Hop Web: generic SVG canvas SPI for plugin graph editors

[mattcasters]

Context

Hop Web already renders pipelines and workflows by generating SVG on the server (PipelineCanvasSvgRenderer / WorkflowCanvasSvgRenderer), caching a revisioned snapshot, serving it via the canvasRender service handler, and overlaying it in the browser (canvas-svg.js). Clicks stay on the RAP SWT path with server-side AreaOwner hit testing; hover uses CanvasInteractionHandler.

That stack is hard-wired to pipeline and workflow graphs:

• CanvasSvgFacade only exposes renderPipeline / renderWorkflow
• CanvasSvgFacadeImpl.syncAreaOwnersToGraph only knows HopGuiPipelineGraph / HopGuiWorkflowGraph
• CanvasInteractionHandler.handleHover same hard-coding
• CanvasFacadeImpl.setDataInternal treats any non-WorkflowMeta as PipelineMeta (ClassCastException for custom models)

Plugins that ship custom canvas editors (e.g. Data Vault / Business Vault / dimensional modelers, other graph file types) cannot reuse the existing Hop Web SVG transport without forking RAP code.

Related consumer: mattcasters/hop-data-vault#119

Goal (minimal for 2.19.0 feature freeze)

Expose a small, backward-compatible SPI so any registered graph can:

1. Publish a pre-rendered CanvasSvgRenderResult snapshot (SVG + area owners + view/graph ports)
2. Receive hover events and area-owner updates without being a pipeline/workflow graph
3. Call CanvasFacade.setData with non-pipeline/workflow meta without crashing (common props only)

Pipeline/workflow behavior must remain unchanged.

Proposed API (minimal)

IWebCanvasGraph (ui module)

public interface IWebCanvasGraph {
  void replaceAreaOwners(List<AreaOwner> owners);
  void handleWebCanvasHover(int graphX, int graphY, int screenX, int screenY);
}

• Implement on existing HopGuiPipelineGraph / HopGuiWorkflowGraph (methods already exist)
• RAP hover + area sync use instanceof IWebCanvasGraph

CanvasSvgFacade.publishSnapshot(...)

public static void publishSnapshot(
    Canvas canvas,
    CanvasSvgRenderResult result,
    float magnification,
    DPoint offset,
    Point canvasSize);

• Extract existing private snapshot store path used by pipeline/workflow render
• Plugins render with their own painters + SvgGc, then publish

CanvasFacadeImpl guard

• If meta is neither PipelineMeta nor WorkflowMeta, set common props (theme, grid, mag, offset, viewport) only; do not cast to PipelineMeta

Non-goals for this PR

• Plugin-shipped JavaScript / custom RAP remote types
• Rewriting modelers inside Hop core
• Changing client canvas-svg.js interaction model

Acceptance

• Opening a custom graph that calls CanvasFacade.setData(canvas, mag, offset, customModel) does not ClassCast under RAP
• A graph implementing IWebCanvasGraph can registerCanvas + publishSnapshot and appear in Hop Web via existing client overlay
• Pipeline and workflow Hop Web paint/hover/drag unchanged
• Desktop RCP facades remain no-ops for new methods

Why 2.19.0

Needed before feature freeze so external plugins (Data Vault and others) can wire Hop Web modelers against the 2.19 line without waiting for 2.20.