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
}