Your ruby.wasm Doesn't Work on Mobile. Here's Why, #rubykaigi 2026 LT

You ship a ruby.wasm app. It works on your MacBook. You bring it to a conference booth, hand it to a stranger on their iPhone — it crashes. The next person's Android freezes with no error. A third player comes back from a break and their quiz progress is gone. I hit all three at SITCON 2026 which is the only open source conference for students, running a Ruby quiz game powered by Ruby 4.0 WASM in-browser, designed to introduce Ruby to people who had never written a line of it. Each failure has a root cause and a fix under 10 lines. I'll show you all three.

Transcript

1. Your ruby.wasm Doesn’t Work on Mobile. Here’s Why. RubyKaigi 2026 · Lightning Talk Mu-Fan Teng (Owaru Ryudo) · Ruby Taiwan
2. Who Am I Mu-Fan Teng 🏢 Chief Organizer, Ruby Taiwan 💎 Rubyist for 19 years 🚀 Rails by day, pushing Ruby everywhere else by night 💎 🎤 ❤️ RubyKaraoke lover 𝕏 @ryudoawaru 🇹🇼 𝕏 @rubytaiwan a.k.a. Owaru Ryudo Ruby Evangelist of Taiwan credit by @headius
3. The Problem with Conference Booths 😴 Another Google Form. Fill it out, get a sticker. 🏷️ I run Ruby booths at Taiwanese dev conferences constantly 📋 Booth activity = a Google Form 🏃 Fill out your name → get a sticker → leave ❌ Attendees never actually touch Ruby
4. Then came SITCON 2026 📅 March 28, 2026 👥 ~1,000 attendees 💡 What if they could write real Ruby on their own phone, right now? Taiwan's largest student tech conference mostly students who had never written Ruby
5. What We Built sitcon2026-booth.ruby.tw ✅ Jekyll static site — zero server ✅ Ruby 4.0 WASM running in-browser ✅ Players type Ruby one-liners; WASM executes live ✅ 10 questions, difficulty gradient (easy → hard) ✅ Shareable scorecard (Web Share API) ✅ Completion certificate with proof code ✅ No install. No account. Just open your phone. github.com/rubytaiwan/sitcon2026-booth
6. Game Flow ① Quiz Phase ② Result Phase ③ Form Phase ④ Proof Phase 10 questions (easy → hard) Max 3 attempts each Answer revealed on failure Verified by rubyVM.eval() 1080×1080 Canvas scorecard Web Share API Share to IG or download Name, email, phone, school POST via Cloudflare Worker → self-hosted Listmonk Non-blocking on failure QR code + proof code RUBY-{score}-{hash8} Show staff → claim prize + SITCON city game check-in
7. 3 Bugs. 3 Root Causes. 📱 💥 iOS 17: crashes on first eval 🤖 ⏳ Android: hangs forever 📱 🔄 iOS mid-quiz: state gone
8. Bug 1 — iOS Safari 18.x RangeError: Maximum call stack size exceeded Symptom: Crashes on the very first eval. Desktop Safari ✅ · iOS Chrome ✅ · iOS Safari 18 💥 Root cause: Ruby WASM is compiled with Asyncify (Binaryen transform for async I/O). The first eval that triggers a lazy require of stdlib ( stringio , strscan ) pushes the Asyncify-instrumented stack past Safari’s limit. It’s the module loading, not your code, that blows the stack. // Part 1: Pre-warm during initRuby() — force lazy stdlib into memory now try { rubyVM.eval('require "stringio"; require "strscan"'); } catch(_) {} // Part 2: In runRuby() — catch overflow, retry after one macrotask break try { result = rubyVM.eval(code).toString(); } catch (e) { if (e instanceof RangeError || e.message?.includes('call stack')) { await new Promise(r => setTimeout(r, 50)); // unwind Asyncify frames result = rubyVM.eval(code).toString(); // succeeds } }
9. Bug 2 — Android Chrome / Firefox Silent WASM streaming compile hang Symptom: Loading spinner runs forever. compileStreaming never resolves, never rejects. Zero console errors. Root cause: Some Android browser/OS combinations silently cancel the streaming pipe for the 31 MB .wasm binary — even when the server sends the correct application/wasm MIME type. Bonus: Added a two-step progress indicator: 1/2 compiling… → 2/2 initializing… so players know the page is alive. try { module = await WebAssembly.compileStreaming(fetch(wasmUrl)); } catch (_) { // Android browsers sometimes silently fail streaming — fall back const buf = await (await fetch(wasmUrl)).arrayBuffer(); module = await WebAssembly.compile(buf); // universally supported ✅ }
10. Bug 3 — iOS Safari Page Eviction Mid-session state wipe Symptom: Mid-quiz, the page silently reloads — progress gone. Some players never left the browser; others triggered it by switching apps. Not pinned to a specific iOS version. Root cause: iOS Safari aggressively reclaims memory by evicting tabs and triggering a full page reload. With a 31 MB WASM binary in memory, even active tabs are at risk. Key detail: Store {defIndex, raw} pairs (index into task bank + generated secret value), not rendered HTML — rebuild-safe after reload. function saveState() { sessionStorage.setItem('quizState', JSON.stringify({ questionIndex, score, questions // questions = [{defIndex, raw}] })); } // On init() — before starting fresh, check for saved state: const saved = sessionStorage.getItem('quizState'); if (saved) restoreFrom(JSON.parse(saved)); // Clear state only when the player reaches the proof phase (game complete)
11. ruby.wasm Is Ready for Community Outreach 🍎 iOS Safari 18 🤖 Android Chrome 💾 iOS Eviction ⚠️ None of these appear in the ruby.wasm documentation. Pre-warm stdlib + macrotask retry arrayBuffer fallback for compileStreaming sessionStorage every state transition
12. Go Ship It. 📱 SOURCE CODE github.com/rubytaiwan/sitcon2026-booth TRY IT NOW · 中文 / EN / JA sitcon2026-booth.ruby.tw Fork it · customize the questions · bring it to your community 🎉