MUTATION_OBSERVER poor performance (observe method)

[rafalpospiech]

There is a huge performance drop while MUTATION_OBSERVER is enabled.
I have component on my page that uses SVG to display some content.
This content is reactive and works on vue.
There are a lot of mutations going on so MUTATION_OBSERVER is running like crazy all the time.
I don't want to disable observer globally.
Is there a way to exclude from observation some parts of the page? by class or id?

I'm using Font Awesome Free 5.5.0

[tagliala]

Hi!

Thanks for being part of the Font Awesome Community.

@robmadole could you please take a look at this?

[rafalpospiech]

Mutation observer is attached to my <g> element inside svg component.
This component does not use font-awesome icons at all.

[robmadole]

@rafalpospiech the MutationObserver by it's very nature needs to examine changes to the DOM to see if there is are any changes that it needs to generate icons for. There is currently no way to tell it to ignore certain parts of the page. You can control where the mutation observers starts, though. I don't know if this will help you or not.

You mentioned that you are using Vue. Can you switch to our Vue component?

[rafalpospiech]

No, Vue is used just by this one component :/

[rafalpospiech]

But take a look at this example (line 23)

https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver#Example

You can observe specific nodes only without subtree and with childnodes only.

So theoretically we can get all elements on the page and observe only those that doesn't have an attribute let say data-fa-no-observer or something like that with multiple observers I think.

If observer will be on each element there could be an option to specify which elements to observe and which exclude from observation.
For example

{
/* ... */
observer:{
  include:[],
  exclude:[]
}
/* ... */
}

Or by default observe all excluding those with data-fa-no-observer.
This would improve the performance a lot.

[robmadole]

@rafalpospiech when brand new elements are added to the page how would we detect those?

And with regard to your include and exclude options. Those elements would still need to be observed are you thinking that we could just bail out of processing them sooner?

[dguettler]

I know this is an old thread, and I don't have a solution to offer. Just adding an example I have been observing.
XHR call is adding a rather long dropdown menu, prior to using the fontawesome.js file this took roughly 1-2 seconds to render upon call completion.
When using frontawesome.js (5.2 in this case) rendering takes about 15 seconds.

[DibyodyutiMondal]

Same issue with angular 15 and fontawesome 6

While the icons are used in a static part of the page, the main portion (which is a calendar) renders many, many many dom elements. Adding fontawesome drops performance from 3s for a complex year view to an entire minute and a half!

[dbrgn]

I'm having this problem as well in a SvelteKit application that builds a page that contains hundreds of icons. The FontAwesome mutation observer blocked the UI thread for around 8s on page load.

I resolved it by using FontAwesome in pure CSS mode (CSS + WebFont, no JS).

[robmadole]

@dbrgn you can also take a look at https://github.com/FortAwesome/svelte-fontawesome

[dbrgn]

@robmadole I tried, but FortAwesome/svelte-fontawesome#7

In the end, I'm actually happier with a pure-CSS mode, I don't need any of the dynamic features 🙂

[davidAtInleague]

I just hit this drawing some SVGs in a Vue app. Would it be feasible/useful to expose something like FontAwesome.disconnectGlobalObserver() and FontAwesome.restartGlobalObserver() (thin wrappers over MutationObserver's disconnect/observe methods)? And then switch it off/on via Vue's beforeUpdate and updated lifecycle hooks.

edit: I'm using 6.5.2, and I see noAuto from

import { noAuto } from "@fortawesome/fontawesome-svg-core";

That looks like it will disable the observer, don't see anything regarding turning it back on though.