vue this.$ error undefined problem and solution

1. Background and problem analysis

background

Use ref to register a child component. The parent component can call functions in the child component through this.$.

When this.$ is called when the page is initialized, it prompts undefined.

analyze

At first I thought the method failed, but later I found that because the ref itself was created as a rendering result, you cannot access them during the initial rendering - they do not exist yet! $refs is not responsive either, so you should not try to use it to do data binding in templates.

In other words, ref can only call this.$refs after the page is loaded. If you use v-if and v-for to render the page, then you will not get this.$refs before the page is rendered at the beginning, so you have to wait until the page is rendered before you can get it.

Check out the data analysis

In , ref belongs to a special attribute that registers reference information for DOM elements or child components. The parent component can access the methods or data of the child component through this.$ . However, in some cases, developers may encounter a problem: trying to access this.$refs when the page is initialized, but the result is undefined.

The root cause of this problem lies in how ref works. ref is created during component rendering, and if you try to access them during the initial rendering phase of the component, it will fail because they have not been generated at this time. Additionally, the $refs object is not responsive and should not be used for data binding in templates.

Especially when using v-if or v-for directives, these directives dynamically render elements based on conditions, meaning that elements or child components registered with ref do not exist before the condition is established. Therefore, access via this.$refs is only possible after Vue has completed the necessary DOM rendering.

2. Solution

1. Make sure that the DOM is loaded and then called

To resolve this, you need to make sure that ref is called after the DOM is fully loaded and rendered.

Vue provides lifecycle hooks such as mounted, and $nextTick methods to handle such asynchronous updates.

Life cycle understanding:

• The created hook is executed after the instance is created, at which time the component's data has been initialized, but the DOM has not yet been rendered.
• The mounted hook is executed after the component is mounted to its DOM element, and the DOM can be accessed.

Get the DOM node stage

use$nextTick:

methods: {
  demoMethod() {
    this.$nextTick(() => {
      ...
    });
  }
}

2. Avoid subcomponents being called immediately upon first loading

In some cases, you may not want the child components to perform certain operations when they are first loaded. This can be done by the component'sdataDefine a state flag in the function to implement it.

Define the first call status:

data() {
  return {
    uploadStatus: {
      isFirstLoad: true
    }
  };
}

• Applicable to a specific scene, then the first rendering is not executed,
• It should be noted that the defined variables need to be under the child component,
• Otherwise, it wouldn't be available

In a template or method, this status flag can be checked and based on it decide whether to execute a specific logic.

But be aware that this state variable must be defined in the subcomponent that needs to be checkeddataIn option, otherwise the parent component still cannot access it on first rendering.

3. Use events or callbacks to ensure timing

Another strategy is to use Vue's custom event system. The child component can issue events at appropriate times to notify the parent component to execute the relevant method.

In subcomponents

mounted() {
  this.$emit('component-loaded');
}

In parent component

methods: {
  handleComponentLoaded() {
    // Call the method of the child component here  }
}

<child-component @component-loaded="handleComponentLoaded"></child-component>

This way, only if the subcomponent is issuedcomponent-loadedThe parent component will not be called after the eventhandleComponentLoadedmethod, thus ensuring the correct call timing.

By adopting these strategies, it is possible to effectively solve the rendering timing in the applicationthis.$refsAccess issues

Summarize

The above is personal experience. I hope you can give you a reference and I hope you can support me more.