joplin/packages/app-desktop/services/plugins/hooks/useViewIsReady.ts

import { useEffect, useState } from 'react';

export default function useViewIsReady(viewRef:any) {
	// Just checking if the iframe is ready is not sufficient because its content
	// might not be ready (for example, IPC listeners might not be initialised).
	// So we also listen to a custom "ready" message coming from the webview content
	// (in UserWebviewIndex.js)
	const [iframeReady, setIFrameReady] = useState(false);
	const [iframeContentReady, setIFrameContentReady] = useState(false);

	useEffect(() => {
		function onIFrameReady() {
			setIFrameReady(true);
		}

		function onMessage(event:any) {
			const data = event.data;

			if (!data || data.target !== 'UserWebview') return;

			if (data.message === 'ready') {
				setIFrameContentReady(true);
			}
		}

		const iframeDocument = viewRef.current.contentWindow.document;

		if (iframeDocument.readyState === 'complete') {
			onIFrameReady();
		}

		viewRef.current.addEventListener('dom-ready', onIFrameReady);
		viewRef.current.addEventListener('load', onIFrameReady);
		viewRef.current.contentWindow.addEventListener('message', onMessage);

		return () => {
			viewRef.current.removeEventListener('dom-ready', onIFrameReady);
			viewRef.current.removeEventListener('load', onIFrameReady);
			viewRef.current.contentWindow.removeEventListener('message', onMessage);
		};
	}, []);

	return iframeReady && iframeContentReady;
}