The method to correctly use the watch function in the Vue 3 Composition API is: 1. Import the watch function and listen to the return value of the ref or getter function, and execute a callback when the value of the listener changes; 2. If you listen to object or array properties, deep: true should be set to achieve deep listening; 3. To distinguish between watch and watchEffect, watch needs to explicitly specify the listening target, and watchEffect automatically tracks dependencies; 4. It is often used to monitor routing parameters, form verification, dynamic search and other scenarios. For example, listening for count changes can be written as watch(count, (newVal) => {...}), and listening for object properties, use watch(() => user.value.age, (newAge) => {...}), or you can add the deep option to listen for the entire object changes.
Using Watcher in Vue 3's Composition API is not to write a watch option directly like the Options API, but to use watch function. Although the form has changed, the core logic is still the same: listen for changes in a certain value and then perform side effects.
The key is to understand how to use it, when, and the difference between it and watchEffect .
How to use the watch function correctly?
Vue 3 provides a watch function that listens to and reacts to changes in responsive data. You can import it from vue :
import { watch } from 'vue'The basic way to use is to pass a responsive reference (ref) or return value getter function , and then pass a callback function to handle changes:
const count = ref(0)
watch(
() => count.value,
(newVal, oldVal) => {
console.log('count changed:', newVal)
}
)If you are passing a ref, you can directly pass it in:
watch(count, (newVal, oldVal) => {
console.log('count changed:', newVal)
}) But note: this method can only listen for changes in .value , and cannot deeply listen for object properties.
The difference between watch and watchEffect
Many people will confuse watch and watchEffect . Their differences are actually quite clear:
- watchEffect : Automatically track dependencies, and will be rerun as long as the responsive data used internally changes.
- watch : The value to be listened to needs to be explicitly specified, and it will only be triggered when that value changes.
For example:
const count = ref(0)
const name = ref('Alice')
watchEffect(() => {
console.log(count.value)
})
// Changing the name will not trigger watchEffect
name.value = 'Bob' // No output And watch only listens to the content you specified:
watch(count, () => {
console.log('count changed')
})
name.value = 'Bob' // will not triggerso:
- If you want to listen for specific data changes, use
watch - If you just want to do something after some data changes and don't care which one has changed, use
watchEffect
How to listen for changes in objects or arrays?
By default, watch is shallow-listening. That is to say, if you listen to an object or array, it will only be triggered when the entire object is replaced, and modifying a certain property of the object will not trigger the watcher.
To listen for changes inside the object, you can add a deep: true option:
const user = ref({ name: 'Alice', age: 20 })
watch(
() => user.value,
(newUser, oldUser) => {
console.log('user changed')
},
{ deep: true }
)Or, if you only care about a certain attribute:
watch(
() => user.value.age,
(newAge) => {
console.log('age changed to:', newAge)
}
)This is more accurate and has better performance.
Several common usage scenarios in actual development
- Listen to changes in routing parameters and refresh data
- Try verification after form field changes
- Dynamic search or filter based on user input
- Side effects are performed after data loading is completed
For example, listen for $route.params.id in a component:
import { useRoute } from 'vue-router'
export default {
setup() {
const route = useRoute()
watch(
() => route.params.id,
(newId) => {
// Get new data according to newId fetchData(newId)
}
)
}
}Basically that's it. Vue 3's Composition API turns watcher into a form of function calls. Although it may not be used to it at the beginning, once you master it, it will be more flexible and clear. The key is to figure out the details of what you want to monitor, how you want to monitor, and whether you want to monitor deeply.
The above is the detailed content of How to use watchers with Composition API?. For more information, please follow other related articles on the PHP Chinese website!
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