08 - System Runner

Docs / Samples / 08 - System Runner

Understand system execution order, groups, and the system runner pipeline.

Prerequisites

  • .NET SDK 8.0+ or Unity 2021.3+
  • ZenECS Core package installed
  • Understanding of System Runner

Run It

.NET Standalone

cd Packages/com.zenecs.core/Samples~/08-SystemRunner
dotnet run

Unity

  1. Open Unity project
  2. Open scene: Packages/com.zenecs.core/Samples~/08-SystemRunner/Scene.unity
  3. Press Play

Code Walkthrough

System Groups

Systems run in specific groups:

[FixedGroup]  // Deterministic simulation
public class PhysicsSystem : ISystem
{
    public void Run(IWorld world, float deltaTime) { }
}

[FrameGroup]  // Variable-timestep updates
public class CameraSystem : ISystem
{
    public void Run(IWorld world, float deltaTime) { }
}

Execution Order

Control system execution order:

[FixedGroup]
[OrderAfter(typeof(InputSystem))]
[OrderBefore(typeof(RenderSystem))]
public class GameplaySystem : ISystem
{
    public void Run(IWorld world, float deltaTime) { }
}

Complete Example

using ZenECS.Core;
using ZenECS.Core.Systems;

// Input system (runs first)
[FixedGroup]
public sealed class InputSystem : ISystem
{
    public void Run(IWorld world, float deltaTime)
    {
        // Process input
    }
}

// Gameplay system (runs after input)
[FixedGroup]
[OrderAfter(typeof(InputSystem))]
public sealed class GameplaySystem : ISystem
{
    public void Run(IWorld world, float deltaTime)
    {
        // Game logic
    }
}

// Render system (runs last)
[FixedGroup]
[OrderAfter(typeof(GameplaySystem))]
public sealed class RenderSystem : ISystem
{
    public void Run(IWorld world, float deltaTime)
    {
        // Rendering
    }
}

// Register systems (order doesn't matter, attributes control execution)
var world = kernel.CreateWorld(null, "GameWorld");
world.AddSystems([
    new RenderSystem(),    // Registered first
    new InputSystem(),      // Registered second
    new GameplaySystem()    // Registered third
]);

// Execution order (controlled by attributes):
// 1. InputSystem
// 2. GameplaySystem
// 3. RenderSystem

Execution Flow

Frame Structure

BeginFrame (Update)
  ├─ FrameInput Systems
  └─ FrameSync Systems

FixedStep (FixedUpdate) × N
  ├─ FixedInput Systems
  ├─ FixedDecision Systems
  ├─ FixedSimulation Systems
  └─ FixedPost Systems

LateFrame (LateUpdate)
  ├─ FrameView Systems
  └─ FrameUI Systems

See Also

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