Implementation principle of v model in vue

Vue is one of the most popular front-end JavaScript frameworks today. Its data-driven view design concept is very practical in development. In order to simplify the binding of view data, Vue provides the v-model directive, through which the value of the form element can be two-way bound to the data in the Vue instance. Although the v-model instruction is very simple to use, its implementation principle is still worth exploring in depth. This article will introduce in detail the implementation principle of v model in vue.

1. The role of v-model

The main function of the v-model instruction in vue is to bidirectionally bind the form data and the data in the vue instance, that is, the user is in the form After entering the data, the entered value is automatically synchronized with the data in the vue instance. v-model is mainly used for form elements, including input, textarea, select, etc.

2. Usage of v-model

Using v-model, we can easily bind form elements to data in the Vue instance. For example, the following code implements an input box and a data binding:

<div id="app">
  <input type="text" v-model="message" />
  <p>{{message}}</p>
</div>

<script>
  var app = new Vue({
    el: '#app',
    data: {
      message: ''
    }
  })
</script>

After this code is run, enter any character in the input box, and the value in the input box will be displayed below.

3. The implementation principle of v-model

The implementation principle of the v-model instruction in Vue is actually implemented through some methods of combining binding events and properties.

1. Bind input events

Under the v-model directive, we can use a specific event for data binding. In the simplest case, for example, using v-model to bind a text input box, the actual bound event is an input event. Within Vue, the code implementation is roughly as follows:

input: function (el, value) {
    el.value = value == null ? '' : value;
    el.addEventListener('input', function (e) {
        value = e.target.value;
    });
    return function () {
        el.removeEventListener('input', function (e) {
            value = e.target.value;
        });
    }
}

In the above code, el is the element to which the event is bound. When the instruction is initially used, el.value is used to determine whether the content of the input box is in the initial state. . When an input event is monitored, the latest value is assigned to the value variable. When the component is destroyed, the event listener is also removed.

2. Bind prop

The input event listener will only update the view in one direction. Here you also need to bind prop to update the data to achieve two-way binding. In Vue, commonly used props include value and checked. In our example, since we need to bind the text input box, we bind the value attribute here. Within Vue, the code is implemented as follows:

bindValue: function (el, prop, value) {
    if (prop === 'value') {
        el.value = value == null ? '' : value;
    }
    return function () {
        if (prop === 'value') {
           // do something 
        }
    }
}

In the above code, prop is the attribute used to represent a specific value in the form element. At this time, we generally use the v-bind instruction for it. Bind value.

3. Bind update event

Vue data is updated asynchronously. At this time, the data may be updated but the view is not updated. In order to avoid this situation, after binding the input event and prop, we also need to perform some update events. The following is the implementation method in the source code of vue:

update: function (el) {
    el.dispatchEvent(new Event('input'));
},

The function of this function is to "force" trigger the input event on the element to update the view. Due to the mechanism of Vue binding events, when the data in the input box changes, the input event will be triggered first. At this time, the value variable listening to the input event will be updated, and this variable and the data in the Vue instance are bound to each other, so The update input event automatically updates the data in the Vue instance.

4. Application scenarios of v-model

In actual development, v-model is very practical and can greatly simplify the amount of our code. It is suitable for all front-end libraries and frameworks, and v-model is also widely used in vue. When using v-model to bind form data, we can get the form data directly in the Vue instance without going through the DOM API. At the same time, v-model also supports multiple data types of input boxes and default values, such as checkbox input boxes and radio input boxes, which is very flexible. Of course, it should be noted that v-model only applies to form elements, not ordinary HTML elements.

5. Summary

In Vue, the implementation principle of the v-model instruction is not complicated. It can achieve two-way data binding through some methods of binding events and properties. The application scenarios of v-model are also very wide and its status plays a decisive role. When using v-model, we need to pay attention to its scope of application and the default value binding of form elements. That’s all for now, I hope it can be helpful to everyone.

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