MonoGame.Samples/RockRain/Core/Particle.cs

#region File Description
//-----------------------------------------------------------------------------
// Particle.cs
//
// Microsoft XNA Community Game Platform
// Copyright (C) Microsoft Corporation. All rights reserved.
//-----------------------------------------------------------------------------
#endregion

#region Using Statements
using System;
using Microsoft.Xna.Framework;
#endregion

namespace RockRainIphone.Core
{
    /// <summary>
    /// particles are the little bits that will make up an effect. each effect will
    /// be comprised of many of these particles. They have basic physical properties,
    /// such as position, velocity, acceleration, and rotation. They'll be drawn as
    /// sprites, all layered on top of one another, and will be very pretty.
    /// </summary>
    public class Particle
    {
        // Position, Velocity, and Acceleration represent exactly what their names
        // indicate. They are public fields rather than properties so that users
        // can directly access their .X and .Y properties.
        public Vector2 Position;
        public Vector2 Velocity;
        public Vector2 Acceleration;

        private Random random;

        // how long this particle will "live"
        private float lifetime;
        public float Lifetime
        {
            get { return lifetime; }
            set { lifetime = value; }
        }

        // how long it has been since initialize was called
        private float timeSinceStart;
        public float TimeSinceStart
        {
            get { return timeSinceStart; }
            set { timeSinceStart = value; }
        }

        // the scale of this particle
        private float scale;
        public float Scale
        {
            get { return scale; }
            set { scale = value; }
        }

        // its rotation, in radians
        private float rotation;
        public float Rotation
        {
            get { return rotation; }
            set { rotation = value; }
        }

        // how fast does it rotate?
        private float rotationSpeed;
        public float RotationSpeed
        {
            get { return rotationSpeed; }
            set { rotationSpeed = value; }
        }

        // is this particle still alive? once TimeSinceStart becomes greater than
        // Lifetime, the particle should no longer be drawn or updated.
        public bool Active
        {
            get { return TimeSinceStart < Lifetime; }
        }

        
        // initialize is called by ParticleSystem to set up the particle, and prepares
        // the particle for use.
        public void Initialize(Vector2 position, Vector2 velocity, Vector2 acceleration,
            float lifetime, float scale, float rotationSpeed)
        {
            // Initialize randomizer
            random = new Random(GetHashCode());

            // set the values to the requested values
            this.Position = position;
            this.Velocity = velocity;
            this.Acceleration = acceleration;
            this.Lifetime = lifetime;
            this.Scale = scale;
            this.RotationSpeed = rotationSpeed;
            
            // reset TimeSinceStart - we have to do this because particles will be
            // reused.
            this.TimeSinceStart = 0.0f;

            // set rotation to some random value between 0 and 360 degrees.
            this.Rotation = RandomBetween(0, MathHelper.TwoPi);
        }

        //  a handy little function that gives a random float between two
        // values. This will be used in several places in the sample, in particilar in
        // ParticleSystem.InitializeParticle.
        public float RandomBetween(float min, float max)
        {
            return min + (float)random.NextDouble() * (max - min);
        }

        // update is called by the ParticleSystem on every frame. This is where the
        // particle's position and that kind of thing get updated.
        public void Update(float dt)
        {
            Velocity += Acceleration * dt;
            Position += Velocity * dt;

            Rotation += RotationSpeed * dt;

            TimeSinceStart += dt;
        }
    }
}