Android sending message to BLE A2DP device - android

I have a device (ESP32) that has a Bluetooth module. I found some library to use on it and it acts as A2DP sink, works perfectly with Spotify app. My objective is to be able to send some small packages of data to it from my Android phone (device name, notifications, etc) while Spotify app is streaming media to it. What would be the best way of doing that? I tried to approach this with MediaSession.setMetadata method, but it seems it only works with actual media playback.

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Android bluetooth app to connect phone to smart speaker full-duplex?

I'm wondering if anyone knows of an app that will allow an android phone to connect to a smart speaker (like Sonos One) via bluetooth, and have the two devices function like an intercom with full-duplex (2-way) audio. There is an app called Intercom on the Play Store, but it will only connect to another phone, not a smart speaker. There are walkie-talkie apps, but these also connect two phones (and require push-to-talk).
Only the Sonos Move has proper Bluetooth support, and only a few audio profiles. You need to use WiFi, and even then you have no access to the microphone on the Sonos.
Thanks for all the come-backs. If anyone trying to solve this same problem runs across this thread, try searching Amazon for "full duplex wireless intercom." I found eMACROS Portable Wireless Intercom System on there. Sound isn't as good as a Sonos or Echo, but it works.

Bluetooth audio streaming between android devices

I made a research on the same topic and found that android devices are a2dp sources and the audio can be streamed only from an a2dp source to an a2dp sink. A2dp sink can be a bluetooth headset or a bluetooth speaker.
But my question is then how the android app named "Bluetooth Music Player" works?
It allows streaming from one mobile to another. So in this case the listening mobile device must act as a sink. How this is possible?
Are they using some other profile instead of a2dp?
Ok, that may be a different profile what they are using. Because the application needs to be installed in the client side also.
But how it becomes possible to stream voice from a bluetooth microphone to an android device?
Please help.
Without knowing details about the mentioned Bluetooth Music Player, it seems to use simple Bluetooth data connection, otherwise you would not need to install a client on playing/sending device.
To stream audio from microphone to another device, you can record it on your sending device and send it to the receiving device. You will need to implement a protocol for that purpose.
OR
You can implement an alternative A2DP sink service. This is, what the sink is: a device with a Bluetooth Protocol Stack with an implementation of A2DP Sink.
Edit:
For the case you detailed by your comments, the sending device should be left as-is, without installing any app. That implicitly means that your solution must make use of out-of-the-box Bluetooth functionality of that Android device.
What you can use here is therefor limited to those profiles that Android typically support, which is HSP, HFP and A2DP. Since you obviously want to stream music, A2DP would be your choice.
On the device supposed to receive the audio stream and do the playback, you have to implement a service providing the A2DP sink as an self implemented BluetoothService opening a BluetoothServerSocket on RFCOMM as described in Android documentation.
You will have to spend much effort implementing this, and I am not sure if you will need a license for this.

How to use Android device as a bluetooth headset for another mobile phone?

I want to make my android device working as a bluetooth headset.
I search the Android APIs, but I just find some interface which
can make Android device working as a masters, not as a device(bluetooth headset).
I'm also ready to modify the source codes of Android OS, and rebuild the OS.
What I hope is, when Android devices connect to a mobile phone with bluetooth, the Android devices can work as a bluetooth headset.
I don't know whether there are interfaces can do this, or I should modify the Android OS?
Thank you!
You need to implement HFP profile in your android device, normally phone role is AG(Audio gateway) and headset roles is HF(Hands free) , think its as a server and client role. Connect RFCOMM channel to phone using UUID defined for HF , have SDP record updated,changes will be required to handle AT Commands as well.
These will be the initial changes, once done you will need to handle audio packets(SCO packets) for voice calls

How to make an Android phone as bluetooth headset

I have 2 Android Phones, and I want to connect them together via bluetooth, and make one of them a bluetooth headset.
So, my question is how to program the second phone to change its role profile to "Bluetooth Headset" when establishing the connection ?
I've found this post but my question is a little bit different
Thank for any advice or idea :)
You cannot on Android. At least using the built-in Bluetooth A2DP profile.
Your phone will only act as a transmitter (source), not a receiver (sink).
Please look at these two answers I gave on why this is not possible :
Is it possible to send/receive voice from PC to Android device over Bluetooth
Receive audio via Bluetooth in Android
You can, on the other hand, develop two apps each acting as a client/server using your own protocol. The server could be streaming audio content to the client via Bluetooth for instance.
Another way to consider this, since using an Android device as an A2DP sink is not feasible on every phone, is building these two apps using your wifi connection. It could be much more rapid and the sound sound be of a much better quality.

Adobe AIR for mobile: Using Bluetooth audio as "Microphone"

I'm developing an AIR for Android application, and am current sending audio to fms servers via standard NetStream/Microphone options. I (ignorantly) assumed that attaching a bluetooth device would be pretty simple, and connecting it would make it show up as a native "Microphone". Unfortunately, it does not.
I don't think it is even possible to use Netstream.publish and publish raw bytes, so the only hope is that there's a way to use NativeProcess + Java to create a native microphone "handle" that AIR can pick up on.
Has anyone run into this issue?
I think one possible solution would be using NetConnection.send() instead of Netstream.publish().
You should get sound data from your BT microphone. I am not sure if you can get using AIR. You may need to use an android service that gets the sound data and feeds your AIR app via a file, a UDP port or an invoke etc.
When you get some sound data, encode it so flash can play it (Speex, Nellymoiser, etc) You can do the encoding in your Android service as well.
Whenever your AIR app receives a sound data, send it to your streaming server via NetConnection.Send().
Extend your streaming server to process sound data received. You can embed it into a flv stream, or send to other flash clients if it is a chat app.
Other than that, I can't find a way to have a "microphone handle" for your BT microphone. I once thought of creating a virtual device on Android, but I couldn't find any solution.

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