Share to PWA from mobile

If you have a website that can be installed as a PWA on your mobile device it has also the ability to receive data from the "share to" functionality on mobile phones. Depending on what you're trying to do you don't need to create a native app.

This article shows how I made use of this for unnote.

There are 3 things that are needed as part of your application to make this work:

1. Update your manifest to support "share to"
2. Handle the "share to" request in your service worker
3. Handle the shared data in your app

Let's go through each part in some detail.

1. Manifest

You need to add a new entry share_target to your manfest.json. The following manifest allows any text data and all types of images to be sent to your application.

"share_target": {
  "action": "/share-target/",
  "method": "POST",
  "enctype": "multipart/form-data",
  "params": {
    "title": "title",
    "text": "text",
    "url": "url",
    "files": [
      {
        "name": "image",
        "accept": ["image/*"]
      }
    ]
  }
}

1. action is the URL that will receive the data. This will be explained in the next chapter about the service worker.
2. params represent the different ways the mobile phone sends the data
   1. title is provided when a document is shared
   2. text contains the contents of the shared document
   3. url is provided when a resource with a URL (as e.g. a website) is shared
   4. files is provded when a binary file is shared. The definition accepts an array of mimetypes that your aplication should accept.

Depending on what data your application should be able to receive you don't need to configure all the params. files is only needed when you want to support binary data.

2. Service Worker

The following is a minimal service worker that intercepts the URL /share-target/ which was previously configured in the manifest. It then puts the data in the caches, which is a global CacheStorage in the browser.

self.addEventListener('fetch', (event) => {
  if (event.request.url.endsWith('/share-target/') && event.request.method === 'POST') {
    return event.respondWith(
      (async () => {
        const formData = await event.request.formData();
        const image = formData.get('image');
        const title = formData.get('title');
        const text = formData.get('text');
        const url = formData.get('url');

        const cache = await caches.open('media');

        if (image) {
          await cache.put('shared-image', new Response(image));
        } else {
          await cache.put('shared-data', new Response(JSON.stringify({ title, text, url })));
        }
        return Response.redirect('/#/handle-share-target', 303);
      })(),
    );
  }
});

The service worker handles images and text data differently, and uses two separate cache keys. The text data is stored as JSON, while the image is stored as a Response.

Note that the intercepted request won't make it to your server. The last statement makes a client side redirect to where your application will actually handle the received data.

3. Application

The last part is the actual application fetching the data from the cache and doing whatever you want to do with it. The following two functions are helpers to extract the json data and the binary image data from the cache.

export async function getBlob(name) {
  const cache = await caches.open('media')

  const image = await cache.match(name)
  if (image) {
    const blob = await image.blob()
    await cache.delete(name)

    return blob
  }
}

export async function getJson(name) {
  const cache = await caches.open('media')

  const data = await cache.match(name)
  if (data) {
    const json = await data.json()
    await cache.delete(name)

    return json
  }
}

The next part is the application specific part. The following shows how to use the previous functions to retrieve the data.

const blob = await getBlob('shared-image')

You could e.g. convert the blob to an image or resize the image.

The last piece of code shows how to extract the properties from the cached JSON.

const { title, text, url }  = await getJson('shared-data')

Links

• unnote's manifest.json
• unnote's service worker
• unnote's data handling
• Handling shared data from other apps (MDN)
• Web Share Demo