Add support for listening to joystick connection/disconnection.

Fixes bug causing NullPointerException when removing joysticks.
Allows adding joysticks after the application has started.

jme3-core/src/main/java/com/jme3/input/JoystickConnectionListener.java

@@ -0,0 +1,7 @@
+package com.jme3.input;
+
+public interface JoystickConnectionListener {
+
+    void connectionChanged(int joystickId, JoystickState action);
+
+}

jme3-core/src/main/java/com/jme3/input/JoystickState.java

@@ -0,0 +1,47 @@
+package com.jme3.input;
+
+/**
+ * Response for joystick callback events.
+ * @author jayfella
+ */
+public enum JoystickState {
+
+    // a list of connected/disconnected codes from various contexts.
+
+    // using the JoystickState.fromCode(int) method, if the code matches
+    // it will return the enum value.
+
+    CONNECTED(new int[] {
+            0x40001 // GLFW.GLFW_CONNECTED / LWJGL3
+    }),
+
+    DISCONNECTED(new int[] {
+            0x40002 // GLFW.GLFW_DISCONNECTED / LWJGL3
+    }),
+
+    UNKNOWN(new int[0]);
+
+    private int[] codes;
+
+    JoystickState(int[] codes) {
+        this.codes = codes;
+    }
+
+    private int[] getCodes() {
+        return codes;
+    }
+
+    public static JoystickState fromCode(int value) {
+
+        for (JoystickState state : values()) {
+            for (int code : state.getCodes()) {
+                if (value == code) {
+                    return state;
+                }
+            }
+        }
+
+        return UNKNOWN;
+    }
+
+}

jme3-lwjgl3/src/main/java/com/jme3/input/lwjgl/GlfwJoystickInput.java

@@ -57,6 +57,8 @@ public class GlfwJoystickInput implements JoyInput {
 
     private final Map<JoystickButton, Boolean> joyButtonPressed = new HashMap<>();
 
+    private InputManager inputManager;
+
     private boolean initialized = false;
 
     @Override
@@ -66,8 +68,24 @@ public class GlfwJoystickInput implements JoyInput {
         }
     }
 
+    public void fireJoystickConnectionEvent(int jid, int event) {
+        inputManager.fireJoystickConnectionEvent(jid, event);
+    }
+
+    public void reloadJoysticks() {
+        joysticks.clear();
+
+        if (inputManager != null) {
+            Joystick[] joysticks = loadJoysticks(inputManager);
+            inputManager.setJoysticks(joysticks);
+        }
+    }
+
     @Override
     public Joystick[] loadJoysticks(final InputManager inputManager) {
+
+        this.inputManager = inputManager;
+
         for (int i = 0; i < GLFW_JOYSTICK_LAST; i++) {
             if (glfwJoystickPresent(i)) {
                 final String name = glfwGetJoystickName(i);
@@ -126,23 +144,33 @@ public class GlfwJoystickInput implements JoyInput {
     @Override
     public void update() {
         for (final Map.Entry<Integer, GlfwJoystick> entry : joysticks.entrySet()) {
+
             // Axes
             final FloatBuffer axisValues = glfwGetJoystickAxes(entry.getKey());
 
-            for (final JoystickAxis axis : entry.getValue().getAxes()) {
-                final float value = axisValues.get(axis.getAxisId());
-                listener.onJoyAxisEvent(new JoyAxisEvent(axis, value));
+            // if a joystick is added or removed, the callback reloads the joysticks.
+            // when the callback is called and reloads the joystick, this iterator may already have started iterating.
+            // To avoid a NullPointerException we null-check the axisValues and bytebuffer objects.
+            // If the joystick it's iterating over no-longer exists it will return null.
+
+            if (axisValues != null) {
+                for (final JoystickAxis axis : entry.getValue().getAxes()) {
+                    final float value = axisValues.get(axis.getAxisId());
+                    listener.onJoyAxisEvent(new JoyAxisEvent(axis, value));
+                }
             }
 
             // Buttons
             final ByteBuffer byteBuffer = glfwGetJoystickButtons(entry.getKey());
 
-            for (final JoystickButton button : entry.getValue().getButtons()) {
-                final boolean pressed = byteBuffer.get(button.getButtonId()) == GLFW_PRESS;
+            if (byteBuffer != null) {
+                for (final JoystickButton button : entry.getValue().getButtons()) {
+                    final boolean pressed = byteBuffer.get(button.getButtonId()) == GLFW_PRESS;
 
-                if (joyButtonPressed.get(button) != pressed) {
-                    joyButtonPressed.put(button, pressed);
-                    listener.onJoyButtonEvent(new JoyButtonEvent(button, pressed));
+                    if (joyButtonPressed.get(button) != pressed) {
+                        joyButtonPressed.put(button, pressed);
+                        listener.onJoyButtonEvent(new JoyButtonEvent(button, pressed));
+                    }
                 }
             }
         }

jme3-lwjgl3/src/main/java/com/jme3/system/lwjgl/LwjglContext.java

@@ -65,6 +65,7 @@ import com.jme3.util.LWJGLBufferAllocator.ConcurrentLWJGLBufferAllocator;
 import org.lwjgl.PointerBuffer;
 import org.lwjgl.Version;
 import org.lwjgl.glfw.GLFW;
+import org.lwjgl.glfw.GLFWJoystickCallback;
 import org.lwjgl.opencl.APPLEGLSharing;
 import org.lwjgl.opencl.CL10;
 import org.lwjgl.opencl.KHRGLSharing;
@@ -233,6 +234,14 @@ public abstract class LwjglContext implements JmeContext {
             joyInput.initialize();
         }
 
+        GLFW.glfwSetJoystickCallback(new GLFWJoystickCallback() {
+            @Override
+            public void invoke(int jid, int event) {
+                joyInput.reloadJoysticks();
+                joyInput.fireJoystickConnectionEvent(jid, event);
+            }
+        });
+
         renderable.set(true);
     }