Invoking a child component's function while displaying the child component within a v-if directive in Vue

I'm looking to execute the method of a child component.

It seems like developers often use the $nextTick function to handle data processing once all child components have been rendered. However, I'm facing an issue with calling the child component's method when it's being rendered using the v-if directive.

You can check out this example for reference.

// JavaScript code...
.child{ display:inline-block; padding:10px; background:#eaeaea; }
<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
            <div id="app">
              <div v-if="if_child">
                <child ref="child" class="child"></child>  
              </div>
              <button type="button" @click="showChild">
                Toggle Child
              </button>
            </div>

When attempting to call the callFunction() method of the child component in the showChild() function, an error is thrown:

Uncaught TypeError: Cannot read property 'callFunction' of undefined

It appears that the error is occurring because the function is being called before the child component is fully rendered. Any suggestions on how to resolve this would be greatly appreciated! Thank you.

Answer №1

According to the question, the best solution is to use $nextTick.

Vue optimizes rendering by batching changes together. When you modify reactive data like if_child, Vue doesn't immediately render the changes. Instead, it queues up components that need to be rendered and updates them all at once after all data modifications are completed.

This approach has two benefits: rendering can be resource-intensive, and delaying rendering ensures that components are in a stable state before being displayed.

The term "rendering" is slightly deceptive; it encompasses more than just drawing elements on the screen. It also involves creating and removing child components.

The update of $refs occurs right after a component renders. This process happens at the beginning of the next tick, which is why we utilize $nextTick to wait for it.

Vue.component('child', {
  template: `
    <div class="child">
      I'm a child
    </div>
  `,
  
  methods: {
    callFunction () {
      console.log("I'm called");
    }
  }
});

var vm = new Vue({
  el: '#app',
  
  data: {
    if_child: false
  },
  
  methods: {
    showChild () {
      this.if_child = !this.if_child;
      
      this.$nextTick(() => {
        const child = this.$refs.child;
        
        if (child) {
          child.callFunction();
        }
      });
    }
  }
});
.child {
  display: inline-block;
  padding: 10px;
  background: #eaeaea;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<div id="app">
  <div v-if="if_child">
    <child ref="child" class="child"></child>  
  </div>
  <button type="button" @click="showChild">
    Toggle Child
  </button>
</div>

This section is crucial:

showChild () {
  this.if_child = !this.if_child;

  this.$nextTick(() => {
    const child = this.$refs.child;

    if (child) {
      child.callFunction();
    }
  });
}

You might be wondering about the necessity of the if (child) {. It's there because the button toggles the value of if_child. Despite the method name, showChild, clicking the button creates the child component initially and destroys it upon subsequent clicks.

If you don't provide a callback to $nextTick, it will return a promise instead. This allows for usage with async/await:

async showChild () {
  this.if_child = !this.if_child;

  await this.$nextTick();

  const child = this.$refs.child;

  if (child) {
    child.callFunction();
  }
}

Answer №2

Why isn't it functioning properly?

The reason it's not working is because when the v-if condition is false, the child component doesn't exist yet. Vue hasn't created it, so the ref for the child element remains undefined and the callFunction won't execute.

Have you considered using the Vue.nextTick API?

I attempted to implement it in the code, but it only works initially. After the first attempt, the child element's ref becomes undefined again because the component is destroyed when if_child returns false.

How can this issue be resolved?

I have found two solutions that can help overcome this problem:

1 - Use v-show on the child component instead of v-if. This way, the child will always be available and rendered even if hidden with a display : none style when the condition is false;

2 - If you prefer to use v-if, you can introduce another variable that changes after the DOM has finished rendering (using the nextTick API). The child component can then watch this variable and trigger the function accordingly. Here is an example implementation:

Vue.component('child', {
  props: ['exe'],
  watch: {
    exe() {
      this.callFunction()
    }
  },
  template: `
  <div class="child">
    I'm a child
    </div>
  `,
  methods: {
    callFunction: function() {
      console.log("I'm called");
    }
  }
});

var vm = new Vue({
  el: '#app',
  data: {
    if_child: false,
    executeTheChildFunction: false,
  },
  methods: {
    showChild() {
      this.if_child = !this.if_child;
      //Calling child's function
      this.$nextTick(function() {
        this.executeTheChildFunction = !this.executeTheChildFunction;
      })
    }
  }
})
.child {
  display: inline-block;
  padding: 10px;
  background: #eaeaea;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<div id="app">
  <div v-if="if_child">
    <child id="child" class="child" :exe="executeTheChildFunction"></child>
  </div>
  <button type="button" @click="showChild">
    Toggle Child
  </button>
</div>

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