Joystick: Only send joy events when focused.

This changes makes it so that you only receive joystick (and implicitly
gamecontroller) input events when your application has keyboard focus.
If you'd like to still receive events when your application is in the
background, set the SDL_JOYSTICK_ALLOW_BACKGROUND_EVENTS hint to "1".

This fixes http://bugzilla.libsdl.org/show_bug.cgi?id=1892

include/SDL_hints.h

@@ -215,6 +215,20 @@ extern "C" {
 #define SDL_HINT_GAMECONTROLLERCONFIG "SDL_GAMECONTROLLERCONFIG"
 
 
+/**
+ *  \brief  A variable that lets you enable joystick (and gamecontroller) events even when your app is in the background.
+ *
+ * The default value is "0".
+ *
+ *  The variable can be set to the following values:
+ *    "0"       - Disable joystick & gamecontroller input events when the
+ *                application is in the background.
+ *    "1"       - Enable joystick & gamecontroller input events when the
+ *                application is in the backgroumd.
+ */
+#define SDL_HINT_JOYSTICK_ALLOW_BACKGROUND_EVENTS "SDL_JOYSTICK_ALLOW_BACKGROUND_EVENTS"
+
+
 /**
  *  \brief If set to 0 then never set the top most bit on a SDL Window, even if the video mode expects it.
  *      This is a debugging aid for developers and not expected to be used by end users. The default is "1"

src/joystick/SDL_joystick.c

@@ -25,6 +25,7 @@
 #include "SDL_events.h"
 #include "SDL_sysjoystick.h"
 #include "SDL_assert.h"
+#include "SDL_hints.h"
 
 #if !SDL_EVENTS_DISABLED
 #include "../events/SDL_events_c.h"
@@ -451,6 +452,22 @@ SDL_JoystickQuit(void)
 }
 
 
+static SDL_bool
+SDL_PrivateJoystickShouldIgnoreEvent()
+{
+    const char *hint;
+    if (SDL_GetKeyboardFocus() != NULL) {
+        return SDL_FALSE;
+    }
+
+    hint = SDL_GetHint(SDL_HINT_JOYSTICK_ALLOW_BACKGROUND_EVENTS);
+    if (hint && *hint == '1') {
+        return SDL_FALSE;
+    }
+
+    return SDL_TRUE;
+}
+
 /* These are global for SDL_sysjoystick.c and SDL_events.c */
 
 int
@@ -469,6 +486,15 @@ SDL_PrivateJoystickAxis(SDL_Joystick * joystick, Uint8 axis, Sint16 value)
     }
     joystick->axes[axis] = value;
 
+    /* We ignore events if we don't have keyboard focus, except for centering
+     * events.
+     */
+    if (SDL_PrivateJoystickShouldIgnoreEvent()) {
+        if (!(joystick->closed && joystick->uncentered)) {
+            return 0;
+        }
+    }
+
     /* Post the event, if desired */
     posted = 0;
 #if !SDL_EVENTS_DISABLED
@@ -497,6 +523,16 @@ SDL_PrivateJoystickHat(SDL_Joystick * joystick, Uint8 hat, Uint8 value)
     /* Update internal joystick state */
     joystick->hats[hat] = value;
 
+    /* We ignore events if we don't have keyboard focus, except for centering
+     * events.
+     */
+    if (SDL_PrivateJoystickShouldIgnoreEvent()) {
+        if (!(joystick->closed && joystick->uncentered)) {
+            return 0;
+        }
+    }
+
+
     /* Post the event, if desired */
     posted = 0;
 #if !SDL_EVENTS_DISABLED
@@ -523,6 +559,11 @@ SDL_PrivateJoystickBall(SDL_Joystick * joystick, Uint8 ball,
         return 0;
     }
 
+    /* We ignore events if we don't have keyboard focus. */
+    if (SDL_PrivateJoystickShouldIgnoreEvent()) {
+        return 0;
+    }
+
     /* Update internal mouse state */
     joystick->balls[ball].dx += xrel;
     joystick->balls[ball].dy += yrel;
@@ -568,6 +609,12 @@ SDL_PrivateJoystickButton(SDL_Joystick * joystick, Uint8 button, Uint8 state)
         return 0;
     }
 
+    /* We ignore events if we don't have keyboard focus, except for button
+     * release. */
+    if (SDL_PrivateJoystickShouldIgnoreEvent() && event.type == SDL_JOYBUTTONDOWN) {
+        return 0;
+    }
+
     /* Update internal joystick state */
     joystick->buttons[button] = state;
 
@@ -605,7 +652,6 @@ SDL_JoystickUpdate(void)
         if ( joystick->closed && joystick->uncentered )
         {
             int i;
-            joystick->uncentered = 0;
 
             /* Tell the app that everything is centered/unpressed...  */
             for (i = 0; i < joystick->naxes; i++)
@@ -617,6 +663,7 @@ SDL_JoystickUpdate(void)
             for (i = 0; i < joystick->nhats; i++)
                 SDL_PrivateJoystickHat(joystick, i, SDL_HAT_CENTERED);
 
+            joystick->uncentered = 0;
         }
 
         SDL_updating_joystick = NULL;