Chromium owns scroll/compositor/lazy-load/image-cache; Rust owns
scan/SQLite/thumbnail-decode behind a gallery:// custom protocol.
Hard-won gotchas (see in-code comments):
- Android: no native custom-scheme support → wry rewrites the URL to
http://gallery.localhost/... (custom_protocol_workaround). The JS
MUST request that form; the handler receives the reverted
gallery://localhost/... and must strip the authority.
- tauri_build::build() in build.rs must run UNCONDITIONALLY (build
scripts run on the host: a cfg(target_os) gate silently disables it,
which skips the rustc-cfg=mobile emission → no mobile_entry_point →
.so with no JNI symbols → UnsatisfiedLinkError).
- build.rs must list frontend files for rerun-if-changed (a dir-level
directive only watches the dir mtime, not content edits) — the .so
bakes the frontend at compile time and cargo otherwise ships stale
UI with a 'successful' build.
- logcat drops ~75% of lines under load: use the file trace.
- fetch() is CORS-restricted (page origin http://tauri.localhost);
<img> is not — add ACAO:* anyway.
Emulator (5853 real files): grid renders, lazy thumbs via
gallery:// with on-disk cache (87 files 35-72KB), FR log clean at
60Hz, viewer opens full-res with compositor-only pan/zoom. The single
'broken' cell is a 17-byte fake test JPEG (correct behavior).
The A/B/C harness series: sltest (Slint) / egtest (egui) / tttest
(Tauri 2 → wry → Android WebView/Chromium). Identical scene to
sltest: 8580 full-width rows, 150 px + 2 px divider, alternating
colors, virtualized (26-element DOM pool), custom touch drag +
exponential-decay fling (sltest decay constant 7.42), rAF-driven
frame loop.
FR logging: the rAF callback records gap/cost exactly like the Rust
harnesses; lines batch (~120) and cross the Tauri event bridge once
per second (JNI round trip kept off the per-frame path). Rust side
drains to a dedicated writer thread →
/sdcard/Android/data/wow.gmp.tttest/files/Documents/fr.log
(Tauri document_dir = getExternalFilesDir(DOCUMENTS) — the framework
CREATES it; a raw std::fs mkdir of the app's external tree is
SELinux-denied on Android 11+). Same FR-line format → fr_capture.py
is the referee (its launch check now falls back to the package's
LAUNCHER activity via monkey for non-NativeActivity apps).
Gotchas learned (see frontend/dist/index.html comments):
- touch-action is NOT inherited: touches land on .row, so each row
must declare touch-action:none or the WebView claims the pan and
cancels the page's drag (only a few px register).
- Fling velocity must be a per-EVENT EMA; frame-based tracking dies
when touch events sample slower than rAF (adb swipe ≈ 17 Hz).
- Drag sign: finger up → offset grows (offset -= finger dy).
- Tauri v2 'brownfield' is the NORMAL direct-IPC mode (not an
app-provides-content mode); the mobile schema rejects
security.pattern entirely.
- Custom #[tauri::command]s hit the two-pass app-command ACL
chicken-and-egg; the core:default event bridge avoids it.
- Rust log output needs a logcat backend (log/std +
__android_log_write) or it is invisible.
Emulator (60 Hz, scripted swipes): 1319 frames, median 16.70 ms,
5 isolated drops (0.38%), all at swipe-injection moments, render
cost med 2 ms. aarch64 phone build compiles (11 MB APK); phone
testing pending (personal-device policy).
Build: harnesses/tttest/scripts/build.sh [x86_64|aarch64] —
cargo-tauri CLI (v2.12.4, prebuilt) drives the gradle build; signed
with the debug keystore.
