Let's talk about why the Vue listener is executed twice

Vue.js is a popular front-end JavaScript framework that provides us with a complete set of tools and features for developing efficient and easy-to-maintain web applications. One of the important functions is the listener, which is responsible for observing changes in data in the Vue instance. When the data changes, the listener will automatically update the view to achieve a responsive user interface.

In actual use, we sometimes find that the Vue listener is executed twice. This problem has caused confusion and trouble among many developers. So, why does the Vue listener execute twice?

Reason 1: Initial rendering and data changes

First of all, it is worth noting that the Vue listener is executed twice because the data bound to this listener is different from the data during the initial rendering. Triggered once when changing. For example, when we bind a data in a Vue component, and the data changes when the component is rendered, then the listener will be executed both during the initial rendering and when the data changes.

Specifically, Vue will execute the rendering function during the initial rendering. This rendering function will generate a virtual Dom and update the view based on this virtual Dom, which may trigger the execution of the Vue listener. When the data changes, Vue will execute the rendering function again, generate a new virtual Dom, and update the view. This process may also trigger the execution of the listener.

Reason 2: The way the Watcher is bound to the tag

Secondly, the Vue listener is executed twice because of its different binding methods. Vue's listeners are usually implemented through Watcher objects, and Watcher objects can be created through different binding methods. Under different binding methods, the number of execution times of the listener may be different.

For example, when we use the v-model directive to bind data, Vue will automatically bind the data to a Watcher object and perform two-way binding on the data. When the data changes, the Watcher object automatically triggers the listener function. Therefore, when we use v-model to bind data in the template, the listener function will be executed twice.

In addition, if we bind multiple tags to the same data in the template, the listener function will also be executed multiple times. For example, when we use v-model to bind the same data on both the input tag and the textarea tag, the listener function will be executed twice.

How to avoid?

So, how to prevent the Vue listener from executing twice? In fact, there is no fixed solution to this problem, because there are many reasons for the listener to be executed twice, and different situations may require different solutions. However, we can try the following solutions:

1. Reduce monitoring of data. If the view does not need to be updated when the data changes, then there is no need to listen for it.
2. Avoid making changes to data on initial render. Data can be initialized in the created life cycle to avoid data changes after mount.
3. Reasonable use of tags to bind data. You can use computed properties, methods, etc. to replace the v-model directive.

Summary

Vue’s listener is an important part of Vue’s responsiveness and one of the core features of the Vue framework. Executing the listener twice is not an error, but is related to rendering, binding methods, etc. By understanding the causes and solutions, we can make better use of listeners and build efficient and stable Vue applications.

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