Libgdx Input Handling

libGDX Input Handling

Reference for polling vs event-driven input, InputProcessor, InputMultiplexer, GestureDetector, coordinate unprojection, and platform input quirks.

Polling vs Event-Driven

Polling — check input state each frame in render():

// isKeyPressed = true EVERY frame while held → continuous actions (movement)
if (Gdx.input.isKeyPressed(Input.Keys.W)) player.y += speed * delta;
if (Gdx.input.isKeyPressed(Input.Keys.SPACE)) fireBullet();

// isKeyJustPressed = true ONLY on the first frame the key goes down → one-shot actions
if (Gdx.input.isKeyJustPressed(Input.Keys.ESCAPE)) togglePause();
if (Gdx.input.isKeyJustPressed(Input.Keys.SPACE)) jump(); // fires once per press

// Touch/mouse
if (Gdx.input.isTouched()) {
    int screenX = Gdx.input.getX();   // screen coords, Y-down
    int screenY = Gdx.input.getY();
}
if (Gdx.input.isTouched(1)) { /* second finger */ }
if (Gdx.input.justTouched()) { /* first frame of touch only */ }

isKeyPressed vs isKeyJustPressed: Using isKeyJustPressed for movement causes stuttering (only moves one frame per key press). Using isKeyPressed for a one-shot action like jumping causes repeated firing every frame the key is held.

Event-driven — implement InputProcessor and register it:

Gdx.input.setInputProcessor(myProcessor);

Use polling for continuous actions (movement, held buttons). Use event-driven for discrete actions (clicks, key presses, gestures).

InputProcessor Interface

All 9 methods — every one returns boolean:

public interface InputProcessor {
    boolean keyDown(int keycode);                                          // physical key pressed
    boolean keyUp(int keycode);                                            // physical key released
    boolean keyTyped(char character);                                      // Unicode character generated
    boolean touchDown(int screenX, int screenY, int pointer, int button);  // finger/mouse down
    boolean touchUp(int screenX, int screenY, int pointer, int button);    // finger/mouse up
    boolean touchCancelled(int screenX, int screenY, int pointer, int button); // touch cancelled by OS
    boolean touchDragged(int screenX, int screenY, int pointer);           // move while touching
    boolean mouseMoved(int screenX, int screenY);                          // mouse move (desktop only)
    boolean scrolled(float amountX, float amountY);                        // scroll wheel / touchpad
}

Return value: true = event consumed (stops propagation in InputMultiplexer). false = event not handled (passes to next processor).

keyDown vs keyTyped: keyDown(int keycode) receives Input.Keys constants — use for game controls (movement, actions). keyTyped(char character) receives the Unicode character produced — use for text input. Never compare keyTyped's char against Input.Keys constants (e.g., character == Input.Keys.A compares a char against int 29, which is silently wrong).

touchDragged vs mouseMoved: touchDragged only fires while a finger/button is held down. mouseMoved fires on mouse movement with no buttons pressed (desktop only). Do not use touchDragged for hover/mouseover detection — it won't fire without a touch.

scrolled direction: amountY is positive for scroll-down, negative for scroll-up (on most platforms). Values are typically -1 or +1 per scroll notch, not pixel deltas. Common mistake: inverting the sign when implementing zoom (scroll-up should zoom in, and amountY is negative when scrolling up).

There is NO onClick(), onTap(), or onKeyPress() method. Clicks are touchDown + touchUp. For tap detection, use GestureDetector.

InputAdapter

Convenience class with all 9 methods returning false. Extend it and override only what you need — same pattern as ApplicationAdapter:

Gdx.input.setInputProcessor(new InputAdapter() {
    @Override
    public boolean keyDown(int keycode) {
        if (keycode == Input.Keys.ESCAPE) { togglePause(); return true; }
        return false;
    }

    @Override
    public boolean touchDown(int screenX, int screenY, int pointer, int button) {
        Vector3 world = new Vector3(screenX, screenY, 0);
        camera.unproject(world);      // REQUIRED — see Coordinate Conversion below
        spawnEntity(world.x, world.y);
        return true;
    }
});

InputMultiplexer

Chains multiple InputProcessors. Dispatches events in order; stops when one returns true.

