Detect when particle is emit? Or last-pose vertex stream option?

I have a custom particle shader where I set some custom last-pose particle data via "SetCustomParticleData" etc.

Problem is I need to know if its a particles first frame (which doesn't have last pose data) so I can set defaults.
Unity doesn't seem to have any way to hook into this event.

Also why doesn't Unity have an option to pass in the last-pose in a vertex stream like it does with normal objects? Like really? There is so much extra work needed because of this missing feature.

 

Ok found work-around in the code below. "lastParticle = particle;" is the line that detect when a new particle spawned.

Code (CSharp):

1. for (int i = 0; i != particleCount; ++i)
2. {
3.     var particle = particles[i];
4.     var lastParticle = lastParticles[i];
5.     lastParticle.remainingLifetime -= Time.deltaTime;
6.     if (lastParticle.remainingLifetime < particle.remainingLifetime || i >= lastParticleCount) lastParticle = particle;
7. // ...

A big performance boost could be had if Unity would add an option for the last position as a vertex stream option to avoid all this hackery.