fix: send XBlock visibility status to the LMS

package.json

@@ -52,6 +52,7 @@
     "history": "5.3.0",
     "joi": "^17.11.0",
     "js-cookie": "3.0.5",
+    "lodash": "^4.17.21",
     "lodash.camelcase": "4.3.0",
     "prop-types": "15.8.1",
     "query-string": "^7.1.3",

src/courseware/course/sequence/Unit/hooks/useIFrameBehavior.js

@@ -1,6 +1,7 @@
 import { getConfig } from '@edx/frontend-platform';
 import React from 'react';
 import { useDispatch } from 'react-redux';
+import { throttle } from 'lodash';
 
 import { StrictDict, useKeyedState } from '@edx/react-unit-test-utils';
 import { logError } from '@edx/frontend-platform/logging';
@@ -84,6 +85,49 @@ const useIFrameBehavior = ({
 
   useEventListener('message', receiveMessage);
 
+  // Send visibility status to the iframe. It's used to mark XBlocks as viewed.
+  React.useEffect(() => {
+    if (!hasLoaded) {
+      return undefined;
+    }
+
+    const iframeElement = document.getElementById(elementId);
+    if (!iframeElement || !iframeElement.contentWindow) {
+      return undefined;
+    }
+
+    const updateIframeVisibility = () => {
+      const rect = iframeElement.getBoundingClientRect();
+      const visibleInfo = {
+        type: 'unit.visibilityStatus',
+        data: {
+          topPosition: rect.top,
+          viewportHeight: window.innerHeight,
+        },
+      };
+      iframeElement.contentWindow.postMessage(
+        visibleInfo,
+        `${getConfig().LMS_BASE_URL}`,
+      );
+    };
+
+    // Throttle the update function to prevent it from sending too many messages to the iframe.
+    const throttledUpdateVisibility = throttle(updateIframeVisibility, 100);
+
+    // Update the visibility of the iframe in case the element is already visible.
+    updateIframeVisibility();
+
+    // Add event listeners to update the visibility of the iframe when the window is scrolled or resized.
+    window.addEventListener('scroll', throttledUpdateVisibility);
+    window.addEventListener('resize', throttledUpdateVisibility);
+
+    // Clean up event listeners on unmount.
+    return () => {
+      window.removeEventListener('scroll', throttledUpdateVisibility);
+      window.removeEventListener('resize', throttledUpdateVisibility);
+    };
+  }, [hasLoaded, elementId]);
+
   /**
   * onLoad *should* only fire after everything in the iframe has finished its own load events.
   * Which means that the plugin.resize message (which calls setHasLoaded above) will have fired already

src/courseware/course/sequence/Unit/hooks/useIFrameBehavior.test.js

@@ -27,6 +27,11 @@ jest.mock('react-redux', () => ({
   useDispatch: jest.fn(),
 }));
 
+jest.mock('lodash', () => ({
+  ...jest.requireActual('lodash'),
+  throttle: jest.fn((fn) => fn),
+}));
+
 jest.mock('./useLoadBearingHook', () => jest.fn());
 
 jest.mock('@edx/frontend-platform/logging', () => ({
@@ -61,7 +66,10 @@ const dispatch = jest.fn();
 useDispatch.mockReturnValue(dispatch);
 
 const postMessage = jest.fn();
-const frame = { contentWindow: { postMessage } };
+const frame = {
+  contentWindow: { postMessage },
+  getBoundingClientRect: jest.fn(() => ({ top: 100 })),
+};
 const mockGetElementById = jest.fn(() => frame);
 const testHash = '#test-hash';
 
@@ -84,6 +92,10 @@ describe('useIFrameBehavior hook', () => {
   beforeEach(() => {
     jest.clearAllMocks();
     state.mock();
+    global.document.getElementById = mockGetElementById;
+    global.window.addEventListener = jest.fn();
+    global.window.removeEventListener = jest.fn();
+    global.window.innerHeight = 800;
   });
   afterEach(() => {
     state.resetVals();
@@ -262,6 +274,53 @@ describe('useIFrameBehavior hook', () => {
         });
       });
     });
+    describe('visibility tracking', () => {
+      it('sets up visibility tracking after iframe has loaded', () => {
+        state.mockVals({ ...defaultStateVals, hasLoaded: true });
+        useIFrameBehavior(props);
+
+        const effects = getEffects([true, props.elementId], React);
+        expect(effects.length).toEqual(2);
+        effects[0](); // Execute the visibility tracking effect.
+
+        expect(global.window.addEventListener).toHaveBeenCalledTimes(2);
+        expect(global.window.addEventListener).toHaveBeenCalledWith('scroll', expect.any(Function));
+        expect(global.window.addEventListener).toHaveBeenCalledWith('resize', expect.any(Function));
+        // Initial visibility update.
+        expect(postMessage).toHaveBeenCalledWith(
+          {
+            type: 'unit.visibilityStatus',
+            data: {
+              topPosition: 100,
+              viewportHeight: 800,
+            },
+          },
+          config.LMS_BASE_URL,
+        );
+      });
+      it('does not set up visibility tracking before iframe has loaded', () => {
+        state.mockVals({ ...defaultStateVals, hasLoaded: false });
+        useIFrameBehavior(props);
+
+        const effects = getEffects([false, props.elementId], React);
+        expect(effects).toBeNull();
+
+        expect(global.window.addEventListener).not.toHaveBeenCalled();
+        expect(postMessage).not.toHaveBeenCalled();
+      });
+      it('cleans up event listeners on unmount', () => {
+        state.mockVals({ ...defaultStateVals, hasLoaded: true });
+        useIFrameBehavior(props);
+
+        const effects = getEffects([true, props.elementId], React);
+        const cleanup = effects[0](); // Execute the effect and get the cleanup function.
+        cleanup(); // Call the cleanup function.
+
+        expect(global.window.removeEventListener).toHaveBeenCalledTimes(2);
+        expect(global.window.removeEventListener).toHaveBeenCalledWith('scroll', expect.any(Function));
+        expect(global.window.removeEventListener).toHaveBeenCalledWith('resize', expect.any(Function));
+      });
+    });
   });
   describe('output', () => {
     describe('handleIFrameLoad', () => {