Misaki
d8a7b07624
- Refactor D3D12 backend and RenderGraph module - Update graphics RHI and core rendering components - Add Random.hlsl shader include - Regenerate API documentation and update user guides
178 lines
7.9 KiB
HTML
178 lines
7.9 KiB
HTML
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GhostEngine
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<h1 id="systems-overview">Systems Overview</h1>
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<p>Systems are where all your ECS logic lives. They interact with data by iterating over entities that match an <code>EntityQuery</code>.</p>
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<h2 id="the-isystem-interface">The <code>ISystem</code> Interface</h2>
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<p>At its core, a system implements <code>ISystem</code>, which provides three lifecycle methods:</p>
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<pre><code class="lang-csharp">public interface ISystem
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{
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void Initialize(ref readonly SystemAPI systemAPI);
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void Update(ref readonly SystemAPI systemAPI);
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void Cleanup(ref readonly SystemAPI systemAPI);
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}
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</code></pre>
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<p>The <code>SystemAPI</code> struct provides access to the <code>World</code> and <code>TimeData</code> for the current frame.</p>
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<h2 id="systembase"><code>SystemBase</code></h2>
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<p>For most gameplay code, you should inherit from the abstract class <code>SystemBase</code>, which provides convenient wrappers and handles "Should Update" logic automatically.</p>
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<pre><code class="lang-csharp">public class MovementSystem : SystemBase
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{
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private Identifier<EntityQuery> _query;
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protected override void OnInitialize(ref readonly SystemAPI systemAPI)
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{
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// Build a query for entities that have both Position and Velocity
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_query = new QueryBuilder()
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.WithAllRW<Position>()
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.WithAll<Velocity>()
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.Build(World);
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// Tell the system base to ONLY run OnUpdate if this query has at least 1 entity
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RequireQueryForUpdate(_query);
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}
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protected override void OnUpdate(ref readonly SystemAPI systemAPI)
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{
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float dt = systemAPI.Time.DeltaTime;
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ref var query = ref World.ComponentManager.GetEntityQueryReference(_query);
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foreach (var chunk in query.GetChunkIterator())
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{
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var positions = chunk.GetComponentDataRW<Position>();
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var velocities = chunk.GetComponentData<Velocity>();
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for (int i = 0; i < chunk.EntityCount; i++)
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{
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positions[i].Value += velocities[i].Value * dt;
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}
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}
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}
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}
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</code></pre>
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<h3 id="automatic-update-triggers">Automatic Update Triggers</h3>
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<p>Notice the <code>RequireQueryForUpdate(_query)</code> call. <code>SystemBase</code> will skip calling <code>OnUpdate</code> if the query results are completely empty, saving processing time. <code>SystemBase</code> also invokes <code>OnStartRunning</code> and <code>OnStopRunning</code> when the system transitions between having matching entities and not.</p>
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<h2 id="system-execution-order">System Execution Order</h2>
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<p>By default, systems execute in the order they are added to a <code>SystemGroup</code>. To enforce execution order regardless of initialization sequence, use the <code>[UpdateAfter]</code> and <code>[UpdateBefore]</code> attributes.</p>
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<pre><code class="lang-csharp">[UpdateAfter(typeof(PhysicsSystem))]
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[UpdateBefore(typeof(RenderSystem))]
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public class MovementSystem : SystemBase
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{
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// ...
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}
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</code></pre>
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<p>When you call <code>group.SortSystems()</code>, the <code>SystemGroup</code> uses Kahn's algorithm to perform a topological sort and resolves these dependencies. If circular dependencies are detected, the engine will throw an exception.</p>
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<h2 id="system-groups">System Groups</h2>
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<p><code>SystemGroup</code>s implement <code>ISystem</code> themselves, meaning you can nest groups within groups to build complex hierarchical update loops (e.g. <code>FixedUpdateGroup</code>, <code>PresentationGroup</code>). GhostEngine provides a <code>DefaultSystemGroup</code> that is automatically instantiated by the <code>SystemManager</code>.</p>
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