SDL core start and stop sequences - CustomSDL/sdl_android_launcher GitHub Wiki

Communication between Java and C++

As SDL Core is implemented as a separate C++ shared library, Android application (usually written on Java) should be able to load it dynamically during the runtime and should be able to send and receive data from that library in some way. In the current implementation, the IPC mechanism is used for data exchange between Java and C++ parts. Both Java and C++ parts have a separate interface called IpcSender and IpcReceiver which should implement and describe how exactly this communication should work. Particularly, there is an IPC implementation that uses Local sockets for communication, however, it can be done in either way (for example by using pipes, shared memory, queues, etc).

Launching SDL Core

To start SDL core dynamically, there was implemented a separate SdlLauncherService which loads all dependent core libraries and SDL Core itself. Then, it uses JNI to invoke the C++ function which initiates the SDL Core lifecycle. For newer Android versions, SDL Core is running in a foreground service to avoid unexpected process shutdown done by the Android system. This sequence is described by the following diagram - sources

Connection establishing

Once SDL Core is started, it also initiates the IpcReceiver control channel for each supported transport (currently for BT and BLE). At the same time, Launcher initiates the IpcSender control channel for the same transports and tries to connect to Core's receivers. These control channels are used by Launcher to notify SDL Core that a certain device was connected/disconnected, so SDL core can open/close the pair of IpcSender and IpcReceiver used for sending/receiving data from the connected device. The Control channel uses a simple JSON format for all its messages. The pair of IpcSender and IpcReceiver for the corresponding device connection uses an SDL Protocol format.

Stopping SDL Core

SDL core is running as a separate service until the user requests to stop it by clicking a separate button on the Main Activity. The diagram below describes what is going on when the user clicks that button - sources