Support particle start delay time (#2431)

* feat: support particle start delay time

packages/core/src/particle/ParticleGenerator.ts

@@ -121,6 +121,8 @@ export class ParticleGenerator {
   private _firstActiveTransformedBoundingBox = 0;
   @ignoreClone
   private _firstFreeTransformedBoundingBox = 0;
+  @ignoreClone
+  private _playStartDelay = 0;
 
   /**
    * Whether the particle generator is contain alive or is still creating particles.
@@ -187,6 +189,8 @@ export class ParticleGenerator {
       if (this.useAutoRandomSeed) {
         this._resetGlobalRandSeed(Math.floor(Math.random() * 0xffffffff)); // 2^32 - 1
       }
+
+      this._playStartDelay = this.main.startDelay.evaluate(undefined, this.main._startDelayRand.random());
     }
   }
 
@@ -266,8 +270,20 @@ export class ParticleGenerator {
     const { main, emission } = this;
     const duration = main.duration;
     const lastPlayTime = this._playTime;
+    const deltaTime = elapsedTime * main.simulationSpeed;
 
-    this._playTime += elapsedTime * main.simulationSpeed;
+    // Process start delay time
+    if (this._playStartDelay > 0) {
+      const remainingDelay = (this._playStartDelay -= deltaTime);
+      if (remainingDelay < 0) {
+        this._playTime -= remainingDelay;
+        this._playStartDelay = 0;
+      } else {
+        return;
+      }
+    }
+
+    this._playTime += deltaTime;
 
     this._retireActiveParticles();
     this._freeRetiredParticles();

packages/core/src/particle/modules/MainModule.ts

@@ -63,6 +63,9 @@ export class MainModule implements ICustomClone {
   _maxParticleBuffer = 1000;
   /** @internal */
   @ignoreClone
+  readonly _startDelayRand = new Rand(0, ParticleRandomSubSeeds.StartDelay);
+  /** @internal */
+  @ignoreClone
   readonly _startSpeedRand = new Rand(0, ParticleRandomSubSeeds.StartSpeed);
   /** @internal */
   @ignoreClone