Passing element to composable function in Vue composition API?

I’ll preface this question by saying I’m very new to the composition API so I’m still trying to figure out the best practices. I’m writing a useResizeObserver composable function using Vue’s composition API in order to track dimension changes on a target element. Because I have to wait for the element to be mounted, I can’t call my composable from top-level in the setup() method on my component as most examples show, but have to call it from onMounted() instead, where I have access to my element’s ref.

onMounted(async () => {
      elementDimensions.value = useResizeObserver(modalContent.value);
}

This forced me to have another ref that I named elementDimensions actually declared at the top level of the setup() function, which I initialise to null.

setup(props) {
    const elementDimensions = ref(null);
    const modalContent = ref(null);
    ...
}

Here is the useResizeObserver function itself:

export default function useResizeObserver(element) {
  const elementHeight = ref(0);
  const elementWidth = ref(0);

  const resizeObserver = new ResizeObserver(entries => {
    elementHeight.value = entries[0].contentRect.height;
    elementWidth.value = entries[0].contentRect.width;
  });

  if (element) {
    resizeObserver.observe(element);
  }

  onUnmounted(() => {
    resizeObserver.disconnect();
  });

  return { elementHeight, elementWidth };
}

This actually works well to give me what I want but I’m wondering if there’s a better way to achieve this. The way things are implemented right now, I end up with “nested” refs, since I end up wrapping elementHeight and elementWidth (which are already refs) into another ref inside the component (elementDimensions). Is there a better recommended way of doing this when you need to pass an element to a composable from onMounted()?

Answer

Composition functions are supposed to be used directly in setup, any other uses depend on the implementation and need to be verified.

Refs are basically objects that allow to pass a value by reference rather than by value. One of their uses is to pass a ref early and access when a value is up-to-date.

It should be:

setup(props) {
   const modalContent = ref(null);
   const elementDimensions = useResizeObserver(modalContent);
  ...
}

and

export default function useResizeObserver(element) {
  const elementHeight = ref(0);
  const elementWidth = ref(0);
  let resizeObserver;

  onMounted(() => {
    if (element.value) {
      resizeObserver = new ResizeObserver(...);
      resizeObserver.observe(element.value);
    }
  }

  onUnmounted(() => {
    if (resizeObserver)
      resizeObserver.disconnect();
  });

  return { elementHeight, elementWidth };
}