SDL_sysjoystick_c.h

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Hash : 2be75c6a
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Date : 2020-03-13T19:08:45

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!

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src/joystick/virtual/SDL_sysjoystick_c.h

/*
  Simple DirectMedia Layer
  Copyright (C) 1997-2020 Sam Lantinga <slouken@libsdl.org>

  This software is provided 'as-is', without any express or implied
  warranty.  In no event will the authors be held liable for any damages
  arising from the use of this software.

  Permission is granted to anyone to use this software for any purpose,
  including commercial applications, and to alter it and redistribute it
  freely, subject to the following restrictions:

  1. The origin of this software must not be misrepresented; you must not
     claim that you wrote the original software. If you use this software
     in a product, an acknowledgment in the product documentation would be
     appreciated but is not required.
  2. Altered source versions must be plainly marked as such, and must not be
     misrepresented as being the original software.
  3. This notice may not be removed or altered from any source distribution.
*/
#include "../../SDL_internal.h"

#ifndef SDL_JOYSTICK_VIRTUAL_H
#define SDL_JOYSTICK_VIRTUAL_H

#if SDL_JOYSTICK_VIRTUAL

#include "SDL_joystick.h"

/**
 * Data for a virtual, software-only joystick.
 */
typedef struct joystick_hwdata
{
    SDL_JoystickType type;
    SDL_bool attached;
    const char *name;
    SDL_JoystickGUID guid;
    int naxes;
    Sint16 *axes;
    int nballs;
    struct {
        Sint16 xrel;
        Sint16 yrel;
    } *balls;
    int nbuttons;
    Uint8 *buttons;
    int nhats;
    Uint8 *hats;
    SDL_JoystickID instance_id;
    SDL_bool opened;
    struct joystick_hwdata *next;
} joystick_hwdata;

int SDL_JoystickAttachVirtualInner(SDL_JoystickType type,
                                   int naxes,
                                   int nballs,
                                   int nbuttons,
                                   int nhats);

int SDL_JoystickDetachVirtualInner(int device_index);

int SDL_JoystickSetVirtualAxisInner(SDL_Joystick * joystick, int axis, Sint16 value);
int SDL_JoystickSetVirtualBallInner(SDL_Joystick * joystick, int ball, Sint16 xrel, Sint16 yrel);
int SDL_JoystickSetVirtualButtonInner(SDL_Joystick * joystick, int button, Uint8 value);
int SDL_JoystickSetVirtualHatInner(SDL_Joystick * joystick, int hat, Uint8 value);

#endif  /* SDL_JOYSTICK_VIRTUAL */
#endif  /* SDL_JOYSTICK_VIRTUAL_H */

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kc3-lang/SDL/src/joystick/virtual/SDL_sysjoystick_c.h

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src/joystick/virtual/SDL_sysjoystick_c.h