Introduction and summary of the usage of vue2.0 listening attributes

We often need to monitor changes in a certain attribute value at work. At this time, we need to use the monitoring attribute watch. Here I summarize the use of the watch attribute in three scenarios and hope it will be helpful to you:

1. Basic version monitoring:

The scenario is as follows: Enter your age in the input box. If the age is 0-15 years old, the prompt message: You are still a child. If the age is 15-25 years old, Prompt message: You are already a teenager. If you are over 25 years old, prompt message: You have grown up. According to the needs of the monitoring attribute, we get the following code:

<template>
  <p id="app">
    年齡：<input type="number" v-model="age"><br>
    提示信息：<span>{{infoMsg}}</span>
  </p>
</template>

<script>
export default {
  data() {
    return {
      age: "",
      infoMsg:""
    }
  },
  watch:{
    age:function(val,oldval){
      if(val>0 && val<15){
        this.infoMsg="你还是个小孩"
      }else if(val>15 && val<25){
        this.infoMsg="你已经是个少年"
      }else{
        this.infoMsg="你已经长大了"
      }
    }
  }
}
</script>

Here we need to Special explanationThe following is: the meaning of the two parameters in the monitoring attribute method: the first val is For the value of the property being monitored, oldval is the value before the property changes. Since this is a formal parameter, the name of the parameter can be defined by yourself, but the meaning of the parameter will not change.

2. Advanced version monitoring:

Let’s change the requirements: the basic rules remain unchanged, but due to changes in the background database (draw a circle and curse him) we need to submit a request like this The data structure

 data() {
    return {
      info: {
        age: ""
      },
      infoMsg: ""
    };
  },

Since we are monitoring changes in the age value of the attribute in the object info, we need to use in-depth monitoring. The specific code is as follows:

<script>
export default {
  data() {
    return {
      info: {
        age: &quot;&quot;
      },
      infoMsg: &quot;&quot;
    };
  },
  watch: {
    info: {
      handler: function(val, oldval) {
        var that = this;
        if (val.age > 0 && val.age < 15) {
          that.infoMsg = "你还是个小孩";
        } else if (val.age > 15 && val.age < 25) {
          that.infoMsg = "你已经是个少年";
        } else {
          that.infoMsg = "你已经长大了";
        }
      },
      deep: true
    }
  }
};
</script>

You need to pay attention here There are two points: 1. The function here cannot be replaced by an arrow function. If an arrow function is used, the point of this will be global. 2. You will notice that an additional attribute added here is deep, which means whether to enable deep monitoring. If it is enabled, the value is true, otherwise it is false.

3. The most advanced monitoring:

Regarding the second case, I don’t know if you have found a problem: what we monitor is the change of a certain attribute in an object, but if this object When there are many values ​​in it, although you can also use the second method to monitor, this will be a huge waste of resources. This is a monitoring flaw. As a rigorous programmer (smiling face ), such a thing must not be allowed to happen, so we can optimize the above code in combination with computed (computed attribute):

<template>
  <p id="app">
    年齡：<input type="number" v-model="info.age"><br>
    提示信息：<span>{{infoMsg}}</span>
  </p>
</template>

<script>
export default {
  data() {
    return {
      info: {
        age: "",
        name: "",
        hobit: ""
      },
      infoMsg: ""
    };
  },
  computed: {
    ageval: function() {
      return this.info.age;
    }
  },
  watch: {
    ageval: {
      handler: function(val, oldval) {
        var that = this;
        if (val > 0 && val < 15) {
          that.infoMsg = "你还是个小孩";
        } else if (val > 15 && val < 25) {
          that.infoMsg = "你已经是个少年";
        } else {
          that.infoMsg = "你已经长大了";
        }
      },
      deep: true
    }
  }
};
</script>

From the above we can see that this time we are monitoring the calculated attribute ageval, The calculated attribute returns the value of age in the info object. Comparing with the second code, we can find that one of the two codes is monitoring the object info, and the other is the value of age in the info object. The performance is greatly enhanced. This This is the result we expected.

Related recommendations:

Use Vue.js to monitor property changes

Computed properties and data monitoring of Vue.js

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