Fixed bug 5028 - Virtual Joysticks (new joystick backend)

David Ludwig

I have created a new driver for SDL's Joystick and Game-Controller subsystem: a Virtual driver.  This driver allows one to create a software-based joystick, which to SDL applications will look and react like a real joystick, but whose state can be set programmatically.  A primary use case for this is to help enable developers to add touch-screen joysticks to their apps.

The driver comes with a set of new, public APIs, with functions to attach and detach joysticks, set virtual-joystick state, and to determine if a joystick is a virtual-one.

Use of virtual joysticks goes as such:

1. Attach one or more virtual joysticks by calling SDL_JoystickAttachVirtual.  If successful, this returns the virtual-device's joystick-index.
2. Open the virtual joysticks (using indicies returned by SDL_JoystickAttachVirtual).
3. Call any of the SDL_JoystickSetVirtual* functions when joystick-state changes.  Please note that virtual-joystick state will only get applied on the next call to SDL_JoystickUpdate, or when pumping or polling for SDL events (via SDL_PumpEvents or SDL_PollEvent).


Here is a listing of the new, public APIs, at present and subject to change:

------------------------------------------------------------

/**
 * Attaches a new virtual joystick.
 * Returns the joystick's device index, or -1 if an error occurred.
 */
extern DECLSPEC int SDLCALL SDL_JoystickAttachVirtual(SDL_JoystickType type, int naxes, int nballs, int nbuttons, int nhats);

/**
 * Detaches a virtual joystick
 * Returns 0 on success, or -1 if an error occurred.
 */
extern DECLSPEC int SDLCALL SDL_JoystickDetachVirtual(int device_index);

/**
 * Indicates whether or not a virtual-joystick is at a given device index.
 */
extern DECLSPEC SDL_bool SDLCALL SDL_JoystickIsVirtual(int device_index);

/**
 * Set values on an opened, virtual-joystick's controls.
 * Returns 0 on success, -1 on error.
 */
extern DECLSPEC int SDLCALL SDL_JoystickSetVirtualAxis(SDL_Joystick * joystick, int axis, Sint16 value);
extern DECLSPEC int SDLCALL SDL_JoystickSetVirtualBall(SDL_Joystick * joystick, int ball, Sint16 xrel, Sint16 yrel);
extern DECLSPEC int SDLCALL SDL_JoystickSetVirtualButton(SDL_Joystick * joystick, int button, Uint8 value);
extern DECLSPEC int SDLCALL SDL_JoystickSetVirtualHat(SDL_Joystick * joystick, int hat, Uint8 value);

------------------------------------------------------------

Miscellaneous notes on the initial patch, which are also subject to change:

1. no test code is present in SDL, yet.  This should, perhaps, change.  Initial development was done with an ImGui-based app, which potentially is too thick for use in SDL-official.  If tests are to be added, what kind of tests?  Automated?  Graphical?

2. virtual game controllers can be created by calling SDL_JoystickAttachVirtual with a joystick-type of SDL_JOYSTICK_TYPE_GAME_CONTROLLER, with naxes (num axes) set to SDL_CONTROLLER_AXIS_MAX, and with nbuttons (num buttons) set to SDL_CONTROLLER_BUTTON_MAX.  When updating their state, values of type SDL_GameControllerAxis or SDL_GameControllerButton can be casted to an int and used for the control-index (in calls to SDL_JoystickSetVirtual* functions).

3. virtual joysticks' guids are mostly all-zeros with the exception of the last two bytes, the first of which is a 'v', to indicate that the guid is a virtual one, and the second of which is a SDL_JoystickType that has been converted into a Uint8.

4. virtual joysticks are ONLY turned into virtual game-controllers if and when their joystick-type is set to SDL_JOYSTICK_TYPE_GAMECONTROLLER.  This is controlled by having SDL's default list of game-controllers have a single entry for a virtual game controller (of guid, "00000000000000000000000000007601", which is subject to the guid-encoding described above).

5. regarding having to call SDL_JoystickUpdate, either directly or indirectly via SDL_PumpEvents or SDL_PollEvents, before new virtual-joystick state becomes active (as specified via SDL_JoystickSetVirtual* function-calls), this was done to match behavior found in SDL's other joystick drivers, almost all of which will only update SDL-state during SDL_JoystickUpdate.

6. the initial patch is based off of SDL 2.0.12

7. the virtual joystick subsystem is disabled by default.  It should be possible to enable it by building with SDL_JOYSTICK_VIRTUAL=1



Questions, comments, suggestions, or bug reports very welcome!

src/joystick/SDL_gamecontrollerdb.h

@@ -707,6 +707,9 @@ static const char *s_ControllerMappings [] =
     "05000000de2800000611000001000000,Steam Controller,a:b0,b:b1,back:b6,guide:b8,leftshoulder:b4,leftstick:b9,lefttrigger:a2,leftx:a0,lefty:a1,rightshoulder:b5,righttrigger:a3,start:b7,x:b2,y:b3,",
     "050000005e040000e0020000df070000,Xbox Wireless Controller,a:b0,b:b1,back:b8,dpdown:h0.4,dpleft:h0.8,dpright:h0.2,dpup:h0.1,leftshoulder:b4,leftstick:b6,lefttrigger:a2,leftx:a0,lefty:a1,rightshoulder:b5,rightstick:b7,righttrigger:a5,rightx:a3,righty:a4,start:b9,x:b2,y:b3,",
 #endif
+#if defined(SDL_JOYSTICK_VIRTUAL)
+    "00000000000000000000000000007601,Virtual Joystick,a:b0,b:b1,x:b2,y:b3,back:b4,guide:b5,start:b6,leftstick:b7,rightstick:b8,leftshoulder:b9,rightshoulder:b10,dpup:b11,dpdown:b12,dpleft:b13,dpright:b14,leftx:a0,lefty:a1,rightx:a2,righty:a3,lefttrigger:a4,righttrigger:a5",
+#endif
 #if defined(SDL_JOYSTICK_EMSCRIPTEN)
     "default,Standard Gamepad,a:b0,b:b1,back:b8,dpdown:b13,dpleft:b14,dpright:b15,dpup:b12,guide:b16,leftshoulder:b4,leftstick:b10,lefttrigger:b6,leftx:a0,lefty:a1,rightshoulder:b5,rightstick:b11,righttrigger:b7,rightx:a2,righty:a3,start:b9,x:b2,y:b3,",
 #endif