In the Composition API of Vue 3, the computed function can be used to implement responsive computed properties; 1. The basic writing method is to pass a function that returns a value to create read-only properties, such as computed(() => count.value * 2); 2. When it is necessary to be readable and writable, you can use the object form with get and set, such as updating firstName and lastName by assigning fullName.value; 3. When using it in a template, no .value is required, Vue will be automatically unpacked; computed is more suitable for derived values, while watch is suitable for side-effect operations. Reasonable choice can improve the simplicity of the code and the clarity of responsive logic.
Using computed attributes in Vue 3's Composition API is actually similar to the Options API, but the writing method is more flexible and the organizational logic is clearer. You can use computed
function to create a responsive calculated value, which will be automatically updated based on the dependent responsive data.
Below are some common usage methods and scenarios.
Basic writing method: use computed
to create read-only attributes
The easiest way to use it is to introduce computed
and then pass in a function that returns a value:
import { ref, calculated } from 'vue' export default { setup() { const count = ref(1) const double = computed(() => count.value * 2) return { count, double } } }
double
here is a calculation property. When count
changes, double
will automatically recalculate. This writing is suitable for most "read-only" situations.
Readable and writable computed properties (with getter and setter)
If you need to modify the value of the calculated attribute and inversely affect the original data, you can write it as an object with get
and set
:
const firstName = ref('John') const lastName = ref('Doe') const fullName = computed({ get: () => `${firstName.value} ${lastName.value}`, set: (value) => { const parts = value.split(' ') firstName.value = parts[0] lastName.value = parts[1] } })
This way you can update both firstName
and lastName
by assigning fullName.value = 'Jane Smith'
. This trick is useful in form linkage or two-way binding.
Notes on using computed in templates
When using the value returned computed
directly in the template, be careful to call .value
like using a normal variable, because it is implemented based on ref
:
<template> <div>Count: {{ count }}</div> <div>Double: {{ double }}</div> </template>
Although double
is a ref
, there is no need to add .value
to the template, Vue will automatically unpack.
The difference between watch: When should I use computed?
- computered is more suitable for derive new values ??based on existing responsive data.
- Watch is more suitable for executing side effects, such as asynchronous requests, operating DOMs, etc.
For example, if you want to do a search filtering function:
- Use
computed
to filter the list based on the input content; - Use
watch
to listen for input changes and initiate API requests.
So remember: try not to use watch if you can solve problems with computed , because it is more efficient and more declarative.
Basically that's it. Using computed well can make your code more concise and responsive logic clearer. What is not complicated but easy to ignore is to remember to distinguish between read-only and writable usage scenarios, and whether .value is needed in the template.
The above is the detailed content of How to use computed properties with Composition API?. For more information, please follow other related articles on the PHP Chinese website!

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