feat: File Upload Progress

[ottomated]

see #14773

Builds on #14775 to use XHRs for file upload progress in forms:

<script>
  import { uploadForm } from './upload.remote';
</script>

<form {...uploadForm} enctype="multipart/form-data">
  <input {...uploadForm.fields.file.as('file')} />
  <p>Progress: {(uploadForm.fields.file.progress().uploaded / (uploadForm.fields.file.progress().total).toFixed(2)}%</p>
  <button>Upload</button>
</form>

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[rChaoz]

Can't this lead to 0/0 issues during SSR? Using your example to calculate the percentage

Update: just saw your comment above, a non-zero SSR total might be good

[elliott-with-the-longest-name-on-github]

@ottomated Want to resolve the conflicts? I can give it a review after that's done.

[ottomated]

@elliott-with-the-longest-name-on-github should be good!

[elliott-with-the-longest-name-on-github]

This looks pretty darn good to me. I'll have another maintainer check it as a backup, but I'm pretty excited about it! Just a minor nit to start off

[elliott-with-the-longest-name-on-github]

Suggested change

	const file_offset_start = 1 + 4 + 2 + encoded_header.length + encoded_file_offsets.length;
	const file_offset_start = HEADER_BYTES + encoded_header.length + encoded_file_offsets.length;

[elliott-with-the-longest-name-on-github]

Some thoughts on API below...

Responding to this: #14878 (comment)

Alternatively, could we provide a percent getter that protects users from 0/0? So instead of actually having to do the division yourself, you just use progress().percent, which is 0 when size is 0 and is a float of 0.0 to 1.0 otherwise?

Additionally, does progress() need to be a function, or could it be a property? I haven't figured out if this is just a limitation of our proxy property chain stuff. If it's possible for progress to be a getter rather than a function, I think I'd like that better...

[elliott-with-the-longest-name-on-github]

We discussed this today and I think Rich and Simon will be stopping by to take a look, but overall: We definitely think there should be a precalculated "percentage complete" property, and we'd all prefer if .progress could be an accessor as opposed to a function call -- but I haven't had time to investigate if that's possible. We were also noodling over whether we should provide any sort of API for rolling up file upload progress, eg. if you wanted to show progress for the entire form.

[ottomated]

I would love it to be an accessor but the established pattern (.value, .issues) is a function call. This might be related to the whole proxy thing that allows fields to be named progress?

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-14878-svelte.vercel.app/

[ottomated]

nasty nasty merge conflicts from #15657 should be fixed