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Each explosion that you can see on the stage is a self-contained Explosion object. It s essentially just a single Sprite. An instance of the Explosion class is added to the stage when a bullet hits the cave wall. The explosion particles that you can see flying away from the center of the explosion are all contained inside that single explosion Sprite. The particles are created by the Explosion class as MovieClip objects. var particle:MovieClip = new MovieClip(); The MovieClip class is dynamic. That means you can add new properties to MovieClip objects whenever you need them, like this: particle.vx = (Math.random() * _speedLimit) - _speedLimit * 0.5; particle.vy = (Math.random() * _speedLimit) - _speedLimit * 0.5; particle.fadeRate = Math.random() * _fadeRate + 0.02; vx, vy, and fadeRate aren t properties of the MovieClip class. I created them on the spur of the moment in this code because I needed them. Dynamic properties don t need to be defined in a class ahead of time. You can make them up as you go along. Dynamic classes are very convenient. Using them, you can keep all the code for the explosion in one class without needing to create a separate Particle class that defines the custom properties you need. If you don t need the precision or detail of a full MVC system, dynamic properties are a quick-and-easy solution that save you from having to write a lot of unnecessary supporting code. The Object class is also dynamic, and you ll see how we put it to similar use in the examples ahead.

You can make any of your own custom classes dynamic by adding the dynamic keyword to the class constructor, like this:

dynamic public function ClassName() {

This means that you can add any new properties to instances of this class. Those new properties don t need to be defined inside the class itself.

The ParticleExplosion application class creates an instance of Explosion every time a bullet hits the cave wall. Each time it creates an explosion instance, it does three things: Stores a reference to the explosion instance in an _explosions array. Adds the explosion instance to the stage. Adds an event listener to the explosion instance. The event listener is used to find out when the explosion is finished, so that it can be removed from the stage and removed from the _explosions array. Here s an abridged version of the for loop in the application class that adds the explosion. (The rest of the code in the application class is identical to the code from the examples in 5.) for(var i:int = 0; i < _bulletModels.length; i++) { // Update the bullet //Check for a collision between the cave and the bullet if(_caveBitmapData.hitTest ( new Point(_caveBitmap.x, _caveBitmap.y), 255, new Point(_bulletModels[i].xPos, _bulletModels[i].yPos) ) ) { // Cut a hole in the cave bitmap //Create the explosion var explosion:Explosion = new Explosion(); explosion.x = _bulletModels[i].xPos; explosion.y = _bulletModels[i].yPos; //1. Add the explosion to the stage addChild(explosion);

//2. Push the explosion instance into the _explosions array _explosions.push(explosion); //3. Add a listener to the explosion to find //out when it finishes explosion.addEventListener ("explosionFinished", removeExplosion); //Remove the bullet } } It s now the job of the Explosion class to make the explosion effect. It creates all the particles, animates them, and removes them from the display list when their alpha reaches zero. When the Explosion class creates a particle, it adds it to the _particles array. for (var i:uint = 0; i< _numberOfParticles; i++) { //Create the particle var particle:MovieClip = new MovieClip(); // create the particle's properties //Push the particle into the _particles array _particles.push(particle); } It removes the particle from that array when the particle is completely invisible. If there are no particles left in the array, it sends an "explosionFinished" event. if(_particles[i].alpha <= 0) { removeChild(_particles[i]); _particles.splice(i, 1); i--; if(_particles.length == 0) { dispatchEvent(new Event("explosionFinished")); } } This is the event that the ParticleExplosion application class is listening for. As soon as it hears the event, it runs the code in its removeExplosion method.