`penumbra.saveZip()` Background Fetch integration for rich download UI

[eligrey]

We could add an optional resources field onto ZipOptions consisting of uninitialized RemoteResource descriptors. A Service Worker could handle downloading these resources with Background Fetch which enables us to create rich download UIs and persistent downloads that continue when the tab is closed.

Rich download UI example screenshot:

Background Fetch example (from https://developers.google.com/web/updates/2018/12/background-fetch)

navigator.serviceWorker.ready.then(async (swReg) => {
  const bgFetch = await swReg.backgroundFetch.fetch('my-fetch', ['/ep-5.mp3', 'ep-5-artwork.jpg'], {
    title: 'Episode 5: Interesting things.',
    icons: [{
      sizes: '300x300',
      src: '/ep-5-icon.png',
      type: 'image/png',
    }],
    downloadTotal: 60 * 1024 * 1024,
  });
});

Limitations

This feature might not be compatible with use cases that involve an unknown amount of async paginated resources.