URL protocol handler registration for PWAs

# Background on schemes (aka. protocols)

A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. Each URI begins with a scheme name that refers to a specification for assigning identifiers within that scheme. As such, the URI syntax is a federated and extensible naming system wherein each scheme's specification may further restrict the syntax and semantics of identifiers using that scheme. Schemes are also known as protocols. You can see some examples of schemes below.

tel:+1-816-555-1212
mailto:Jane.Doe@example.com
news:comp.infosystems.www.servers.unix
https://web.dev/

The term Uniform Resource Locator (URL) refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism (e.g., its network location).

# Background on the registerProtocolHandler() method

The secure-content-only Navigator method registerProtocolHandler() lets sites register their ability to open or handle particular URL schemes. Therefore, sites need to call the method like so: navigator.registerProtocolHandler(scheme, url). The two parameters are defined as follows:

• scheme: A string containing the protocol the site wishes to handle.
• url: A string containing the URL of the handler. This URL must include %s, as a placeholder that will be replaced with the escaped URL to be handled.

The scheme must either be one of the safelisted schemes (for example, mailto, bitcoin, or magnet) or begin with web+, followed by at least one or more lowercase ASCII letters after the web+ prefix, for instance, web+coffee.

To make this clearer, here is a concrete example of the flow:

1. The user visits a site at https://coffeeshop.example.com/ that makes the following call: navigator.registerProtocolHandler('web+coffee', 'coffee?type=%s').
2. At a later point, while visiting https://randomsite.example.com/, the user clicks on a link such as <a href="web+coffee:latte-macchiato">All about latte macchiato</a>.
3. This causes the browser to navigate to the following URL: https://coffeeshop.example.com/coffee?type=web%2Bcoffee%3A%2F%2Flatte-macchiato. The search string URL-decoded then reads ?type=web+coffee://latte-macchiato.

# What this proposal is about

The present URL protocol handler registration for PWAs proposal is about offering protocol handler registration as part of a PWA installation through its manifest. After registering a PWA as a protocol handler, when a user clicks on a hyperlink with a specific scheme such as mailto , bitcoin, or web+music from a browser or a platform-specific app, the registered PWA will open and receive the URL. It is important to note that both the proposed manifest-based registration and the traditional registerProtocolHandler() play very similar roles in practice, while still allowing the possibility for complementary user-experiences:

• Similarities include requirements around the list of schemes allowed to be registered, and the name and format of parameters, etc.
• Differences in the manifest-based registration are subtle, but might be useful to enhance the experience for PWA users. For example, manifest-based PWA registration may not require an additional user action apart from the user-initiated installation of the PWA.

# Use cases

• In a word processing PWA, the user in a document encounters a link to a presentation like web+presentations://deck2378465. When the user clicks on the link, the presentation PWA automatically opens in the correct scope and shows the slide deck.
• In a platform-specific chat app, the user in a chat message receives a link to a magnet URL. Upon clicking the link, an installed torrent PWA launches and starts downloading.
• The user has a music streaming PWA installed. When a friend shares a link to a song like web+music://songid=1234&time=0:13 and the user clicks on it, the music streaming PWA will automatically launch in a standalone window.

# How to use URL protocol handler registration for PWAs

The API for URL protocol handler registration is modeled closely on navigator.registerProtocolHandler(). Just this time the information is passed declaratively via the web app manifest in a new property called "protocol_handlers" that takes an array of objects with the two required keys "protocol" and "url". The code snippet below shows how to register web+tea and web+coffee. The values are strings containing the URL of the handler with the required %s placeholder for the escaped URL.

{
  "protocol_handlers": [
    {
      "protocol": "web+tea",
      "url": "/tea?type=%s"
    },
    {
      "protocol": "web+coffee",
      "url": "/coffee?type=%s"
    }
  ]
}

# Multiple apps registering for the same protocol

If multiple applications register themselves as handlers for the same scheme, for example, the mailto protocol, the operating system will show the user a picker and let them decide which of the registered handlers to use.

