Skip to main content

watch event listener in vue2

 what is watch? It is an expression or method that monitors changes to a Vue instance. The callback function gets the new value and the old value as arguments, replaced by a function. Simply put: the role of watch can monitor the change of a value, and then call the method that needs to be executed because of the change. You can dynamically view the changes of the instance through watch, and you can also make further operations.

The properties of the watch can be configured:

  • immediate:true/false//Executed once when the instance object is initialized
  • deep:true //will monitor all properties in the object
  • handler()//Some custom operations can be performed in this function
  • Note that you should not use arrow functions to define watcher functions
  • The watch monitoring method can only monitor one variable at a time. Simply using watch cannot monitor multiple variables, but we return an object through the computed computed property, and then use watch to monitor the method of this computed property. As long as the value changes, the watch will be triggered. 

Next, look at the actual code:

    <div id="app">
        <input type="text" v-model="id"><br/>
        <input type="text" v-model="name"><br/>
        <button @click="IdAdd">clicl</button>
        <p>{{result}}</p>

    </div>

    <script>
     Vue.config.productionTip=false
        var vm = new Vue({
            el: '#app',
            data: {
                id:1001,
              name:"aaa",
               obj:{
                    hobby:"sing"
               }
            },
            methods: {
                IdAdd(){
                    this.obj.hobby="swim";
                    this.id+=100;
                }
            },
            computed:{
                result(){
                    this.obj.hobby="swim";
                    return this.id+"---"+this.name;
                }
            },
            watch:{
/*                 result:{
                    immediate:true,
                    deep:true,
                    handler(oldValue,newValue){
                        console.log("changed...",oldValue,newValue);
                    }
                },  */                            
                "obj.hobby":{
                    handler(oldValue,newValue){
                        console.log("changed...Hobby",oldValue,newValue);
                    }  
                },
/*                 'IdAdd':{
                    handler(){
                        console.log("changed...ID",oldValue,newValue);
                    },
                } */
            }
        });
    </script>

Another way of writing watch

        vm.$watch("obj.hobby",{
           
            immediate:true,
            deep:true,
            handler(oldValue,newValue){
                console.log("changed...Hobby",oldValue,newValue)
            }
        })


Labels:

Comments

Post a Comment

Popular posts from this blog

Native ajax usage

 What is AJAX? AJAX stands for "Asynchronous Javascript And XML" (Asynchronous JavaScript and XML), which refers to a web development technology for creating interactive web applications for data interaction between browsers and servers. AJAX uses asynchronous data transfer (HTTP requests) between the browser and the web server, which allows web pages to request small amounts of information from the server instead of the entire page. AJAX describes a web application architecture that mainly uses scripts to manipulate HTTP. The main feature of AJAX applications is to use scripts to manipulate HTTP and web servers for data exchange without causing page reloads. Simply put, without reloading the entire web page, AJAX loads data in the background and displays it on the web page, which can partially refresh the page. The user has a better experience using the web page, but cannot go back.
Post a Comment
Read more

Using ref to bind elements in Vue

 Vue provides us with a property to manipulate dom, ref. When bound to the dom element, it is used similar to the id. It is called through this.$refs, and it can also be bound to the component. It is very troublesome to obtain DOM elements in the previous native way. It needs to be obtained with such a long AP as: documentgetElementXXX/querySXXX.... Then jQuery was born to solve this pain point. The use of $("id") makes Our operation has become easier. But there are other problems, jQuery is just a library (plug-in) and cannot be used to build a framework. Now Vue can help us solve this problem. Vue also has its own method to obtain DOM, which is ref. It can get not only DOM elements but also components. If you add ref to the dom element, you can get the dom element, and if you add it to the component, you can get the component instance, and you can use the component method.
Post a Comment
Read more

What is the difference between the box-size property content-box and border-box in the css box model?

 The box model is a very important concept in CSS layout, it includes content area, padding, border, and margin. Box models can be divided into two types: standard box models and IE box models. The box model, as the name suggests, is used to hold things, and the things it holds are the content of HTML elements. In other words, every visible HTML element is a box.
Post a Comment
Read more