Document `PostTick` and `PreTick` for Simulation

[Doprez]

We should add a section to the Physics manual to show how to subscribe to the physics tick events in order to mimic what people are used to for FixedUpdate.

• This section should get the simulation
• subscribe to both PostTick and PreTick
• add some comments as to how they run

Example:

using Stride.Engine;
using Stride.Physics;

public class FixedUpdate : StartupScript
{
    public override void Start()
    {
        var sim = this.GetSimulation();
        sim.PreTick += Sim_PreTick;
        sim.PostTick += Sim_PostTick;
    }

    private void Sim_PreTick(Simulation sender, float tick)
    {
        // Run before the Simulation is updated
    }

    private void Sim_PostTick(Simulation sender, float tick)
    {
        // Run after the Simulation is updated
    }

    public override void Cancel()
    {
        var sim = this.GetSimulation();
        sim.PreTick -= Sim_PreTick;
        sim.PostTick -= Sim_PostTick;
    }
}

This should be a pretty simple manual page I think but we should also add it to the Stride for Unity® developers somewhere that would mention FixedUpdate from Unity.

more things to think about:

• The Bepu physics implementation does not have these events. This will be a Bullet physics doc.
• C# events kind of suck for performance apparently. Maybe we should give an example on how to make a single event call that can call multiple scripts with some sort of custom processor?
• Maybe for Bepu we can show how to use the SimulationUpdateComponent which works very similar to Unitys Monobehaviour with the Fixedupdate replaced by SimulationUpdate and AfterSimulationUpdate methods.

[VaclavElias]

Do we need to unsubscribe?

[Doprez]

Yes, absolutely worth mentioning maybe in the Cancel method that is overridable.

edit: Added to the example above.