Resident MIDI Keyboard for Android
ResidentMIDIKeyboard for Android is a virtual MIDI keyboard application for Android that runs “anywhere” on screen as a System Alert Window. It supports MIDI 1.0 and 2.0 (June 2023 Updates, in “almost” standard compliant way).
It works both as (1) a MIDI client keyboard that can connect to MIDI output devices, as well as (2) a virtual MIDI keyboard that DAWs can connect to it to receive MIDI input events.
It is fully open source. For more details, have a look at the README on the GitHub project.