InputMultiplexer multiplexer = new InputMultiplexer();
multiplexer.addProcessor(stage);          // Stage FIRST — UI eats events before game
multiplexer.addProcessor(gameInput);      // game input SECOND
Gdx.input.setInputProcessor(multiplexer);

Stage MUST be first. If game input is first, it may return true and the Stage never receives the event — UI buttons stop working. Stage returns true only when a UI actor handles the event, so game input still receives clicks on empty areas.

Key Constants

com.badlogic.gdx.Input.Keys:

Touch / Mouse

Parameters in touchDown/touchUp/touchDragged:

• screenX, screenY — screen coordinates (Y-down, origin at top-left). NOT world coordinates.
• pointer — finger index for multi-touch (0 = first finger, 1 = second, etc.). Always 0 for mouse.
• button — which mouse button. On touch devices, always LEFT.

Button constants (Input.Buttons):

Coordinate Conversion (CRITICAL)

touchDown coordinates are screen pixels (Y-down, top-left origin). To interact with game world objects, you MUST unproject through the camera:

// Reuse a single Vector3 — do NOT allocate in touchDown every frame
private final Vector3 touchPos = new Vector3();

@Override
public boolean touchDown(int screenX, int screenY, int pointer, int button) {
    touchPos.set(screenX, screenY, 0);
    camera.unproject(touchPos);           // converts to world coords
    placeTower(touchPos.x, touchPos.y);   // now in world space
    return true;
}

Using raw screenX/screenY for game logic is always a bug. The Y axis is flipped (screen Y-down vs world Y-up), and camera position/zoom are ignored.

With a Viewport (preferred when using FitViewport, ExtendViewport, etc. with letterboxing/pillarboxing — accounts for viewport offset automatically):

private final Vector2 touchPos2 = new Vector2();

@Override
public boolean touchDown(int screenX, int screenY, int pointer, int button) {
    touchPos2.set(screenX, screenY);
    viewport.unproject(touchPos2);            // Vector2, not Vector3
    placeTower(touchPos2.x, touchPos2.y);
    return true;
}

Note: viewport.unproject() takes a Vector2. camera.unproject() takes a Vector3. Don't mix them up.

GestureDetector

Wraps an InputProcessor to detect complex gestures. GestureDetector implements InputProcessor — so it plugs directly into setInputProcessor() or InputMultiplexer.

GestureDetector detector = new GestureDetector(new GestureDetector.GestureAdapter() {
    @Override
    public boolean tap(float x, float y, int count, int button) {
        // count = tap count (double-tap = 2)
        return true;
    }

    @Override
    public boolean fling(float velocityX, float velocityY, int button) {
        // swipe gesture; velocity in pixels/sec
        return true;
    }

    @Override
    public boolean longPress(float x, float y) {
        return true;
    }

    @Override
    public boolean pan(float x, float y, float deltaX, float deltaY) {
        // finger drag
        return true;
    }

    @Override
    public boolean pinch(Vector2 initialPointer1, Vector2 initialPointer2,
                         Vector2 pointer1, Vector2 pointer2) {
        return true;
    }

    @Override
    public boolean zoom(float initialDistance, float distance) {
        // pinch-to-zoom; compute ratio = distance / initialDistance
        return true;
    }
});

// Works directly in InputMultiplexer:
multiplexer.addProcessor(stage);
multiplexer.addProcessor(detector);     // GestureDetector IS an InputProcessor
multiplexer.addProcessor(gameInput);
Gdx.input.setInputProcessor(multiplexer);

GestureListener methods: touchDown, tap, longPress, fling, pan, panStop, zoom, pinch, pinchStop. Use GestureAdapter for empty defaults.

Swipe detection: Use fling(velocityX, velocityY, button). Compare velocity components to determine direction (e.g., Math.abs(velocityX) > Math.abs(velocityY) → horizontal swipe). There is no onSwipe(), SwipeListener, or SwipeDetector class — fling() is the swipe handler.

