feat(flowchart): support interactive embedding in markdown with readonly mode and frontend evaluation

flowchart/src/App.svelte

@@ -18,6 +18,9 @@
     if (params.get('iframe') === 'true') {
       fcState.isIframeMode = true;
     }
+    if (params.get('readonly') === 'true') {
+      fcState.isReadonly = true;
+    }
     
     // Listen to messages from parent window
     window.addEventListener('message', (event) => {
@@ -72,9 +75,14 @@
 
 <main>
   <Icons />
+  {#if !fcState.isReadonly}
   <Topbar />
   <Toolbar />
+  {/if}
+  
   <Canvas />
+  
+  {#if !fcState.isReadonly}
   <Properties />
   
   <div id="zoom-controls">
@@ -89,6 +97,7 @@
       <svg><use href="#icon-fit"/></svg>
     </button>
   </div>
+  {/if}
 </main>
 
 <style>

flowchart/src/lib/flowchartState.svelte.ts

@@ -34,6 +34,7 @@ export class FlowchartState {
   panY = $state<number>(0);
   
   isIframeMode = $state<boolean>(false);
+  isReadonly = $state<boolean>(false);
   initialData = $state<any>(null);
   
   isDragging = $state<boolean>(false);

frontend/src/lib/actions/renderFlowchartEmbeds.ts

@@ -0,0 +1,71 @@
+/**
+ * Find `.flowchart-embed` divs (generated by backend from ```flowchart fences)
+ * and replace them with live Flowchart iframes. Uses IntersectionObserver
+ * for lazy loading.
+ */
+
+function loadFlowchartEmbed(div: HTMLElement) {
+	const width = div.dataset.width || '100%';
+	const height = div.dataset.height || '400px';
+	const dataEl = div.querySelector('.flowchart-data');
+	const flowchartData = dataEl?.textContent?.trim() || '';
+
+	const wrapper = document.createElement('div');
+	wrapper.className = 'flowchart-embed-wrapper';
+	wrapper.style.width = width;
+	wrapper.style.height = height;
+    wrapper.style.position = 'relative';
+
+	const iframe = document.createElement('iframe');
+    // Using cb to avoid caching issues during development
+	iframe.src = `/flowchart/?iframe=true&readonly=true&cb=${Date.now()}`;
+	iframe.style.width = '100%';
+	iframe.style.height = '100%';
+	iframe.style.border = 'none';
+	iframe.title = 'Flowchart Editor';
+
+	const loadData = () => {
+		if (!flowchartData) return;
+		try {
+            iframe.contentWindow?.postMessage({
+                type: 'FLOWCHART_LOAD',
+                payload: JSON.stringify({
+                    initialData: flowchartData
+                })
+            }, '*');
+		} catch (e) {
+			console.error('Flowchart embed load error:', e);
+		}
+		const loadingEl = div.querySelector('.flowchart-embed-loading');
+		if (loadingEl) loadingEl.remove();
+	};
+
+	window.addEventListener('message', (event) => {
+        if (event.source === iframe.contentWindow && event.data?.type === 'FLOWCHART_READY') {
+            loadData();
+        }
+    });
+
+	wrapper.appendChild(iframe);
+	div.appendChild(wrapper);
+	div.dataset.rendered = 'true';
+}
+
+export function renderFlowchartEmbeds(container: HTMLElement) {
+	const embeds = container.querySelectorAll('.flowchart-embed:not([data-rendered])');
+	if (embeds.length === 0) return;
+
+	const observer = new IntersectionObserver(
+		(entries) => {
+			entries.forEach((entry) => {
+				if (entry.isIntersecting) {
+					loadFlowchartEmbed(entry.target as HTMLElement);
+					observer.unobserve(entry.target);
+				}
+			});
+		},
+		{ rootMargin: '200px' }
+	);
+
+	embeds.forEach((el) => observer.observe(el));
+}

frontend/src/routes/lesson/[slug]/+page.svelte

@@ -26,6 +26,7 @@
 	import { highlightAllCode } from '$actions/highlightCode';
 	import { setupTryButtons } from '$actions/setupTryButtons';
 	import { renderCircuitEmbeds } from '$actions/renderCircuitEmbeds';
+	import { renderFlowchartEmbeds } from '$actions/renderFlowchartEmbeds';
 	import { renderMath, autoRenderMath } from '$lib/actions/renderMath';
 	import { tick, untrack } from 'svelte';
 	import type { LessonContent } from '$types/lesson';
@@ -295,12 +296,14 @@
 					setupTryButtons(contentEl, handleTryCode);
 					highlightAllCode(contentEl);
 					renderCircuitEmbeds(contentEl);
+					renderFlowchartEmbeds(contentEl);
 					autoRenderMath(contentEl);
 				}
 				if (tabsEl) {
 					setupTryButtons(tabsEl, handleTryCode);
 					highlightAllCode(tabsEl);
 					renderCircuitEmbeds(tabsEl);
+					renderFlowchartEmbeds(tabsEl);
 					autoRenderMath(tabsEl);
 				}
 			});

services/lesson_service.py

@@ -241,6 +241,42 @@ def _process_circuit_embeds(text):
     return pattern.sub(_replacer, text)
 
 
+def _process_flowchart_embeds(text):
+    """Replace ```flowchart[,width][,height] code fences with embeddable HTML divs.
+
+    Supported formats:
+        ```flowchart          -> width=100%, height=400px
+        ```flowchart,500px    -> width=100%, height=500px
+        ```flowchart,80%,500px -> width=80%, height=500px
+    """
+    pattern = re.compile(
+        r'```flowchart(?:,([^\s,`]+))?(?:,([^\s,`]+))?\s*\n(.*?)```',
+        re.DOTALL,
+    )
+
+    def _replacer(match):
+        param1 = match.group(1)
+        param2 = match.group(2)
+        # One param = height only; two params = width, height
+        if param1 and param2:
+            width, height = param1, param2
+        elif param1:
+            width, height = '100%', param1
+        else:
+            width, height = '100%', '400px'
+        data = html_module.escape(match.group(3).strip())
+        return (
+            f'<div class="flowchart-embed" '
+            f'data-width="{html_module.escape(width)}" '
+            f'data-height="{html_module.escape(height)}">'
+            f'<pre class="flowchart-data" style="display:none">{data}</pre>'
+            f'<div class="flowchart-embed-loading">Memuat flowchart...</div>'
+            f'</div>'
+        )
+
+    return pattern.sub(_replacer, text)
+
+
 def _extract_section(content, start_marker, end_marker):
     """Extract text between markers and return (extracted, remaining_content)."""
     if start_marker not in content or end_marker not in content:
@@ -379,12 +415,15 @@ def render_markdown_content(file_path):
     lesson_content = parts[0] if parts else lesson_content
     exercise_content = parts[1] if len(parts) > 1 else ""
 
-    # Convert ```circuit fences to embed divs before markdown rendering
+    # Convert ```circuit and ```flowchart fences to embed divs before markdown rendering
     lesson_content = _process_circuit_embeds(lesson_content)
+    lesson_content = _process_flowchart_embeds(lesson_content)
     if exercise_content:
         exercise_content = _process_circuit_embeds(exercise_content)
+        exercise_content = _process_flowchart_embeds(exercise_content)
     if lesson_info:
         lesson_info = _process_circuit_embeds(lesson_info)
+        lesson_info = _process_flowchart_embeds(lesson_info)
 
     lesson_html = md.markdown(lesson_content, extensions=MD_EXTENSIONS)
     exercise_html = md.markdown(exercise_content, extensions=MD_EXTENSIONS) if exercise_content else ""