While Vue Composition API RFC Reference site has many advanced use scenarios with the watch module, there are no examples of how to watch component props.
Neither is it mentioned in Vue Composition API RFC's main page or vuejs/composition-api in Github.
I've created a Codesandbox to elaborate on this issue.
<template>
<div id="app">
<img width="25%" src="./assets/logo.png">
<br>
<p>Prop watch demo with select input using v-model:</p>
<PropWatchDemo :selected="testValue"/>
</div>
</template>
<script>
import { createComponent, onMounted, ref } from "@vue/composition-api";
import PropWatchDemo from "./components/PropWatchDemo.vue";
export default createComponent({
name: "App",
components: {
PropWatchDemo
},
setup: (props, context) => {
const testValue = ref("initial");
onMounted(() => {
setTimeout(() => {
console.log("Changing input prop value after 3s delay");
testValue.value = "changed";
// This value change does not trigger watchers?
}, 3000);
});
return {
testValue
};
}
});
</script>
<template>
<select v-model="selected">
<option value="null">null value</option>
<option value>Empty value</option>
</select>
</template>
<script>
import { createComponent, watch } from "@vue/composition-api";
export default createComponent({
name: "MyInput",
props: {
selected: {
type: [String, Number],
required: true
}
},
setup(props) {
console.log("Setup props:", props);
watch((first, second) => {
console.log("Watch function called with args:", first, second);
// First arg function registerCleanup, second is undefined
});
// watch(props, (first, second) => {
// console.log("Watch props function called with args:", first, second);
// // Logs error:
// // Failed watching path: "[object Object]" Watcher only accepts simple
// // dot-delimited paths. For full control, use a function instead.
// })
watch(props.selected, (first, second) => {
console.log(
"Watch props.selected function called with args:",
first,
second
);
// Both props are undefined so it's just a bare callback func to be run
});
return {};
}
});
</script>
Although my question and code example were initially with JavaScript, I'm using TypeScript. Tony Tom's first answer although working, led to a type error. Which was solved by Michal Levý's answer. So I've tagged this question with typescript afterward.
Here is my polished yet barebones version of the reactive wirings for this custom select component, on top of <b-form-select> from bootstrap-vue (otherwise agnostic-implementation but this underlying component does emit @input and @change events both, based on whether the change was made programmatically or by user interaction).
<template>
<b-form-select
v-model="selected"
:options="{}"
@input="handleSelection('input', $event)"
@change="handleSelection('change', $event)"
/>
</template>
<script lang="ts">
import {
createComponent, SetupContext, Ref, ref, watch, computed,
} from '@vue/composition-api';
interface Props {
value?: string | number | boolean;
}
export default createComponent({
name: 'CustomSelect',
props: {
value: {
type: [String, Number, Boolean],
required: false, // Accepts null and undefined as well
},
},
setup(props: Props, context: SetupContext) {
// Create a Ref from prop, as two-way binding is allowed only with sync -modifier,
// with passing prop in parent and explicitly emitting update event on child:
// Ref: https://v2.vuejs.org/v2/guide/components-custom-events.html#sync-Modifier
// Ref: https://medium.com/@jithilmt/vue-js-2-two-way-data-binding-in-parent-and-child-components-1cd271c501ba
const selected: Ref<Props['value']> = ref(props.value);
const handleSelection = function emitUpdate(type: 'input' | 'change', value: Props['value']) {
// For sync -modifier where 'value' is the prop name
context.emit('update:value', value);
// For @input and/or @change event propagation
// @input emitted by the select component when value changed <programmatically>
// @change AND @input both emitted on <user interaction>
context.emit(type, value);
};
// Watch prop value change and assign to value 'selected' Ref
watch(() => props.value, (newValue: Props['value']) => {
selected.value = newValue;
});
return {
selected,
handleSelection,
};
},
});
</script>
If you take a look at watch typings here it makes it clear the first argument of watch could be an array, function or Ref<T>
props passed to the setup function are a reactive object (likely by readonly(reactive()), it's properties are getters. So what you are doing is passing the value of the getter as the 1st argument of watch, string "initial" in this case. Because Vue 2 $watch API is used under the hood (and same function exists in Vue 3), you are effectively trying to watch non-existent property with name "initial" on your component instance.
Your callback was only called once. The reason it was called at least once is because the new watch API is behaving like the current $watch with the immediate option (UPDATE 03/03/2021 - this was later changed and in release version of Vue 3, watch is lazy same way as it was in Vue 2)
So by accident you are doing the same thing Tony Tom suggested but with the wrong value. In both cases, it is not valid code when you are using TypeScript.
You can do this instead:
watch(() => props.selected, (first, second) => {
console.log(
"Watch props.selected function called with args:",
first,
second
);
});
Here the 1st function is executed immediately by Vue to collect dependencies (to know what should trigger the callback) and 2nd function is the callback itself.
Other way would be to convert props object using toRefs so it's properties would be of type Ref<T> and you can pass them as the first argument of watch.
However, most of the time watching props is not needed. Simply use props.xxx directly in your template (or setup) and let Vue do the rest.