Add an API for installing a handler

[mgiuca]

Currently, handlers are specified declaratively in the web manifest. This is great for indexing, but it means the browser needs to decide when to prompt the user to install a handler. (On first page load? After some time using the site? When the site is added to home screen?)

Instead, we should add an API that lets the web app programmatically request to prompt the user to install the handler. Note: The handler would still need to be declaratively specified; the API would just be a trigger to install. Then the website can provide a contextual button where the user requests to install the handler.

Downsides: We may actually want to leave the decision to browser policy (e.g., in Chrome, when the user clicks "Add to home screen" or "Add to desktop", and tie handling to app installation). This may be giving too much control to the website.

[benfredwells]

I think to start with we would definitely want to leave the decision to the browser.

[adrianhopebailie]

I asked the question of the Web Manifest editors but I wonder why you'd not want to allow a website to at least request that a manifest is installed? [1] This says nothing about how the website handles user consent.

There are use cases for installing an app that go beyond simply adding the app to your home screen or desktop (such as registering a handler).

We're particularly interested in this for the Web Payments WG work and also possibly having "pay" as an action.

So, rather than having a way to register a handler wouldn't it be better to have a way to explicitly request that browser install a manifest?

[1] w3c/manifest#446