# The same app registering for multiple protocols

The same app can register itself for multiple protocols, as you can see in the code sample above.

# App updates and handler registration

Handler registrations are synchronized with the latest manifest version provided by the app. There are two cases:

• An update that adds new handlers triggers handler registration (separate from app installation).
• An update that removes handlers triggers handler unregistration (separate from app uninstallation).

# Protocol handler debugging in DevTools

Navigate to the Protocol Handlers section via the Application > Manifest pane. You can view and test all the available protocols here.

For example, install this demo PWA. In the Protocol Handlers section, type "americano" and click Test protocol to open the coffee page in the PWA.

# Demo

You can see a demo of URL protocol handler registration for PWAs on Glitch.

1. Go to https://protocol-handler.glitch.me/, install the PWA, and reload the app after the installation. The browser has now registered the PWA as a handler for the web+coffee protocol with the operating system.
2. In the installed PWA window, click on the link https://protocol-handler-link.glitch.me/. This will open a new browser tab with three links. Click on the first or the second (latte macchiato or americano). The browser will now show you a prompt and ask if you are fine with the app being a protocol handler for the web+coffee protocol. If you agree, the PWA will open and show the selected coffee.
3. To compare with the traditional flow that uses navigator.registerProtocolHandler(), click the Register protocol handler button in the PWA. Then in the browser tab click the third link (chai). It will likewise show a prompt, but then open the PWA in a tab, not in a browser window.
4. Send yourself a message on a platform-specific application like Skype on Windows with a link like <a href="web+coffee://americano">Americano</a> and click it. It should likewise open the installed PWA.

# Security considerations

Since PWA installation requires the context to be secure, protocol handling inherits this constraint. The list of registered protocol handlers is not exposed to the web in any way so it cannot be used as a fingerprinting vector.

# Non user-initiated navigation attempts

Navigation attempts that are not initiated by the user, but that are programmatic, may not open apps. The custom protocol URL may only be used in top-level browsing contexts, but not, for example, as the URL of an iframe.

# Allowlist of protocols

Just like with registerProtocolHandler() there is an allowlist of protocols that apps can register to handle.

# Consent prompt

On the first launch of the PWA due to an invoked protocol, the user will be presented with a permission dialog. This dialog will display the app name and origin of the app, and ask the user if the app is allowed to handle links from the protocol. If a user rejects the permission dialog, the registered protocol handler will be ignored by the operating system. To unregister the protocol handler, the user needs to uninstall the PWA that registered it. The browser will also unregister the protocol handler if the user selects "Remember my choice" and selects "Disallow".

# Feedback

The Chromium team wants to hear about your experiences with URL protocol handler registration for PWAs.

# Tell us about the API design

Is there something about the API that does not work like you expected? Or are there missing methods or properties that you need to implement your idea? Have a question or comment on the security model? File a spec issue on the corresponding GitHub repo, or add your thoughts to an existing issue.

# Report a problem with the implementation

Did you find a bug with Chromium's implementation? Or is the implementation different from the spec? File a bug at new.crbug.com. Be sure to include as much detail as you can, simple instructions for reproducing, and enter UI>Browser>WebAppInstalls in the Components box. Glitch works great for sharing quick and easy repros.

# Show support for the API

Are you planning to use URL protocol handler registration for PWAs? Your public support helps the Chromium team prioritize features and shows other browser vendors how critical it is to support them.

Share how you plan to use it on the WICG Discourse thread. Send a Tweet to @ChromiumDev using the hashtag #ProtocolHandler and let us know where and how you're using it.

# Useful links

• Explainer
• Spec proposal
• GitHub
• ChromeStatus
• Chromium bug
• TAG review
• Discourse

# Acknowledgements

URL protocol handler registration for PWAs was implemented and specified by Fabio Rocha, Diego González, Connor Moody, and Samuel Tang from the Microsoft Edge team. This article was reviewed by Joe Medley and Fabio Rocha. Hero image by JJ Ying on Unsplash.