The role of watcher in vue

The role of Watcher in Vue is to observe data changes and perform corresponding operations. Specific scenarios include: listening to data changes, triggering UI updates, asynchronous data operations, and implementing custom logic.

The role of Watcher in Vue

In Vue, Watcher is used to observe data changes and execute corresponding The object of the operation. When the observed data changes, Watcher will trigger the corresponding callback function.

The role of Watcher

Watcher is mainly used in the following scenarios:

• Listen to data changes:When the data When a change occurs, the Watcher will be automatically triggered to perform the operations in the callback function.
• Trigger UI update: When the UI needs to be updated after data changes, Vue's responsive update mechanism can be triggered through Watcher.
• Asynchronous data operation: When you need to operate on asynchronously obtained data, you can use Watcher to wait for the data to return and trigger the callback function.
• Implement custom logic: By defining a custom Watcher, more complex business logic can be implemented, such as conditional judgment, data verification, etc.

Usage of Watcher

In Vue, you can use the watch option to define Watcher, the format is as follows:

<code class="js">watch: {
  // 被观察的数据
  propertyName: {
    // 数据发生变化时触发的回调函数
    handler(newValue, oldValue) {
      // 要执行的操作
    },
    // 是否立即执行回调函数（默认 false）
    immediate: true,
  },
}</code>

Example

The following is a simple example that demonstrates how to use Watcher to update the UI:

<code class="js">const App = {
  data() {
    return {
      count: 0,
    };
  },
  watch: {
    count(newValue, oldValue) {
      console.log(`计数从 ${oldValue} 变为 ${newValue}`);
    },
  },
  template: `<p>计数：{{ count }}</p>`,
};</code>

In this example, when count data When a change occurs, the Watcher will trigger the callback function and print out the information about the data change.

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