particles: expose birth_litter() to support burst emission

Closes #524
Closes #340

direct/src/particles/ParticleEffect.py

@@ -36,6 +36,10 @@ class ParticleEffect(NodePath):
             self.addParticles(particles)
         self.renderParent = None
 
+    def birthLitter(self):
+        for p in self.particlesDict.values():
+            p.birthLitter()
+
     def cleanup(self):
         self.removeNode()
         self.disable()
@@ -269,3 +273,4 @@ class ParticleEffect(NodePath):
     clear_to_initial = clearToInitial
     soft_stop = softStop
     soft_start = softStart
+    birth_litter = birthLitter

panda/src/particlesystem/particleSystem.h

@@ -104,6 +104,8 @@ PUBLISHED:
   virtual void write_spawn_templates(std::ostream &out, int indent=0) const;
   virtual void write(std::ostream &out, int indent=0) const;
 
+  void birth_litter();
+
 private:
   #ifdef PSSANITYCHECK
   int sanity_check();
@@ -111,7 +113,6 @@ private:
 
   bool birth_particle();
   void kill_particle(int pool_index);
-  void birth_litter();
   void resize_pool(int size);
 
   pdeque< int > _free_particle_fifo;

tests/particles/test_particlesystem.py

@@ -101,3 +101,38 @@ def test_particle_soft_start():
     system.update(1)
 
     assert system.get_living_particles() == 1
+
+
+def test_particle_burst_emission():
+    effect = ParticleEffect()
+    system = Particles("testSystem", 10)
+    effect.add_particles(system)
+
+    # Setup some dummy nodes, since it seems to want them
+    # We might normally call "start", but that calls "enable", which
+    # seems to assume that "base" exists and has physics and particle managers...
+    system.setRenderParent(NodePath(PandaNode("test")))
+    system.setSpawnRenderNodePath(NodePath(PandaNode("test")))
+    # However, we don't want normal emission, so we now soft-stop it immediately,
+    # before the system has a chance to update and emit.
+    effect.softStop()
+
+    # Now, a sanity-check: assert that we have no particles,
+    # Then update the system, and assert again that we
+    # have no particles. If so, then we're (hopefully)
+    # not emitting normally!
+
+    assert system.getLivingParticles() == 0
+    system.update(1)
+    assert system.getLivingParticles() == 0
+
+    # Now, the real test: emit a particle-burst!
+    effect.birthLitter()
+
+    # And assert that a particle has, in fact, been emitted.
+    assert system.getLivingParticles() == 1
+
+    # Check the snake-case version, too.
+    effect.birth_litter()
+
+    assert system.getLivingParticles() == 2