How to use watch in vue

Watch in Vue is a reactive function used to monitor changes in data attribute values ​​and execute callback functions. The basic usage is watch(property, handler), where property is the property or property array to be monitored, and handler is the callback function. It can also configure option objects such as immediate (immediate call) and deep (deep listening). watch is suitable for situations where you need to react to changes in data property values, such as updating the UI or loading data asynchronously.

Usage of watch in Vue

What is watch

watch is a reactive function in Vue that allows you to monitor changes in data property values ​​and execute callback functions. When the monitored property changes, the callback function is called, where you can perform any necessary updates or operations.

Syntax

watch(property, handler)

• property: The property or array of properties to monitor.
• handler: The callback function that is called when the property value changes. It receives new value and old value as parameters.

Basic usage

To monitor a property, just pass in the property name and a callback function:

watch('count', (newValue, oldValue) => {
  // count 的新值是 newValue，旧值是 oldValue
})

Monitor multiple properties

To monitor multiple properties at once, you can pass an array of properties:

watch(['count', 'name'], (newValue, oldValue) => {
  // 监视的值在 newValue 中作为对象提供，键为属性名
})

Options object

You can use Options object to configure watch behavior:

watch({
  count: {
    handler(newValue, oldValue) {
      // ...
    },
    immediate: true,
    deep: true
  }
})

• immediate: If true, the callback function is called immediately when the monitored property is initialized.
• deep: If true, watch will monitor objects and arrays for depth changes instead of just reference changes.

Advanced usage

Listen to specific attribute paths

Use dot notation to monitor changes in object attribute paths :

watch('user.name', (newValue, oldValue) => {
  // ...
})

Use the return value

The watch callback function can return a function or a Promise containing the unwatch function:

• Function: This function is called when the component is destroyed, you can use it to clear any resources or unsubscribe from events.
• Promise: When the Promise is resolved, the watch will be dismissed.

When to use watch

watch is suitable for situations where you need to react to changes in data attribute values, such as:

• Update UI
• Trigger other calculated properties or methods
• Load data asynchronously
• Form validation

Alternatives

In some cases, computed properties may be an alternative to watch. However, computed properties are derived, meaning their values ​​are calculated from other reactive properties.

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