProp)

            }
        }
    }
</script>

<style>

</style>

Grandchild.vue

<template>
    <div><p>{{myProp}}</p>
    <input v-model="myProp" @input="changed"/>
    </div>
</template>

<script>
    export default{
        props:['myProp'],
        methods:{
            changed(event){
                this.$emit('update:myProp',this.myProp)
            }
        }
    }
</script>

<style>

</style>

But after this you wont help notice the screaming warnings of vue saying

'Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders.'

Again as I mentioned earlier most of the devs don't encounter this issue, because it's an anti pattern. That's why you get this warning.

But in order to solve your issue (according to your design). I believe you have to do the above work around(hack to be honest). I still recommend you should rethink your design and make is less prone to bugs.

I hope it helps.