Use Android as A Bluetooth Keyboard, support OTG Keyboard too

How to Use

1. pair Android and other devices, like Windows or iPhone to get the device name
2. forget this Android on Windows, no need for iPhone
3. open this app, choose Windows or iPhone's name to connect
4. if it's Windows, it will pop pair-dialog up, pair it, no need for iPhone
5. OTG connect this Android with wired keyboard, choose aosp English keyboard as input method on Android
6. no need to repeat above steps for reopen this app, but if the PC's changed, it needs to repeat it again, no need for iPhone

How to Use for Archlinux

1. connect to wifi, bluetooth and wifi on same chip, so need wifi is on
2. pair android and archlinux
3. open app, choose archlinux device name, press home button to put app on backgroup
4. forget device on archlinux (or android, just one is ok)
5. on andriod, app switcher, back to app
6. now it pop pair up dialog, pair it!

Switch Window

'cause android capture Alt Tab, so, use Alt Space to switch window

Remap CapsLock to Left Ctrl

by default, capslock is switched to left ctrl

about Window Key

since android 10, lineageos 17.1 won't release su
but it can get root with adb

enable 'rooted debugging' in developer options
run adb root then adb shell u will get root privilege

before 17.1, you can get su file via https://download.lineageos.org/extras
and just flash it via recovery, then in adb shell run su

connect android to PC via usb cable, swipe down notification bar,
click android system, change 'Charging the device via USB' to 'File Transfer'
adb root
adb remount remount /system as read-write, not readonly
adb shell
nano /system/usr/keylayout/Generic.kl
change 125 to WINDOW

125 META_LEFT is window/super key in the physical keyboard

https://blog.csdn.net/u012587637/article/details/115517908

Reference

https://github.com/ginkage/wearmouse
https://github.com/domi1294/BluetoothHidDemo
https://github.com/AchimStuy/Kontroller
https://www.usb.org/sites/default/files/documents/hut1_12v2.pdf
https://elementalx.org/button-mapper/android-key-codes/

Key code and Scan code

keycode is OS use it to map key scan code is physical keyboard, there're press and release two things, send to OS, make is press, break is release, + 0x80, make code first position make it to 1 will be break code scan code A is 0x04, press code is 0x04, release code is 0x04 + 0x80

send 0x0 will send all key up

notes

-- pair and connect not same thing in archlinux, if you pair it, but it may not be connected, you have to connect by right click on it 'connect',
-- on windows 10, in the settings, after pair, it will auto be connected,
-- the important thing is run this app and then connect it, you can't connect it before this app run,
-- neton, systemctl start lightdm, systemctl start bluetooth; blueman-applet; blueman-manager
-- 1. pair android and archlinux
-- 2. open this app choose archlinux device name
-- 3. forget archlinux device name in android
-- 4. archlinux blueman-manager right click on android device name, choose 'connect'
-- 5. pair each other
-- 6. it works

Latency on iOS/iPadOS

it may have some latency on iOS/iPadOS, search "predict" in Settings,
disable "Predictive" in General -> Keyboard and all the smart things,
disable "Extended Predictions" in Accessibility -> Switch Control
be connected from other devices, not connect to other devices

Weird Connection

the device connect to other devices, it can connect directly
but other devices connect to this device with hid profile,
only the last successed one can connect it directly,
otherwise you need to forget the last successed device on this device,
then use other device connect this device