Using Particle Systems

Now that we’ve defined some example particle systems, let’s see how we can put them into use.

Adding Rain

Let’s start with our RainParticleSystem, and add rain that runs down the screen. Since we don’t need to start/stop the rain for this simple example, all we need to do is construct the particle system and add it to the Game.Components list in the Game.Initialize() method:

    RainParticleSystem rain = new RainParticleSystem(this, new Rectangle(100, -10, 500, 10));
    Components.Add(rain);

Adding Explosions

Let’s say we want an explosion to appear on-screen wherever we click our mouse. We’ll use the ExplosionParticleSystem from the previous section to accomplish this.

First, because we will need to access this system in multiple methods of Game we’ll need to create a field to represent it in our Game class. We’ll also want to keep track of the previous mouse state:

    ExplosionParticleSystem explosions;
    MouseState oldMouseState;

And initialize it in our Game.Initialize() method:

    explosions = new ExplosionParticleSystem(this, 20);
    Components.Add(explosions);

We set the particle system to use up to 20 explosions on-screen at a time (as it takes about a second for an explosion to finish, this is probably more than enough, unless we have a very explosive game).

Next, we add some logic to our Update() method to update the mouse state, and determine if we need to place an explosion:

    MouseState newMouseState = Mouse.GetState();
    Vector2 mousePosition = new Vector2(mouseState.X, mouseState.Y);

    if(newMouseState.Left == ButtonState.Down && oldMouseState.Left == ButtonState.Up) 
    {
        explosions.PlaceExplosion(mousePosition);
    }

Now, whenever we click our mouse, we’ll see an explosion spawn!

Adding a Pixie

Rather than declare a class to represent the mouse and go through that effort, let’s just implement the IParticleEmitter interface directly on our Game. Thus, we need to implement Position and Velocity properties:

    public Vector2 Position { get; set; }

    public Vector2 Velocity { get; set; }

We’ll set these in the Game.Update() method, based on our mouse state:

    Velocity = mousePosition - Position;
    Position = mousePosition;

And we’ll need to add the particle system in the Game.Initialize() method, and set its emitter to the Game instance:

    PixieParticleSystem pixie = new PixieParticleSystem(this, this);
    Components.Add(pixie);

Now the mouse should start dripping a trail of sparks!