[feat] custom load dependencies

[icalvin102]

Closes #1684

Problem

The invalidate function only re-runs load if fetch is used. This is not possible in cases where an APIClient does not allow for customization of the fetching process or when using non REST APIs like GraphQL.

Solution

Add a dependencies array to the LoadOutput (suggested by @Rich-Harris in #1709).
The dependencies will be registered the in the same way fetch does it. This allows not only to invalidate real resources but entire pages or groups of pages.

Example Usage

<script context="module">
import { api } from '$lib/myapi';
export async function load({params}){
    const item = await api.getItem(params.id);
    return { 
        props: { item },
        dependencies: [`/item-${params.id}`, 'custom:item-page']
    }
}
</script>

<script>
import Item from '$lib/Item.svelte';
import { invalidate } from '$app/navigation';
export let item;
</script>

<Item {item} />
<button on:click={()=> invalidate('custom:item-page') }>Reload Page</button>
<button on:click={()=> invalidate(`/item-${item.id}`) }>Reload Item</button>

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🦋 Changeset detected

Latest commit: 652f432

The changes in this PR will be included in the next version bump.

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@sveltejs/kit	Patch

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[dreitzner]

@icalvin102 just a quick question....
Is this problem not solved by page endpoints?
To the best of my understanding this should generate a load function on build.
By moving the $lib/myapi integration to the endpoint there should be no trouble on calling the load again 😉

[icalvin102]

@dreitzner page endpoints will only work if your API is written in sveltekit AFAIK.
Page endpoints wont work in SPA mode and/or when using third party APIs that rely on browser state like cookies that are not accessible from the sveltekit server.

[Rich-Harris]

thank you!