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Summary: Pull Request resolved: https://github.com/facebook/react-native/pull/42397 Changelog: [Internal] Adds a stub `PageTarget` class to serve as the entry point to the modern CDP backend in React Native. The primary method exposed by `PageTarget` is `connect()` which is designed to fit directly as a connection callback passed to `InspectorPackagerConnection::addPage()`. This constructs a `PageTargetSession` containing a `PageAgent` where the actual CDP message handling/routing will occur. For now, `PageAgent` implements no CDP methods, and always responds with a "not implemented" error. Basic unit tests are included, though we might want to migrate to a more integration-style test suite (with fewer mocks and real bindings to RN) once we've implemented more of the protocol. ## What is a Page In Chrome's implementation of CDP, a Page represents a single browser tab. A Chrome DevTools session connects to one Page at a time (though it can potentially inspect multiple JavaScript contexts owned by that Page, such as those found in frames and workers). In our system, a Page will correspond 1:1 to React Native's concept of a *Host* (implemented as `RCTHost`, `RCTBridge`, `ReactHostImpl` or `ReactInstanceManager`, depending on the platform). In all cases, the Host is the object that has a stable identity across reloads, and manages the lifetime of an *Instance* where the JSVM and other application state lives. There can be multiple Hosts in a React Native process, though this is somewhat unusual; those would be treated as independent "tabs" from the perspective of the debugger. NOTE: The concepts of Target, Session and Agent are new (to this codebase) and are *broadly* inspired by the [corresponding Chromium / V8 concepts](https://chromium.googlesource.com/chromium/src/+/master/third_party/blink/public/devtools_protocol/#Agents_Targets-and-Sessions), though some details differ. ## Next steps Each core platform implementation in React Native (iOS Bridgeless, iOS Bridge, Android Bridgeless, Android Bridge), as well as out-of-tree platforms that want to support the new debugger, will need to create and register a `PageTarget` instance. We'll do this piecemeal in subsequent diffs. We'll also gradually add APIs and logic to `PageTarget` / `PageAgent` to allow us to implement some "interesting" CDP methods - some of them directly (e.g. handling reload commands) and others by dispatching to nested agents (e.g. a JS debugging agent powered by Hermes). Reviewed By: huntie Differential Revision: D50936932 fbshipit-source-id: ebe5856d7badb361d4971dd9aabeb9982f8aed1b