Implement CallInvoker as replacement for JSDispatcher

[acoates-ms]

Description

Implements a new CallInvoker API on IReactContext, and some additional helpers on ReactContext, which modifies how users access the jsi::Runtime.

Type of Change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Why

The current implantation of JSDispatcher is thread based. This worked well on the old arch where the JS runtime ran on a dedicated thread. However, in the new architecture there are cases where the JS runtime can end up running on the UI thread. This means that that existing usages of JSDispatcher that are being used to directly interact with the JS runtime, could run into cases where they attempt to access the JS runtime on the JS thread while the runtime is being used on the UI thread.

What

Modifies the new arch code path to no longer create a JSDispatcher with a MessageQueueThread, but instead create a MessageQueueThread directly. This uses more shared code from core.

Previously we exposed JSDispatcher to gain access to the JS thread, and a JsiRuntime property on ReactContext, which allowed access to the jsi::Runtime object. -- Although that API should only have been accessed from the JS thread.

With this change I'm replacing JSDispatcher with a new CallInvoker API. This property exposes the same CallInvoker that react-native uses. CallInvoker allows access to the jsi::Runtime in a more controlled way where it is only accessible on the stack during a callInvoker callback. This prevents access to the jsi::Runtime in an unsafe manner.

With this change RNW will align with core, by restricting access to the jsi::Runtime to locations where it is valid to use it.

Changelog

Should this change be included in the release notes: Yes

Add a brief summary of the change to use in the release notes for the next release.
IReactContext.JSDispatcher, IReactContext.JsiRuntime, and ExecuteJsi are no longer valid in new architecture, and these APIs are deprecated for old architecture. These will be removed in a future release.
Replacing those APIs is a new IReactContext.CallInvoker API. -- Although most applications should use ReactContext.CallInvoker for ease of use. The CallInvoker allows safe access to the jsi::Runtime and aligns with how you access the jsi::Runtime on other platforms.

old:

  ExecuteJsi(reactContext, [](jsi::Runtime &runtime) {
    ... // Interact with jsi::Runtime here
  });

new:

  reactContext.CallInvoker()->invokeAsync([](jsi::Runtime &runtime) {
    ... // Interact with jsi::Runtime here
  });

In addition, a new optional parameter on module initializers has been added. Module initializers can now take a jsi::Runtime& argument that can be used to synchronously interact with the jsi::Runtime during module initialization.

old:

REACT_INIT(Initialize)
void Initialize(msrn::ReactContext const &reactContext) noexcept {
  ExecuteJsi(reactContext, [](jsi::Runtime &runtime) {
    ... // Interact with jsi::Runtime here
  });
}

new:

REACT_INIT(Initialize)
void Initialize(msrn::ReactContext const &reactContext, jsi::Runtime& runtime) noexcept {
  ... // Interact with jsi::Runtime here
}

[Copilot]

Pull Request Overview

This PR implements the new JSDispatcher abstraction on top of the existing CallInvoker infrastructure to avoid potential thread conflicts with the JS runtime in Fabric scenarios. Key changes include replacing CallInvokerWriter calls with JSDispatcherWriter calls in multiple modules, updating project file references, and removing several asynchronous dispatch invocations in favor of direct callback invocations.

Reviewed Changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
vnext/Shared/Shared.vcxitems	Updated file inclusions to reference JSDispatcherWriter and CallInvokerDispatcher instead of CallInvokerWriter.
vnext/Microsoft.ReactNative/TurboModulesProvider.cpp	Changed lambdas to capture the JSDispatcher rather than the old jsInvoker.
vnext/Microsoft.ReactNative/ReactHost/ReactInstanceWin.cpp	Updated initialization to construct a CallInvokerDispatcher and pass it to TurboModuleManager.
vnext/Microsoft.ReactNative/Modules/NativeUIManager.cpp	Removed JSDispatcher.Post calls and now invoke callbacks directly.
Other module files (ImageViewManagerModule, ClipboardModule, AppStateModule, NativeAnimatedModule, AlertModule, AccessibilityInfoModule)	Similar removal of dispatcher posts in favor of direct callback invocation for improved thread safety with the new design.
vnext/Microsoft.ReactNative/JSDispatcherWriter.{h,cpp}	Renamed and restructured the writer implementation to work with the new JSDispatcher abstraction.
vnext/Microsoft.ReactNative/CallInvokerDispatcher.{h,cpp}	New files implementing the dispatcher interface on top of CallInvoker.
Project files and change JSON	Updated references to new file names and modules to reflect the transition.

Comments suppressed due to low confidence (1)

vnext/Microsoft.ReactNative/Modules/NativeUIManager.cpp:1001

• Replacing asynchronous dispatcher posts with direct callback calls alters the execution timing. Please verify that these synchronous invocations do not introduce race conditions or thread-safety issues, particularly if downstream consumers expected the callbacks to be scheduled asynchronously.

callback(0, 0, 0, 0, 0, 0);