问为什么我不能在Start()方法之外使用ParticleSystem.Emit()？

EN

using UnityEngine;

[RequireComponent(typeof(ParticleSystem))]
public class ParticlePool : MonoBehaviour
{

 private int lastParticleIndex = 0;  // keeps track of our oldest particle (for deletion)

 // these will all be inited in Initialize() on Start()
 private ParticleSystem particleSys;             // our object's particle system
 private ParticleSystem.Particle[] particles;    // our reusable array of particles
 private ParticleSystem.EmitParams emitParams;   // reusable emitparams
 private int maxParticles = 0;       // total number of particles in our scene before re-using


 private void Awake()
 {
     Initialize(); // initialize all of our member variables
 }



 public void CreateParticle(Vector3 position, float size, Vector3 velocity, float angularVelocity)
 {
     // if we're at our particle count limit, kill our oldest particle.

     int activeParticles = particleSys.GetParticles(particles);
     /// this thing sucks. Read the Unity docs VERY carefully to understand...
     /// basically the parameter (particles) is an out parameter which will
     /// write out the existing particles in the particle system to our
     /// reusable array. After that, we can directly modify the particles
     /// and then when we're done, write the particles back into the
     /// particle system with ParticleSystem.SetParticles( particles, count )
     
     if (activeParticles >= maxParticles)
     {
         // set lifetime to -1 to kill the particle
         particles[lastParticleIndex].remainingLifetime = -1;
         // we need to reset start lifetime to a normal value, too or the particle will still have infinite lifetime
         particles[lastParticleIndex].startLifetime = 1;

         lastParticleIndex++;    // keep track of oldest particle
         if (lastParticleIndex >= maxParticles) lastParticleIndex = 0;

         // have to re-write
         particleSys.SetParticles(particles, particles.Length);  // write those pesky particles back into our ParticleSystem
     }


     // set up params for this particle, you can use whatever you want (see unity docs for EmitParams for what's available)
     emitParams.angularVelocity = angularVelocity;
     emitParams.position = position;
     emitParams.startSize = size;
     emitParams.velocity = velocity;
     emitParams.startLifetime = float.MaxValue; // float.MaxValue makes the particle's lifetime infinite

     particleSys.Emit(emitParams, 1);
     particleSys.Play();
 }


 void Initialize()
 {
     if (particleSys == null)
         particleSys = GetComponent<ParticleSystem>();

     maxParticles = particleSys.main.maxParticles;

     if (particles == null || particles.Length < particleSys.main.maxParticles)
         particles = new ParticleSystem.Particle[particleSys.main.maxParticles];
 }