From 5417ecea19f2d049696a8974a688ab651d0dc7a7 Mon Sep 17 00:00:00 2001
From: Jan Andrle <andrle.jan@centrum.cz>
Date: Fri, 25 Aug 2023 12:04:58 +0200
Subject: [PATCH] :sparkles: Add `connected`/`disconnected` observers

---
 src/events.js | 118 ++++++++++++++++++++++++++++++++++++++++++++++++++
 test/index.js |   2 +-
 2 files changed, 119 insertions(+), 1 deletion(-)

diff --git a/src/events.js b/src/events.js
index 7866b17..966d4fd 100644
--- a/src/events.js
+++ b/src/events.js
@@ -8,3 +8,121 @@ export function dispatch(event, detail){
 		event= typeof detail==="undefined" ? new Event(event) : new CustomEvent(event, { detail });
 	return element=> element.dispatchEvent(event);
 }
+
+const connections_changes= connectionsChangesObserverConstructor();
+on.connected= function(listener){
+	return function registerElement(element){
+		connections_changes.onConnected(element, listener);
+	};
+};
+on.disconnected= function(listener){
+	return function registerElement(element){
+		connections_changes.onDisconnected(element, listener);
+	};
+};
+
+function connectionsChangesObserverConstructor(){
+	const store= new Map();
+	let is_observing= false;
+	const observer= new MutationObserver(function(mutations){
+		for(const mutation of mutations){
+			if(mutation.type!=="childList") continue;
+			if(observerAdded(mutation.addedNodes)){
+				stop();
+				continue;
+			}
+			if(observerRemoved(mutation.removedNodes))
+				stop();
+		}
+	});
+	return {
+		onConnected(element, listener){
+			start();
+			const listeners= getElementStore(element);
+			listeners.connected.push(listener);
+		},
+		offConnected(element, listener){
+			if(!store.has(element)) return;
+			const ls= store.get(element);
+			const l= ls.connected;
+			l.splice(l.indexOf(listener), 1);
+			cleanWhenOff(element, ls);
+		},
+		onDisconnected(element, listener){
+			start();
+			const listeners= getElementStore(element);
+			listeners.disconnected.push(listener);
+		},
+		offDisconnected(element, listener){
+			if(!store.has(element)) return;
+			const ls= store.get(element);
+			const l= ls.disconnected;
+			l.splice(l.indexOf(listener), 1);
+			cleanWhenOff(element, ls);
+		}
+	};
+	function cleanWhenOff(element, ls){
+		if(ls.connected.length || ls.disconnect.length)
+			return;
+		store.delete(element);
+		stop();
+	}
+	function getElementStore(element){
+		if(store.has(element)) return store.get(element);
+		const out= { connected: [], disconnected: [] };
+		store.set(element, out);
+		return out;
+	}
+	function start(){
+		if(is_observing) return;
+		is_observing= true;
+		observer.observe(document.body, { childList: true, subtree: true });
+	}
+	function stop(){
+		if(!is_observing || store.size) return;
+		is_observing= false;
+		observer.disconnect();
+	}
+	//TODO remount support?
+	function requestIdle(){ return new Promise(function(resolve){
+		(requestIdleCallback || requestAnimationFrame)(resolve);
+	}); }
+	async function collectChildren(element){
+		if(store.size > 30)//TODO limit?
+			await requestIdle();
+		const out= [];
+		if(!(element instanceof Node)) return out;
+		for(const el of store.keys()){
+			if(el===element || !(el instanceof Node)) continue;
+			if(element.contains(el))
+				out.push(el);
+		}
+		return out;
+	}
+	function observerAdded(addedNodes){
+		for(const element of addedNodes){
+			collectChildren(element).then(observerAdded);
+			if(!store.has(element)) return false;
+			
+			const ls= store.get(element);
+			ls.connected.forEach(listener=> listener(element));
+			ls.connected.length= 0;
+			if(!ls.disconnected.length) store.delete(element);
+			return true;
+		}
+	}
+	function observerRemoved(removedNodes){
+		for(const element of removedNodes){
+			collectChildren(element).then(observerRemoved);
+			if(!store.has(element)) return false;
+			
+			const ls= store.get(element);
+			ls.disconnected.forEach(listener=> listener(element));
+			
+			ls.connected.length= 0;
+			ls.disconnected.length= 0;
+			store.delete(element);
+			return true;
+		}
+	}
+}
diff --git a/test/index.js b/test/index.js
index eae8b0d..9e26d95 100644
--- a/test/index.js
+++ b/test/index.js
@@ -25,7 +25,7 @@ function component({ name= "World", surname= "" }= {}){
 	const full_name= S(()=> store.name()+" "+store.surname());
 	on(full_name, console.log);
 	
-	return el("div", { className }).append(
+	return el("div", { className }, on.connected(console.log)).append(
 		el("p").append(
 			el("#text", { textContent: "Hello " }),
 			el("strong", { textContent: full_name }),