Engine calls from the UI¶

Reference for every path a UI element takes to the native engine. NpsAbi is the sole C# P/Invoke surface for NPSEngine.dll (src/Nps.EngineHost/NpsAbi.cs:3, DllName = "NPSEngine" at NpsAbi.cs:43). Everything above it is one of two facades:

• EngineSession (src/Nps.EngineHost/EngineSession.cs:67) — owns the engine handle.
• ViewportController (src/Nps.Shell/ViewportController.cs:117) — owns the G-code viewport handle.

SimulatorApp also uses MeshController (src/Nps.EngineHost/MeshController.cs:71) for mesh snapshots, and the two shell painters split by app: Slicer projects its toolpath through GcodeToolpathPainter, Simulator projects its background mesh through MeshCanvasPainter (its toolpath still paints direct — see the last section). Export names below are verified against the [DllImport] declarations in NpsAbi.cs.

flowchart TD
    UI[Button / slider / scrubber / canvas drag] --> APP[MainWindow.axaml.cs handler]
    APP --> ES[EngineSession]
    APP --> VC[ViewportController]
    APP --> MC[MeshController]
    APP --> PT[GcodeToolpathPainter / MeshCanvasPainter]
    ES --> ABI[NpsAbi P/Invoke]
    VC --> ABI
    MC --> ABI
    PT --> ABI
    ABI --> ENG[NPSEngine native]

HOW IT IS: SlicerApp never calls NpsAbi directly — rg "NpsAbi\." src/SlicerApp returns nothing; even its CLI preflight goes through EngineSession (src/SlicerApp/Program.cs:22,27,35,59). SimulatorApp has two deliberate exceptions, covered in the Simulator-only section below.

EngineSession → NpsAbi exports¶

ViewportController → NpsAbi exports¶

HOW IT IS: the legacy split setters ViewportSetOverlayMode, ViewportSetOverlayValues, ViewportSetOverlayRange are still exported (NpsAbi.cs:422–428) but deprecated — the facade's doc comment calls them legacy in favor of the one-shot ViewportApplyOverlay (ViewportController.cs:495–501). No facade method calls them.

MeshController and the two painters¶

Both painters hoist scratch buffers out of the per-segment loop (GcodeToolpathPainter.cs:54, MeshCanvasPainter.cs:74). Neither owns engine state: the mesh painter renders the MeshController snapshot, the toolpath painter reads ViewportController.MovesSpan (GcodeToolpathPainter.cs:125), a span over the pointer last handed to ViewportSetGcodeMoves.

Simulator-only raw smoke calls (Program.cs)¶

HOW IT IS: SimulatorApp/Program.cs calls NpsAbi directly, before Avalonia starts, on a throwaway engine. The UI then creates a second engine via EngineSession.CreateAndInitialize() (SimulatorApp/MainWindow.axaml.cs:108); the self-check engine is disposed and does not seed the UI session.

The UI starts regardless of the self-check outcome (Program.cs:98–99). Slicer's preflight covers the same ground but stays behind the facade: EnsureAbiMatchesHost, CreateAndInitialize, LoadModel, GenerateDemoToolpath with NumLayers: 0 (src/SlicerApp/Program.cs:22,27,35,59).

HOW IT IS: Simulator's MainWindow.RenderViewport also calls NpsAbi directly at paint time — ProjectOrbitXY and ViewportGetHeatmapColor (SimulatorApp/MainWindow.axaml.cs:768,770,778) — because it has not migrated to GcodeToolpathPainter; the painter's comment notes Simulator "may pass _simMoveIndex + 1 until it also migrates" (GcodeToolpathPainter.cs:78–79). Slicer's render path already goes through the painter.

Quick index: element → call chain¶

For where these calls sit inside each app's interaction flow, see Slicer interactions and the Simulator overview; the sharing analysis lives under Sharing.