Generalize subscriber's next/error/complete algorithm infra, for internally-subscribed Observables #95
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aarongable
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Jan 12, 2024
In the Observable specification, recently WICG/observable#95 generalized the collection of "next", "error", "complete" handlers into an "internal observer" concept that the `Subscriber` interface is responsible for holding. This was an abstraction exercise to make it possible to supply arbitrary observer handlers, instead of just JavaScript callback functions passed into `Observable::subscribe()`, via the `ObserverUnion`. This abstraction is crucial for the Observable API, because it is not only developer JavaScript code that is responsible for subscribing to observables. Native "internal" code can also subscribe to an existing Observable, via the Promise-returning operators [1]. These operators internally subscribe to an Observable and return a Promise that resolves or rejects according to the operator's semantics. The Promise resolves or rejects with values accumulated via the "internal" next, error, and complete handlers. To achieve this in the implementation, we introduce a `blink::ObservableInternalObserver` class mirroring the spec's "internal observer" struct [2]. This is an abstract interface with overridable `Next`, `Error`, and `Complete` algorithms. `blink::Subscriber` holds onto this class, and subclasses of this interface provide the concrete implementations of those algorithms. Because no Promise-returning operators are implemented yet, the only consumer/subclass of this abstract interface is the `ScriptCallbackInternalObserver` that `observable.cc` provides. It is responsible for implementing the logic that much of `blink::Subscriber` currently implements (before this CL). This is typically trivial logic like storing the user-supplied `ObserverUnion` callback functions, calling them if they are non-null, and reporting exceptions if the callbacks throw any errors. With this logic outsourced to the specific `ScriptCallbackInternalObserver`, and no longer hard-coded inside `Subscriber`, Subscriber can be more concerned with logic pertaining to subscription management, and less with Observer-specific logic. Subsequent CLs will implement specific subclasses of this generic interface to provide the Promise-returning operator implementations. [1]: https://wicg.github.io/observable/#promise-returning-operators [2]: https://wicg.github.io/observable/#internal-observer [email protected] Bug: 1485981 Change-Id: I524f0de89dab23a896b1f6f4783c0168b1fa6c0a Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5178410 Commit-Queue: Dominic Farolino <[email protected]> Reviewed-by: Mason Freed <[email protected]> Cr-Commit-Position: refs/heads/main@{#1246676}
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This allows for the Promise-returning operators to subscribe to an Observable with an internally spec-defined set of steps for next/error/complete.
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