GestureDetector coordinates are also screen coordinates. Unproject them through the camera for world-space use.

Mobile Text Input

Gdx.input.setOnscreenKeyboardVisible(true) shows the soft keyboard on mobile (no-op on desktop). Gdx.input.getTextInput() shows a native text input dialog:

Gdx.input.getTextInput(new Input.TextInputListener() {
    @Override public void input(String text) { playerName = text; }
    @Override public void canceled() { /* user dismissed */ }
}, "Enter Name", "Player1", "Your name here");

On desktop this shows a Swing dialog. Non-blocking — callbacks fire asynchronously. For in-game text fields, use Scene2D's TextField widget instead (covered by Scene2D skill).

Accelerometer

Mobile only. Returns device tilt in m/s² (gravity ~ 9.8).

if (Gdx.input.isPeripheralAvailable(Input.Peripheral.Accelerometer)) {
    float ax = Gdx.input.getAccelerometerX();
    float ay = Gdx.input.getAccelerometerY();
    float az = Gdx.input.getAccelerometerZ();
}

Returns 0 on desktop. Always check isPeripheralAvailable() or provide keyboard fallback.

Android Back / Menu Keys

Use setCatchKey() (the modern API). setCatchBackKey() and setCatchMenuKey() are deprecated.

// In create() or show():
Gdx.input.setCatchKey(Input.Keys.BACK, true);   // prevent default "exit app"
Gdx.input.setCatchKey(Input.Keys.MENU, true);    // prevent default menu

// Then handle in your InputProcessor:
@Override
public boolean keyDown(int keycode) {
    if (keycode == Input.Keys.BACK) {
        showPauseMenu();
        return true;
    }
    return false;
}

Do NOT use Gdx.input.setCatchBackKey(true) — it is deprecated. Use setCatchKey(Input.Keys.BACK, true).

Common Mistakes

1. Using screen coordinates for game logic without unprojecting — touchDown gives screen pixels (Y-down). Always camera.unproject() before using with game world.
2. Putting game input before Stage in InputMultiplexer — Stage must be first so UI buttons receive events. Game input goes after Stage.
3. Forgetting to return true from InputProcessor methods — If your processor doesn't return true, InputMultiplexer passes the event to the next processor. Events get handled twice or by the wrong processor.
4. Inventing methods that don't exist on InputProcessor — There is no onClick(), onTap(), onKeyPress(), onTouch(), onSwipe(), onSwipeLeft(), onSwipeRight(), onSwipeUp(), or onSwipeDown(). Use touchDown/touchUp for clicks, GestureDetector for taps/flings. Swipe detection = GestureDetector.fling() — there is no dedicated swipe method or SwipeListener class.
5. Not knowing GestureDetector implements InputProcessor — GestureDetector IS an InputProcessor. Add it directly to InputMultiplexer. No adapter or wrapper needed.
6. Using deprecated setCatchBackKey()/setCatchMenuKey() — Use setCatchKey(Input.Keys.BACK, true) and setCatchKey(Input.Keys.MENU, true) instead.
7. Allocating new Vector3() in every touchDown call — Reuse a field-level Vector3. Allocating per-event creates garbage and triggers GC pauses on Android.
8. Not unprojecting GestureDetector coordinates — GestureDetector's tap/pan/fling/longPress coordinates are also screen coordinates. Unproject them just like touchDown coords.
9. Using isKeyJustPressed for continuous movement — Only true for one frame per press, causing stuttering. Use isKeyPressed for held actions (movement, firing).
10. Using touchDragged for hover/mouseover on desktop — touchDragged only fires while a finger/button is held. Use mouseMoved for mouse hover without clicking.
11. Comparing keyTyped char against Input.Keys constants — keyTyped receives a Unicode character, not a key constant. character == Input.Keys.A compares char to int 29, which is silently wrong. Use keyDown for key identity checks.
12. Inverting scroll direction — scrolled() amountY is positive for scroll-down, negative for scroll-up. When implementing zoom, scroll-up (negative amountY) should zoom in (decrease camera.zoom).

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