XamlToCSharpGenerator.PreviewerHost

Role

The preview helper and preview MCP host for AXSG preview orchestration.

This repo component is not a shipped global tool or public NuGet package. It is an operational host artifact used by:

the VS Code extension preview workflow
custom preview clients
automated preview smoke tests
MCP-based preview orchestration

Run modes

XamlToCSharpGenerator.PreviewerHost supports two modes:

lightweight helper mode over the legacy JSON-line command transport
MCP mode over JSON-RPC stdio via --mcp

Preview MCP mode:

dotnet run --project src/XamlToCSharpGenerator.PreviewerHost -- --mcp

What it owns

This host owns preview-session orchestration, not workspace analysis and not general runtime hot reload hosting.

It is responsible for:

starting preview sessions
relaying preview URL/session metadata
pushing live XAML updates into the active preview session
exposing preview lifecycle state over MCP
exposing in-process preview hot reload completion through axsg.preview.hotReload

It uses the shared contracts from XamlToCSharpGenerator.RemoteProtocol.Preview, so MCP mode and the lightweight helper transport share the same normalized preview operation layer.

MCP surface

When started with --mcp, the host exposes:

Tools

axsg.preview.start
axsg.preview.hotReload
axsg.preview.update
axsg.preview.stop

axsg.preview.hotReload is the preferred mutation tool when the caller needs the in-process live-apply result. axsg.preview.update is the lower-level dispatch-only operation.

Resources

axsg://preview/session/status
axsg://preview/session/events
dynamic axsg://preview/session/current

Dynamic behavior

The host advertises:

notifications/tools/list_changed
notifications/resources/list_changed
resources/subscribe
notifications/resources/updated

That dynamic behavior matters because the active preview session changes the available tool/resource catalog.

What it is not

It is not:

the standalone workspace MCP host
the runtime MCP host inside a watched Avalonia app
the designer host that loads the previewed app itself

Use:

XamlToCSharpGenerator.McpServer.Tool for workspace MCP queries
XamlSourceGenRuntimeMcpServer for live runtime hot reload, hot design, and studio state
XamlToCSharpGenerator.Previewer.DesignerHost as the internal designer-process layer that actually hosts source-generated preview runtime loading

Internal layering

The preview pipeline is split intentionally:

XamlToCSharpGenerator.PreviewerHost
- outer helper process
- MCP or lightweight helper transport
- preview session lifecycle and transport orchestration
XamlToCSharpGenerator.Previewer.DesignerHost
- inner designer host
- source-generated runtime loader integration
- live preview overlay application

This separation lets AXSG reuse the same preview session layer across the VS Code extension, MCP clients, and test harnesses.

Use it when

you are implementing a custom AXSG preview client
you need preview lifecycle resources over MCP
you need in-process preview hot reload completion through MCP
you are testing preview start/update/stop behavior independently from VS Code