How to Pause and Resume Planck.js Physics Loop?

This article provides a direct guide on how to pause and resume a physics simulation loop in Planck.js. You will learn how to manage the simulation’s state by controlling the execution of the world stepping function, allowing you to implement pause menus or gameplay interruptions smoothly.

Planck.js, a 2D physics engine based on Box2D, does not have a native world.pause() or world.resume() method. Instead, the simulation advances only when you explicitly call world.step(timeStep). Controlling whether the simulation is running or paused is entirely managed by wrapping this step function inside your game’s main update loop with a conditional boolean flag.

Step 1: Define a Pause State Flag

To control the simulation, create a mutable boolean variable in your game state to keep track of whether the physics should update.

let isPaused = false;

Step 2: Implement the Conditional Loop

In your game’s animation or tick loop (usually powered by requestAnimationFrame), check the status of the isPaused flag before advancing the physics world.

function gameLoop() {
  // Always request the next frame to keep rendering or UI active
  requestAnimationFrame(gameLoop);

  if (!isPaused) {
    // Advance the physics simulation by the time step (e.g., 1/60Hz)
    world.step(1 / 60);
  }

  // Draw your graphics/sprites based on body positions
  renderCurrentFrame();
}

Step 3: Create Toggle Functions

To change the simulation state during gameplay (such as pressing a “Pause” button or opening an in-game menu), simply flip the value of your flag.

function pauseSimulation() {
  isPaused = true;
}

function resumeSimulation() {
  isPaused = false;
}

// Example event listener for the 'P' key
window.addEventListener('keydown', (event) => {
  if (event.key === 'p' || event.key === 'P') {
    isPaused = !isPaused;
  }
});

Using this approach ensures that while the physics calculations freeze, your main loop can still run. This keeps your application responsive, allowing you to continue rendering UI elements, animating menus, or processing non-physics user inputs while the game world is paused.