How to use a Svelte app as a component in another Svelte app?

Question

Use case: I have a couple of Svelte components that are compiled as a standard Svelte app (i.e., not a web component) that are then used to enhance the functionality of the main Svelte app.

I am instantiating them using the typical app = new SvelteApp({target, props}) method through a wrapper/proxy component (as suggested in this answer), so that I can do something like this:

MainApp.svelte

<script>
    import ComponentWrapper from 'ComponentWrapper.svelte'

    const App1 = window.app1; // this is standalone/compiled Svelte app
</script>

<ComponentWrapper this={App1} prop1="one" prop2="two" />

ComponentWrapper.svelte

<script>
    import { onDestroy } from 'svelte';

    let component;
    export { component as this };

    let target;
    let cmp;

    const create = () => {
        cmp = new component( {
            target,
            props: $$restProps,
        } );
    };

    const cleanup = () => {
        if ( !cmp ) return;
        cmp.$destroy();
        cmp = null;
    };

    $: if ( component && target ) {
        cleanup();
        create();
    }

    $: if ( cmp ) {
        cmp.$set( $$restProps );
    }

    onDestroy( cleanup );
</script>

<div bind:this={target} />

This works fine except that I can't listen to events dispatched by the App1 and App2 using the on:event={} directive on the ComponentWrapper or the div element inside it.

My questions are:

1. Is there a better way to include standalone Svelte apps?

2. How would I make events work? Sure, I can pass a method as a prop and use that inside the standalone apps, but I would rather dispatch/listen to events if that's at all possible.

Answer 1

1. Is there a better way to include standalone Svelte apps?

There is no special difference between an App and a Component.

<script>
const App1 = window.app1
</script>

<App1 prop1="one" prop2="two" />

could work, but that depends if the App uses the same svelte runtime. (Depends on how the apps are bundled)

You might want to try if you could generate the App as lib using @sveltejs/package command.

2. How would I make events work?

You can add listeners to the events and dispatch these:

const dispatch = createEventDispatcher()
cmp.$on('eventname', (event) => {
  dispatch('eventname', event);
});

This wraps the original event with a CustomEvent. I haven't found a way to emit an event as-is. And Svelte doesn't have a $$listeners like Vue. So you need to let the ComponentWrapper.svelte know what events to listen for.

Using props for these like you suggested might be cleaner.