[docs] decision: BiDi events are awaited with expect_* context managers

docs/decisions/17671-bidi-events-awaited-with-expect-context-managers.md

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+# 17671. BiDi events are awaited with `expect_*` context managers
+
+- Status: Proposed
+- Date: 2026-06-11
+- Discussion: https://github.com/SeleniumHQ/selenium/pull/17671
+
+## Context
+
+WebDriver BiDi is event-driven: navigation, network traffic, console output, user
+prompts, and downloads all surface as asynchronous events. Today the only way a binding
+exposes them is a fire-and-forget callback registration — e.g. Python's
+`add_event_handler(event, callback)`, Java's `addListener`, JavaScript's `.on(...)`.
+
+To *wait for* an event that a user action triggers (click a button, wait for the matching
+network response), users must register a callback, stash the event into a shared variable
+or queue, perform the action, then poll/wait. This has two problems:
+
+1. **A time-of-check/time-of-use race.** If the event fires between "perform the action"
+   and "start waiting", it is lost. Correctly ordered code must subscribe *before* the
+   action — which the callback pattern does not make natural or obvious.
+2. **No predicate.** Users hand-write loops that inspect each event to find the one they
+   care about (a specific URL, a console error), reinventing the same filter every time.
+
+Playwright solved this with `with page.expect_event(...) as info: action()`, where the
+listener is armed on `__enter__` (before the action) and `info.value` blocks on
+`__exit__` until a matching event arrives or the timeout elapses. This is the single most
+common asynchronous pattern in browser automation, and Selenium has no equivalent.
+
+The forces: the BiDi event surface already exists in every binding; the wire protocol
+needs nothing new; the only question is the *shape* of the waiting API and whether it is
+consistent across bindings.
+
+## Decision
+
+Every binding exposes an **`expect_*` family of context-manager (or block) helpers** that
+arm a one-shot, predicate-filtered subscription before the user action and resolve to the
+captured event after it.
+
+Normative requirements for all bindings:
+
+- The subscription is registered when the scope is entered, **before** the user action
+  runs, eliminating the race.
+- The helper accepts an optional **predicate/matcher** and an optional **timeout**
+  (defaulting to the binding's standard wait timeout). For URL-shaped events a string
+  **glob** is also accepted.
+- On scope exit the captured event is retrieved synchronously; a timeout raises the
+  binding's standard timeout error.
+- A reusable **`Subscription`** primitive underlies the helpers: register/unregister are
+  decoupled, detach is idempotent and lock-guarded, and a captured-event queue backs the
+  wait. Bindings MAY expose this primitive directly.
+- Existing callback registration (`add_event_handler`/`addListener`/`.on`) stays; this is
+  additive.
+
+Concrete shorthands each binding SHOULD provide (built on the generic primitive):
+`expect_request`, `expect_response`, `expect_console_message`, `expect_navigation`
+(see [17676](17676-navigation-awaited-with-expect-helpers.md)), `expect_user_prompt`
+(see [17672](17672-user-prompts-handled-through-typed-handler-api.md)), `expect_download`,
+and — for tabs/windows the page opens itself — `expect_page` / `expect_popup` over
+`browsingContext.contextCreated`, returning the new context as a handle object
+(see [17681](17681-browsing-contexts-exposed-as-handle-objects.md)).
+
+Code sketch — Python (reference implementation):
+
+```python
+# Generic primitive
+with driver.script.expect_event("log.entryAdded") as info:
+    button.click()
+entry = info.value
+
+# Typed shorthands with predicate or glob
+with driver.network.expect_response("**/api/search**") as info:
+    search_button.click()
+assert info.value.status == 200
+
+with driver.network.expect_request(lambda r: r.method == "POST") as info:
+    submit.click()
+
+with driver.script.expect_console_message(lambda m: m.type == "error") as info:
+    driver.execute_script("console.error('boom')")
+print(info.value.text)
+```
+
+Code sketch — other bindings (idiomatic shape, same semantics):
+
+```java
+// Java — try-with-resources arms before the action
+try (var expectation = driver.network().expectResponse(url -> url.contains("/api/"))) {
+    button.click();
+    Response response = expectation.value();
+}
+```
+
+```javascript
+// JavaScript — async block form
+const response = await driver.network().expectResponse('**/api/**', async () => {
+  await button.click();
+});
+```
+
+## Considered options
+
+- **`expect_*` context managers (chosen)** — arms before the action by construction;
+  matches the dominant Playwright idiom users already know; predicate + timeout built in.
+- **Document "subscribe first, then act" with the existing callbacks** — no new API, but
+  leaves the race as a footgun, provides no predicate or timeout, and every user
+  re-implements the capture-and-wait boilerplate. Rejected: solves nothing structurally.
+- **A future/promise returned from a `waitForEvent(...)` call** — viable in async
+  bindings, but in synchronous bindings it reintroduces the race (the call to start
+  waiting happens after the action) unless wrapped in a block anyway. Rejected as the
+  primary shape; bindings MAY offer it as a secondary convenience where idiomatic.
+
+## Consequences
+
+- Users get race-free, predicate-filtered event waiting with no boilerplate; the most
+  common async pattern becomes a one-liner.
+- Each binding gains a small reusable `Subscription` primitive; implementing it surfaces
+  and forces fixes to event-dispatch thread-safety (callback maps must be lock-guarded,
+  subscribe/unsubscribe I/O must not be held under a dispatch lock).
+- The other `expect_*`-based decisions (navigation, downloads, user prompts, and the
+  `expect_page`/`expect_popup` handles in
+  [17681](17681-browsing-contexts-exposed-as-handle-objects.md)) build on this primitive, so
+  this record should land first.
+- The thread-safety fixes this surfaces (lock-guarded callback maps, non-busy-wait command
+  completion, bounded event dispatch) are also the prerequisite for the multi-thread
+  concurrency contract in [17681](17681-browsing-contexts-exposed-as-handle-objects.md).
+- No deprecations. Existing callback APIs are unaffected.
+
+## Binding status
+
+| Binding    | Status      | Notes / tracking link                                           |
+|------------|-------------|-----------------------------------------------------------------|
+| Java       | pending     |                                                                 |
+| Python     | in progress | `expect_*` + `Subscription` implemented; PR pending             |
+| Ruby       | pending     |                                                                 |
+| .NET       | pending     |                                                                 |
+| JavaScript | pending     |                                                                 |
+
+## Appendix
+
+The BiDi events backing the shorthands already exist in the spec and are emitted by
+current browsers: `network.beforeRequestSent`, `network.responseStarted`,
+`network.responseCompleted`, `log.entryAdded`, `browsingContext.userPromptOpened`,
+`browsingContext.downloadWillBegin`/`downloadEnd`, and the navigation events. No new wire
+protocol is required — this decision is purely about the binding-side waiting API.