SDL core Classic BT sequences and mechanics - CustomSDL/sdl_android_launcher GitHub Wiki

Note, that in Classic BT communication each device can have either a client or server role.

Role of SDL Android Launcher

SDL Android Launcher is implemented to have a Client role.

Scanning for devices

When the user initiates BT communication from SDL Android Launcher, it acquires the list of all currently paired devices. If this list is not empty, then Launcher iterates over this list starting from the beginning and attempting to connect to each device in this list one by one. If Launcher reaches the end of that list, it starts from the beginning again, until it finds the device with SDL-compatible application which is waiting for incoming Bluetooth connection over the specific SDP channel. In case SDL finds such a device, it also prioritizes it and moves that device on top of the list so next time the scanning sequence is initiated, Launcher will try to connect to that device first.

If the list of paired devices is empty, Launcher will start the discovery process and collects all discovered devices into the list. Once the discovery is finished, Launcher will iterate over this list in the same way as described above.

NOTE. The Android discovery mechanism is unstable by itself so it's better to pair your devices before working with Classic BT.

Establishing BLE connection

To establish Classic BT communication, you need at least 2 physical devices - one with SDL Android Launcher installed and one with a Mobile application that supports BT transport. The mobile application should open the BT server socket and wait for incoming connections if it is going to connect to Launcher over Classic BT. Once the BT server socket is active, the client device will be able to connect to the server device during the scanning sequence.

NOTE. Peripheral and central devices should BE paired with each other. Both devices should have Bluetooth enabled.

The process of connection establishing is described by the following sequence diagram - sources

Classic BT data flow

If the connection was successfully established, Launcher can initiate corresponding connection channels in SDL core and perform messages exchange in both directions. This is described by the following sequence diagram - sources

Finalizing BT connection

If the BT connection was not successful or interrupted with the current device, Launcher will try to connect to the next device in the list as described in the scanning sequence. The "move_to_next-connect-move_to_next" sequence is repeated until the user requests to stop BT transport by clicking a separate button on the Main Activity. The following sequence diagram shows what is going on when user clicks this button - sources