2) Yes, but sometimes pressing play wont actually play. I also see this occurring with antenna-pod (FOSS podcast player), so I think this is Android 15 being broken, or they both use the same underlying (non standard?) audio library.
3) The sibling says yes, from checking the github issues, I think its not as "smart" as other players and simply does a fixed offset if you pause for some length of time. Honestly I always figured it ran back a few seconds due to how keyframes [sic] work in m4b files.
4) Yes, I do not have any data on my phone so I basically exclusively download my books to the device and play them offline. Sometimes I do get progress syncing issues, honestly I forget in which direction, possibly it only occurs when I finish a book offline then go online and it resets back to the last "still playing" sync point.
Sometimes I do have issues downloading a book, and it will stall out mid download. I can't say whether this is the app, the phone, the wifi or the server. It's sporadic, possibly fixed in some recentish version, haven't seen it for a while.
You can (independently) adjust the skip ahead/skip back buttons between 5s, 10s and 30s. I just leave it on 10s and mash if I have to. Before ABS I used Voice (FOSS, IMO great but I wanted library management too. Not as feature rich as Smart Audiobook Player, intentionally I think.) which only had one set of skip buttons so I never "missed" this in ABS.
The android app has improved a lot lately, it's perfectly fine to use.
There are also alternative clients, like Lissen.
And the built in equalizer. And the speed shifting. And the file management. It's just a great app.
- bookmarks (I tried to use them, but it's too much work, if I really need to take a note, I better do it in written form)
- UI to write down characters (same: I used it a couple of times, just because it's there, but it's not worth it, use normal note-taking app for note-taking)
- equalizer (I'm sure somebody uses it, but I never truly needed it for audio-books)
And many more.
What actually matters are small details:
- Granular playback speed control. ABS seems to have it done right too, but some players have like 1-1.25-1.5-..., which is not granular enough to get normal conversational talking speed out of somebody way too slow.
- It is possible to enable ID3, but it doesn't force it either.
- It recognizes chapters in whatever I have. m4b, mp3, cue, many files in a single folder, many nested folders, whatever. If it gets things wrong, it is possible to merge books in the library. I guess, the worst case is if I am overzealous with chapters (like marking chapters in a youtube podcastwhich has a mark like every 2 minutes), but then again, it only means "stop at the end of the chapter" becomes not feasible, but otherwise it's fine.
- I guess, every audio-book player does, but still: it does remember positions in multiple books, so I can switch between lectures and fictions and get back to where I stopped.
- It takes into account the current playback speed when estimating how much time is left.
- It is possible to remove buttons you don't need, it's possible to choose an action at the end of the book, it's possible to choose a lot, but defaults are really just fine for me.
- I never used it, but thinking about your question I just found out it has "Skip start/end" functionality. This is just great, I'm excited to try it out ASAP.
Anyway, I probably forget some small details, so you better take a look yourself. It's not perfect, but good enough to study when building something similar. I can also suggest 1 thing that is not in SABP, and, again, it's important to not overload the UI, but I sometimes wished it was there. In SABP it's only possible to either stop after X minutes (with sleep-suggestive volume fade), or at the end of a chapter (without fade). Sometimes I wish I could tell it to stop at the end of a chapter approximately after X minutes (like, if it has a chapter every 7-15 minutes, and you are planning to listen for about 30-40 minutes more, but you don't want it to fade volume an to stop in the middle of a chapter overall). So it's either "approx X minutes" or simply "N more chapters". Neither is possible in SABP.
Also, I don't know it I actually want it, but it seemed nice listening to podcats in NewPipe (SABP doesn't have it). There is an option to skip silence in NewPipe. I didn't actually use it too much, and the last time was a while ago (I pretty much stopped to listen podcasts on it), but I seem to recall it worked nicely.
The android app does allow you to download audio books for offline listening, which I use a lot when driving. Unfortunately the 'playlist' doesn't seem to just autoplay the next book. I don't like interacting with my phone behind the wheel. Not sure if it has an android auto app or not.
I think there are plans to rectify this behavior in a future release though.
1. Does it properly stop the playback when headphones are disconnected?
2. Is there a usable lock-screen widget?
3. Does it auto-rewind after a pause? It's best when the pause duration is taken into account: it shouldn't rewind more than a couple of seconds if you pause for a second, but a whole minute may be better if you left it off yesterday.
4. Does it handle well situations when the server is not accessible? Can you just pre-download a couple of audio-books for your 10 h long flight? I mean, honestly, streaming is never desirable, the only point of a self-hosted server is that you don't have to download and delete your collection manually. It's a tricky problem when the playlist is highly variable (as with music), but for an audio-book player should be a non issue: just pre-fetch a whole book (maybe manually), storage space is a lesser issue than mobile internet.