Great question—adding a new host platform for React Native Fabric is no small feat, but after digging through the React Native source and off-tree projects, there’s a clear (if under-documented) blueprint to follow. Let’s break down what you’ll need to build to integrate Fabric with your C++ GTK/Qt app, leveraging your existing React Native knowledge.

First, remember that Fabric’s design is intentionally platform-agnostic at its core:

• Fabric Core: The C++ layer that handles reconciliation, layout, and communication between JS and native.
• Host Platform Bindings: The code you’ll write to connect Fabric’s abstract interfaces to your GTK/Qt widgets.
• JSI: The bridge between JS and C++—you’ll use this for both rendering commands and business logic communication.

1. Set Up Fabric’s C++ Dependencies in Your Project #

Start by pulling in the necessary React Native C++ modules from the React Native repository:

• Clone the React Native repo and copy the ReactCommon/fabric, ReactCommon/jsi, ReactCommon/cxxreact, and folly directories into your project.
• Configure your build system (CMake, QMake, etc.) to compile these modules with C17 or higher (Fabric relies on modern C features).
• Initialize a JSI runtime: Fabric requires a facebook::jsi::Runtime instance. You can use the default Hermes runtime (compile Hermes from React Native’s source) or adapt an existing JS engine (like V8) if your project already uses it.

2. Implement Fabric’s Host Platform Interfaces #

Fabric defines abstract C++ classes that your platform must implement to handle rendering. The most critical one is HostConfig (found in ReactCommon/fabric/core/hostConfig/HostConfig.h):

• Key HostConfig methods to implement:
  • createView: Map React Native view types (e.g., "View", "Text") to GTK/Qt widget instances. For example, when RN requests a "View", instantiate a GtkBox or QWidget.
  • updateView: Sync React Native properties (e.g., style.backgroundColor, onPress) to your native widget. Convert RN’s style objects to GTK CSS or Qt style sheets, and wire up event handlers.
  • deleteView: Clean up and destroy the native widget when Fabric no longer needs it.
  • commitMount: Handle post-mount setup (e.g., focusing a widget, attaching to a parent container).
• You’ll also need to implement HostComponentFactory to create instances of your platform-specific components, and EventDispatcher to pass native events (like button clicks) back to the JS layer via JSI.

3. Integrate Fabric’s Render Pipeline with Your UI Loop #

• Initialize the Fabric UIManager: Create an instance of facebook::react::UIManager, passing your HostConfig and JSI runtime. This is the core coordinator for rendering.
• Sync Fabric updates to your UI thread: Fabric runs most reconciliation work on a background thread, but UI updates must happen on your GTK/Qt main thread. Use platform-specific mechanisms to queue Fabric’s update commands—for example, g_idle_add for GTK or QMetaObject::invokeMethod for Qt—to ensure safe UI modifications.
• Connect the JS layer: In your React Native JS code, enable Fabric via Metro (fabric: true in metro.config.js) and use the new architecture’s AppRegistry to register your app. Load the JS bundle into your JSI runtime and trigger the initial render via Fabric’s UIManager.

4. Build ViewManager Equivalents for Your Native Widgets #

Just like in mobile React Native, you’ll need to define how RN components map to your native widgets:

• Create a C++ class for each custom widget (e.g., GtkButtonComponent) that inherits from facebook::react::FabricComponent.
• In your HostConfig::createView method, return an instance of this component when RN requests the corresponding view type.
• Implement property and event mapping: For example, map RN’s onPress prop to a GTK clicked signal, then dispatch that event back to JS via EventDispatcher.

5. Implement Bidirectional Communication #

• JS → C++: Use JSI to register global functions that your JS code can call. For example, register a callNativeBusinessLogic function that triggers your C++ app’s core logic.
• C++ → JS: Use the JSI runtime to invoke JS functions directly. For example, when your C++ backend receives data, call a JS callback to update the UI. This works similarly to Turbo Modules but operates at the JSI level.

6. Test and Iterate #

• Start small: First implement a basic View and Text component to validate rendering and property updates.
• Debug with logs: Add detailed logging to track Fabric’s reconciliation steps, JSI calls, and widget lifecycle events.
• Handle edge cases: Pay attention to thread safety, layout calculations (Fabric uses Yoga for layout—you’ll need to pass Yoga’s output to your widgets), and memory management (ensure widgets are properly destroyed when Fabric unmounts them).

• React Native’s Mobile Fabric Implementations: Look at the iOS RCTFabricHostingViewController and Android FabricViewManager code to see how they implement HostConfig and integrate with their native UI toolkits.
• ReactCommon Source Code: The comments in HostConfig.h, UIManager.h, and ComponentFactory.h are surprisingly detailed and will clarify Fabric’s expected behavior.
• Off-Tree Projects: For the project you mentioned, ignore the Node bindings and focus on how it connects Fabric’s C++ core to its native windowing layer—this will show you a working example of a non-mobile Fabric host.

内容的提问来源于stack exchange，提问作者jschuss