Implement unstable_getBoundingClientRect in RN Fabric refs

packages/react-native-renderer/src/ReactFabricHostConfig.js

@@ -56,6 +56,7 @@ const {
   unstable_DiscreteEventPriority: FabricDiscretePriority,
   unstable_getCurrentEventPriority: fabricGetCurrentEventPriority,
   setNativeProps,
+  getBoundingClientRect: fabricGetBoundingClientRect,
 } = nativeFabricUIManager;
 
 const {get: getViewConfigForType} = ReactNativeViewConfigRegistry;
@@ -210,6 +211,20 @@ class ReactFabricHostComponent {
     }
   }
 
+  unstable_getBoundingClientRect(): DOMRect {
+    const {stateNode} = this._internalInstanceHandle;
+    if (stateNode != null) {
+      const rect = fabricGetBoundingClientRect(stateNode.node);
+
+      if (rect) {
+        return new DOMRect(rect[0], rect[1], rect[2], rect[3]);
+      }
+    }
+
+    // Empty rect if any of the above failed
+    return new DOMRect(0, 0, 0, 0);
+  }
+
   setNativeProps(nativeProps: Object) {
     if (__DEV__) {
       warnForStyleProps(nativeProps, this.viewConfig.validAttributes);

packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/InitializeNativeFabricUIManager.js

@@ -149,6 +149,19 @@ const RCTFabricUIManager = {
 
     callback(10, 10, 100, 100);
   }),
+  getBoundingClientRect: jest.fn(function getBoundingClientRect(node) {
+    if (typeof node !== 'object') {
+      throw new Error(
+        `Expected node to be an object, was passed "${typeof node}"`,
+      );
+    }
+
+    if (typeof node.viewName !== 'string') {
+      throw new Error('Expected node to be a host node.');
+    }
+
+    return [10, 10, 100, 100];
+  }),
   measureLayout: jest.fn(function measureLayout(
     node,
     relativeNode,
@@ -181,3 +194,19 @@ const RCTFabricUIManager = {
 };
 
 global.nativeFabricUIManager = RCTFabricUIManager;
+
+// DOMRect isn't provided by jsdom, but it's used by `ReactFabricHostComponent`.
+// This is a basic implementation for testing.
+global.DOMRect = class DOMRect {
+  constructor(x, y, width, height) {
+    this.x = x;
+    this.y = y;
+    this.width = width;
+    this.height = height;
+  }
+
+  toJSON() {
+    const {x, y, width, height} = this;
+    return {x, y, width, height};
+  }
+};

packages/react-native-renderer/src/__tests__/ReactFabricHostComponent-test.internal.js

@@ -200,6 +200,33 @@ describe('measureLayout', () => {
   });
 });
 
+describe('unstable_getBoundingClientRect', () => {
+  test('component.unstable_getBoundingClientRect() returns DOMRect', () => {
+    const [[fooRef]] = mockRenderKeys([['foo']]);
+
+    const rect = fooRef.unstable_getBoundingClientRect();
+
+    expect(nativeFabricUIManager.getBoundingClientRect).toHaveBeenCalledTimes(
+      1,
+    );
+    expect(rect.toJSON()).toMatchObject({
+      x: 10,
+      y: 10,
+      width: 100,
+      height: 100,
+    });
+  });
+
+  test('unmounted.unstable_getBoundingClientRect() returns empty DOMRect', () => {
+    const [[fooRef]] = mockRenderKeys([['foo'], null]);
+
+    const rect = fooRef.unstable_getBoundingClientRect();
+
+    expect(nativeFabricUIManager.getBoundingClientRect).not.toHaveBeenCalled();
+    expect(rect.toJSON()).toMatchObject({x: 0, y: 0, width: 0, height: 0});
+  });
+});
+
 describe('setNativeProps', () => {
   test('setNativeProps(...) invokes setNativeProps on Fabric UIManager', () => {
     const {

scripts/flow/react-native-host-hooks.js

@@ -201,6 +201,14 @@ declare var nativeFabricUIManager: {
     onFail: () => void,
     onSuccess: __MeasureLayoutOnSuccessCallback,
   ) => void,
+  getBoundingClientRect: (
+    node: Node,
+  ) => [
+    /* x:*/ number,
+    /* y:*/ number,
+    /* width:*/ number,
+    /* height:*/ number,
+  ],
   findNodeAtPoint: (
     node: Node,
     locationX: number,