/// <summary>
/// Updates the projectile.
/// </summary>
public bool Update(GameTime gameTime)
{
float elapsedTime = (float)gameTime.ElapsedGameTime.TotalSeconds;
// Simple projectile physics.
position += velocity * elapsedTime;
velocity.Y -= elapsedTime * gravity;
age += elapsedTime;
// Update the particle emitter, which will create our particle trail.
trailEmitter.Update(gameTime, position);
// If enough time has passed, explode! Note how we pass our velocity
// in to the AddParticle method: this lets the explosion be influenced
// by the speed and direction of the projectile which created it.
if (age > projectileLifespan)
{
for (int i = 0; i < numExplosionParticles; i++)
{
explosionParticles.AddParticle(position, velocity);
}
for (int i = 0; i < numExplosionSmokeParticles; i++)
{
explosionSmokeParticles.AddParticle(position, velocity);
}
return(false);
}
return(true);
}
/// <summary>
/// Updates the emitter, creating the appropriate number of particles
/// in the appropriate positions.
/// </summary>
public void Update(GameTime gameTime, Vector3 newPosition)
{
if (gameTime == null)
{
throw new ArgumentNullException("gameTime");
}
// Work out how much time has passed since the previous update.
float elapsedTime = (float)gameTime.ElapsedGameTime.TotalSeconds;
if (elapsedTime > 0)
{
// Work out how fast we are moving.
Vector3 velocity = (newPosition - previousPosition) / elapsedTime;
// If we had any time left over that we didn't use during the
// previous update, add that to the current elapsed time.
float timeToSpend = timeLeftOver + elapsedTime;
// Counter for looping over the time interval.
float currentTime = -timeLeftOver;
// Create particles as long as we have a big enough time interval.
while (timeToSpend > timeBetweenParticles)
{
currentTime += timeBetweenParticles;
timeToSpend -= timeBetweenParticles;
// Work out the optimal position for this particle. This will produce
// evenly spaced particles regardless of the object speed, particle
// creation frequency, or game update rate.
float mu = currentTime / elapsedTime;
Vector3 position = Vector3.Lerp(previousPosition, newPosition, mu);
// Create the particle.
particleSystem.AddParticle(position, velocity);
}
// Store any time we didn't use, so it can be part of the next update.
timeLeftOver = timeToSpend;
}
previousPosition = newPosition;
}