The Element-UI cascading drop-down box realizes parent-child component communication through custom events: the parent component listens to the change event issued by the child component and obtains the selection result. The handleChange method is used to handle selection logic and can perform different operations based on hierarchical differences. In order to obtain intermediate process information, you need to customize events inside the cascading selection box, and one event is triggered at each selection level. For large data sets, it is necessary to optimize the data loading method and use packaging components for complex logic processing. Code readability and maintainability cannot be ignored, and clear naming and annotation can help with post-maintenance.

Vue and Element-UI cascaded drop-down boxes emit usage: more than just parent-child component communication

You must have encountered a cascading selection box when developing with Vue and Element-UI. It is very convenient, but do you really understand its underlying emit mechanism? Many developers simply use it, but ignore its powerful flexibility and potential pitfalls. In this article, we will explore in depth to help you thoroughly grasp the essence of this component and avoid some common mistakes. After reading it, you can not only be proficient in using it, but also customize more complex cascade selection logic by yourself.

Let’s talk about the conclusion first: The cascaded selection box of Element-UI is essentially used to realize parent-child component communication through custom event emit . The parent component listens for events triggered by the child component and gets the selection result. It seems simple, but there are many details to pay attention to in actual applications.

Let’s first review the relevant basics. Vue's $emit method is used to trigger custom events, and the parent component can listen to these events using the v-on directive (or the @ modifier). This mechanism is used inside the Element-UI's cascade selection box, which sends events to the parent component when the user's selection changes.

Let’s take a look at a simple example and understand the core concept:

 <code class="vue"><template> <div> <el-cascader v-model="value" :options="options"></el-cascader> <p>Selected Value: {{ value }}</p> </div> </template> <script> export default { data() { return { value: [], options: [ { value: &#39;zhejiang&#39;, label: &#39;浙江&#39;, children: [ { value: &#39;hangzhou&#39;, label: &#39;杭州&#39;, children: [ { value: &#39;xihu&#39;, label: &#39;西湖&#39; } ] } ] } ] }; }, methods: { handleChange(value) { console.log(&#39;Selected value changed:&#39;, value); // 在這里處理選擇后的邏輯，例如發(fā)送請求到后端} } }; </script></code>

This code shows the most basic usage. @change directive listens to change event of the cascading selection box. When the user's selection changes, the handleChange method will be called, and value parameter is the value selected by the user.

Now, let's go deeper. You may encounter this situation: You need to perform different actions according to the different levels you choose. At this time, relying solely on value parameter may not be enough. change event of Element-UI only provides the final selection result, without information about the intermediate process. To solve this problem, you need to judge and process it inside the handleChange function, or perform finer event distribution inside the cascade selection box.

For a more advanced usage, we can customize events inside the cascading selection box, for example, trigger an event when selecting each level:

 <code class="vue">// 這部分需要修改Element-UI的源碼，或者使用一個包裝組件// 這里只做概念性說明，實際實現(xiàn)比較復雜，需要深入了解Element-UI的源碼// 不推薦直接修改Element-UI源碼，建議使用包裝組件// ... (省略復雜代碼) ...</code>

Remember, it is very risky to modify the Element-UI source code directly and is not recommended. A better approach is to create a wrapper component, handle more complex logic in this component, and then pass information to the upper component through custom events.

Finally, regarding performance optimization, if your cascading selection box has a large amount of data, you need to consider optimizing the data structure and loading method to avoid loading all data at once. Asynchronous loading or virtual scrolling can be used to improve performance. The readability and maintainability of the code are also very important. Using clear naming and annotation can greatly reduce the cost of later maintenance. Remember, writing code is like building a house. Only by laying a solid foundation can you build a high-rise building. Don't sacrifice code quality for the sake of speed.

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