Having trouble starting particle systems from script

Hi Frank!

How GlobalTime and ExternalTimeController work with particle system material

You're right, GlobalTime and ExternalTimeController will help you with this.

ExternalTimeController connects with the variables exposed by the particle system inspector where it says 

meshVis.getMaterial(0).mainPass.externalTimeInput = positiveTime-0.01;

That is: it is looking for a mesh visual in the same SceneObject where the script is added to, accessing its material (the particle system) and modifying it's time (what state the particle system is in). 

Above that, we can see that positiveTime, is a variable based on global.controlTime, which is set by the GlobalTime.js

 global.animTime = global.controlTime - getTime();
 var positiveTime = -animTime * 0.5;

In other words, these two scripts control what state the particle system is in by storing when the lens started (global.controlTime), and then calculating how long time has passed, and finally passing it into the particle material. 

It's important to note, that the particle system has a checkbox to tell it to use externalTime rather than its own time. 

(from: https://lensstudio.snapchat.com/templates/world/particles/)

(Re)starting material

Currently, in ExternalTimeController, there is a function called resetTime which resets the particle system after a certain time has elapsed. This is how you can restart the particle system. We can repurpose this to reset the particle system on command.

Let's say we want the particle system to restart when the user taps. 

What we need to do is to remove the time check if statements  and then bind the reset function to the touch event. 

global.touchSystem.touchBlocking = true;
function resetTime(time)
{
 print("RESET TIME")
 global.controlTime = getTime();
}
var ts = script.createEvent("TouchStartEvent");
ts.bind(resetTime);

Alternatively, we can expose this function to other scripts so that they can call it.

function resetTime(time)
{
 print("RESET TIME");
 global.controlTime = getTime();
}
script.api.resetTime = resetTime;

and then in the other script, we can get a reference to this script component / function

// @input Component.ScriptComponent partcleExternalTimeController

script.partcleExternalTimeController.api.resetTime();

Check out the API Reference section here: https://lensstudio.snapchat.com/guides/scripting/scripting-overview/

That's a great idea on particle system component--I will pass that on to the team :)

Can't wait to see what you come up with!

Jon