How to create a continuous pulse (loop)

Purpose. Make something pulse/blink indefinitely (a “loading” indicator, glow, the breathing of a button). LoopAnimation cycles a value from 0 → 1 repeatedly, with or without easing, without onComplete (it is infinite — you stop it with stop()).

APIs used

• new LoopAnimation(float duration [, float fps, boolean easeInOut]) — each cycle lasts duration seconds and restarts on its own. fps default 60; easeInOut default false (linear). With easeInOut=true, it applies cubic ease-in-out per cycle.
• LoopAnimation.onUpdate(Runnable) — callback on each frame; read getValue() (0–1, progress of the current cycle).
• LoopAnimation.getValue() — current progress of the cycle.
• LoopAnimation.start() / stop() — starts / stops the loop. There is no onComplete or cancel() — it is infinite by nature.
• Component.alpha(float) — applies opacity (for the “breathing” effect).
• Component.repaint() — static; required in onUpdate.

Full example (copy, paste and run)

package com.exemplo;

import dev.crossfly.ui.*;
import dev.crossfly.ui.layout.*;
import dev.crossfly.ui.animation.*;

/** Runnable main screen: pulses a "●" in a loop; stop the loop when the screen closes. */
public class MainWindow extends Window
{
   private LoopAnimation pulse;

   @Override
   public void onShow() throws Exception
   {
      super.onShow();
      setLayout(new VerticalLayout());

      Label dot = new Label("●");
      add(dot);

      // 1 s cycle, 60 fps, with easing (smooth pulse)
      pulse = new LoopAnimation(1f, 60f, true);
      pulse.onUpdate(() ->
         {
            // value goes 0 → 1 and restarts; maps to opacity 0.3 → 1.0
            float v = 0.3f + 0.7f * pulse.getValue();
            dot.alpha(v);
            Component.repaint();
         })
         .start();

      // button that ends the pulse (e.g. when loading finishes)
      add(new Button("Parar").onAction(e -> pulse.stop()));
   }

   @Override
   public void onRemove()
   {
      // Stop it when leaving the screen so you don't leave an orphan loop animating off-screen.
      if (pulse != null) pulse.stop();
      super.onRemove();
   }
}

The snippet below shows only the core. To run it, place it inside the MainWindow skeleton above.

Minimal snippet

// verified in the source: LoopAnimation(duration, fps, easeInOut); no onComplete; stop with stop()
Label dot = new Label("●");
add(dot);

// 1 s cycle, 60 fps, with easing (smooth pulse)
LoopAnimation pulse = new LoopAnimation(1f, 60f, true);
pulse.onUpdate(() ->
   {
      // value goes 0 → 1 and restarts; maps to opacity 0.3 → 1.0
      float v = 0.3f + 0.7f * pulse.getValue();
      dot.alpha(v);
      Component.repaint();
   })
   .start();

// later, to end the pulse (e.g. when loading finishes):
pulse.stop();

Important cautions:

• Keep the reference (pulse) so you can call stop(). A LoopAnimation without stop() runs forever and continuously consumes CPU/GPU (each frame calls repaint()).
• Stop when leaving the screen: call pulse.stop() when the Window is closed (onRemove/hide) so you don’t leave an orphan loop animating off-screen.
• getValue() returns to 0 at the start of each cycle. For a “back-and-forth” pulse (0→1→0), map it with a triangular or sine function over getValue().

Dig deeper

• Value animations · Press and feedback
• Sibling recipe: Fade a component · Move a component