Adding a feature to a closed-source app

11 hours ago

The author uses Audiobookshelf (ABS) for audiobooks and Smart Audiobook Player (SABP) for listening, but they don't sync progress.
To add ABS syncing to the closed-source SABP, the author used LLMs (Claude) to decompile the app with apktool and jadx.
They found a key function (PlayerService.u0()) to hook into for syncing progress, avoiding writing smali code by using Java.
The ABS API was tested manually to understand payloads, revealing an issue with the isFinished flag affecting progress.
A separate Java class (AbsSyncClient) was compiled and integrated into the APK using a smali invoke-static call.
LLM code review caught bugs like unnecessary reflection and a race condition, but missed syncing logic issues corrected by the author.
Configuration was handled via a JSON file in the book directory for server details and book IDs, avoiding UI changes.
The app was resigned with a new key, requiring reinstallation, but settings were preserved.
The experience highlighted that while LLMs make modding easier, open-source software benefits more from contributions due to accessible source code.
The author plans to switch to Lissen, an open-source ABS-compatible app, despite the successful mod.

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