commit aeeea4221bce5c44edeb8adad0296bbd68a4af71
author Dominic Farolino <dom@chromium.org> Thu Dec 14 14:34:04 2023
committer Blink WPT Bot <blink-w3c-test-autoroller@chromium.org> Thu Dec 14 14:53:11 2023
tree 74420db48fb86e5d3599c95df4d8c392022f2e30
parent d18aca5e683fe68a138f6c602d60a02c861d6828

DOM: Observable EventTarget integration 1/N

This CL implements "limited" and "leaky" EventTarget integration with
the Observable API. See below for what "limited" and "leaky" mean.
Concretely, this involves introducing the `on()` method to the
EventTarget interface, so that all EventTargets can return Observables
that listen for events. This is the part that really makes Observables a
"better addEventListener()".

This is the first instance of a natively-constructed Observable, as
opposed to a JS-constructed Observable. This means the subscription
callback passed to the Observable constructor is not just a JS callback
function with user-defined code, but instead is a C++ delegate class,
called `SubscribeDelegate` which has its first concrete implementation
provided by EventTarget (in event_target.cc). The concrete
implementation of this interface that this CL introduces, adds an event
listener to the given EventTarget, upon subscription. The events are
forwarded to the Subscriber's `next()` method. This is what unlocks
more ergonomic event handling with the composable Observable primitive
and all of its (coming) operators.

1. The EventTarget integration is considered "limited" because we do not
support any of the `AddEventListenerOptions` yet, as of this CL. A
subsequent CL will add support for a more restricted version of the
`AddEventListenerOptions`, called `ObservableEventListenerOptions`,
which does not include a `once` option, or an `AbortSignal`, since
Observable operators and subscription is responsible for managing those
aspects. Concretely, an `ObservableEventListenerOptions` will
resolve to an `AddEventListenerOptionsResolved` accordingly. See:
 - https://github.com/WICG/observable/issues/66
 - https://github.com/WICG/observable/pull/67
 - https://github.com/WICG/observable/issues/65

2. The EventTarget integration is considered "leaky" as of this CL,
because there is currently no way to remove an event listener added by
an EventTarget-vended Observable. This will come in a subsequent CL,
which will pass the test that is currently failing in this CL. See
https://github.com/WICG/observable/issues/75 for discussion about
tying the subscription termination to removing an event listener.
From a technical perspective, this is pretty easy — it involves adding
an abort algorithm to `Subscriber#signal` (which has already been wired
up properly by now!) that removes the given per-Subscription
`ObservableEventListener` NativeEventListener from the associated
EventTarget. That implementation has already been sketched out in
https://crrev.com/c/4262153 and the design doc. It will included in a
follow-up CL, to reduce the complexity of this one.

For WPTs:
Co-authored-by: ben@benlesh.com

R=masonf@chromium.org

Bug: 1485981
Change-Id: Iafeddb0894b8eed2be1d95c181fc44d7650c0d47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5073394
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1237501}