When is the WU function called in Vue?

Sorry, but there is no concept called "WU function" in Vue.js.

Perhaps you meant one of the following concepts or functions:

1. setup in Vue 3 Composition API: This is a new component option where reactive logic, state, and methods are placed.

   // Example setup function
   import { ref, onMounted } from 'vue';
   
   export default {
     setup() {
       const count = ref(0); // Reactive state
   
       function increment() {
         count.value++;
       }
   
       onMounted(() => {
         // Lifecycle hook
         console.log('Component mounted');
       });
   
       return { // Return data and functions for use in template
         count,
         increment
       };
     }
   }
   

   setup is called before the component instance is created, immediately after props are resolved and before the instance itself is created.

2. watch in Options API or Composition API: Used to track changes in reactive data.

   // Watch in Options API
   export default {
     data() {
       return {
         message: 'Hello'
       }
     },
     watch: {
       message(newValue, oldValue) {
         console.log(`Message changed from "${oldValue}" to "${newValue}"`);
       }
     }
   }
   

   // Watch in Composition API
   import { ref, watch } from 'vue';
   
   export default {
     setup() {
       const message = ref('Hello');
   
       watch(message, (newValue, oldValue) => {
         console.log(`Message changed from "${oldValue}" to "${newValue}"`);
       });
   
       return {
         message
       };
     }
   }
   

   watch is called when the tracked data source changes.

3. methods in Options API: Used to add functions to the component instance. Called as needed (e.g., on button click).

   // Methods in Options API
   export default {
     methods: {
       greet() {
         console.log('Hello!');
       }
     }
   }
   

   Methods are called when explicitly invoked (e.g., from template @click="greet" or from another method).

If you meant something else, please clarify. It might be a typo or a specific function name in a particular project or library.