wave5-chunk-did-change-incremental-update

Chunk.DidChange — Incremental Update

Intent

Skip processing of unchanged chunks in IJobChunk systems using chunk.DidChange(), enabling incremental updates that recompute only when input data has changed.

Use When

• System computes output (LocalToWorld, world-space bounds, derived data) from inputs that change infrequently
• Large static worlds where most entities don't move between frames
• Expensive derived-value computation that should not run for unchanged entities

Avoid When

• Input changes every frame for all entities — DidChange always true, no benefit
• Output is cheap to compute — optimization overhead exceeds savings

Senior Pattern

[BurstCompile]
public struct ComputeDerivedJob : IJobChunk
{
    [ReadOnly] public ComponentTypeHandle<SourceData> SourceHandle;
    public ComponentTypeHandle<DerivedData> DerivedHandle;
    public uint LastSystemVersion;

    [BurstCompile]
    public void Execute(in ArchetypeChunk chunk, int unfilteredChunkIndex,
        bool useEnableMask, in v128 chunkEnabledMask)
    {
        if (!chunk.DidChange(ref SourceHandle, LastSystemVersion))
            return;  // nothing changed in this chunk since last frame

        var sources = chunk.GetNativeArray(ref SourceHandle);
        var deriveds = chunk.GetNativeArray(ref DerivedHandle);
        var enumerator = new ChunkEntityEnumerator(useEnableMask, chunkEnabledMask, chunk.Count);
        while (enumerator.NextEntityIndex(out var i))
            deriveds[i] = Compute(sources[i]);
    }
}

// In OnUpdate:
var job = new ComputeDerivedJob
{
    SourceHandle = state.GetComponentTypeHandle<SourceData>(isReadOnly: true),
    DerivedHandle = state.GetComponentTypeHandle<DerivedData>(),
    LastSystemVersion = state.LastSystemVersion   // pass per-call, not as a system field
};
state.Dependency = job.ScheduleParallel(myQuery, state.Dependency);

// For hierarchy propagation (children depend on parent LocalToWorld):
bool shouldUpdate = chunk.DidChange(ref ChildTransformHandle, LastSystemVersion)
                 || chunk.DidChange(ref LocalToWorldHandle, LastSystemVersion);