Does uniapp support custom instructions?

With the development of technology, web development has gradually concentrated on front-end frameworks. For front-end developers, frameworks such as Vue, React and Angular have become mainstream choices. In these frameworks, components are an important part of the development process. Components can make the code structure clearer, the functions more convenient, and the reusability higher. Of course, as part of a component, instructions are also an important part of component development. Directives are special attributes used in templates. They can modify the behavior of DOM elements, allowing us to use functions based on DOM operations in templates. So, are custom instructions also supported in uniapp?

uniapp is a cross-platform development framework that can compile a code base to multiple platforms at the same time, such as WeChat applet, Alipay applet, H5, App, etc. The components and instructions in uniapp are designed based on Vue's components and instructions and support most of Vue's functions and syntax. Therefore, uniapp can also implement custom instructions.

Custom directives can add some unique features to the application or modify the default behavior. In uniapp, Vue's instruction API is generally used to customize instructions. The instruction API in Vue defines the following hook function:

• bind: called when the instruction is bound to an element. Only called once, the instruction can obtain the current element, the value bound to the instruction, the expression bound to the instruction, and other information.
• inserted: Called when the bound element is inserted into the parent node (the existence of the parent node does not necessarily have to have been inserted into the document).
• update: Called when the VNode of the component is updated, but it may occur before its child VNode is updated.
• componentUpdated: Called after all the VNode of the component where the instruction is located and its sub-VNodes have been updated.
• unbind: Called when the instruction is unbound from the element.

The usage of custom instructions is prefixed with v- in HTML, followed by the name of the custom instruction. Then register the custom directive through Vue.directive and call the bound hook function.

Let’s take a look at an example of a custom instruction. Suppose we want to add a lazy loading effect to all images on the page, we can use Vue's lazyload plug-in to achieve this. First, install the plug-in in the project and introduce it:

npm install vue-lazyload --save

import VueLazyload from 'vue-lazyload'
Vue.use(VueLazyload)

Then, we can implement lazy loading through the custom instruction v-lazy:

<template>
  <img v-lazy="imgName" />
</template>

<script>
export default {
  data: {
    imgName: 'http://example.com/1.jpg'
  },
  directives: {
    lazy: {
      bind: function (el, binding) {
        // 初始化时使用loading图片
        el.setAttribute('src', 'http://example.com/loading.gif')

        // 使用Vue-lazyload插件加载图片
        VueLazyload.init()
      },
      inserted: function (el, binding) {
        // 图片加载完成后使用这个图片
        el.addEventListener('load', function () {
          el.setAttribute('src', binding.value)
        })
      }
    }
  }
}
</script>

In the example, we define a Instruction v-lazy, the job of this instruction is to replace the src attribute of the img tag with the real image link after loading. When binding the instruction, we set the current src attribute to loading.gif, and then use the VueLazyload plug-in to load the image. When inserting a DOM element, we listen to the load event and replace the current src attribute with the real link once the image is loaded.

In short, in uniapp, we can use the Vue instruction API to customize instructions to achieve some on-demand or customized functions. The registration and usage methods of instructions are basically the same as Vue. Developers can operate DOM elements through hook functions to achieve custom behaviors.

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