How to use watch in Vue to monitor changes in responsive data

With the continuous development of front-end frameworks, Vue has become one of the front-end frameworks that many companies like to adopt, and it is relatively simple. In the process of using Vue to develop applications, responsive data is a feature we often use. In the control of responsive data changes, watch is a very useful feature provided by Vue.

In this article, we will introduce in detail how to use watch to monitor changes in responsive data in Vue, as well as some precautions.

1. What is Watch

watch is a responsive way of monitoring data changes in Vue. It uses Object.defineProperty() to monitor changes in responsive data. When data changes are monitored, relevant logical operations can be performed. In Vue, we can define a watcher in a component or on an object when creating a Vue instance.

2. Use watch

Use the Vue.watch() method to define a function that can be used to monitor changes in specified data, and the specified function completes the specified logical step.

The following is the basic syntax of Vue.watch():

watch: { 
  数据名: function() {
    //执行操作
  }
}

In a Vue instance, we can use the watch attribute to define a listener. For example, in the following code, we declare We have a Vue instance, which has a data attribute message. We use the watch attribute to monitor changes in the message:

var vm = new Vue({ 
  el: '#app', 
  data: { 
    message: 'Hello World!' 
  },
  watch: { 
    message: function (newValue, oldValue) { 
      console.log('message发生了变化，新值为：' + newValue + ',旧值为：' + oldValue); 
    } 
  } 
});

In this example, the initial value of the message is Hello World! If we execute the following code, change the value of message to Hello React! , we will see it printed in the console:

'message发生了变化，新值为：Hello React！,旧值为：Hello World!'

Through the above example, we can find that using the watch function is very simple, just set the properties to be monitored and the logical operations to be performed.

3. watch methodMore advanced tips

In addition to defining a separate observer, you can also use the general observer option with multiple options. The following is the sample code:

watch: { 
  'state.user': { 
    handler: function (val, oldVal) { 
      console.log('new: %s, old: %s', val.name, oldVal.name) 
    }, 
    deep: true 
  } 
}

In this example, we use object notation and property paths to listen for changes in the nested property state.user property. We defined a function in the handler option to handle each change, and used the deep attribute setting to perform deep monitoring on the Vue data object.

In addition to using deep monitoring, there are some other advanced techniques that can be used, such as immediate, returning a function that cancels observation, etc.

4. Notes

When using the watch function, you need to pay attention to the following matters:

• Avoid using deep monitoring, thinking that this will reduce Monitoring efficiency.
• watch can monitor changes in array and object values, but do not directly change the values ​​of arrays or objects. You can use the API methods provided by Vue to operate.
• Do not modify the data being monitored in the listening function, as it may cause an infinite loop.

In short, the watch function is one of the important ways for Vue to monitor responsive data changes. You can use it to easily monitor data changes, grasp the status of the application in time, and further change the display effect of the application. Learning to use watch and following the above precautions will help improve the running efficiency and development efficiency of Vue applications.

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