XAML previewer is a standalone executable file shipped with Avalonia's NuGet package. It communicates with the IDE process via a tcp socket and supports various ways of presenting the rendered XAML

The path could be retrieved from AvaloniaPreviewerNetCoreToolPath and AvaloniaPreviewerNetFullToolPath MSBuild properties set by Avalonia NuGet package.

Previewer requires the final app executable that has BuildAvaloniaApp in the class that contains the application entry point and some information required to load app dependencies. Let's say that said executable is named App.dll (.NET Core) or App.exe (.NET Framework).

IDE is required to run previewer with depsfile and runtimeconfig from the final app executable. Previewer should be run with app build output directory set as the current directory.

dotnet exec --depsfile ./App.deps.json --runtimeconfig ./App.runtimeconfig.json $(AvaloniaPreviewerNetCoreToolPath)

Just run the executable from AvaloniaPreviewerNetFullToolPath msbuild property with the application build output directory set as the current directory.

If you went through the previous steps, the previewer should now print something like:

Usage: --transport transport_spec --session-id sid --method method app

Example: --transport tcp-bson:https://127.0.0.1:30243/ --session-id 123 --method avalonia-remote MyApp.exe

--transport allows you to configure the messaging transport. Currently only TCP transport with BSON-serialized messages is supported. The IDE process is supposed to listen on some port and pass tcp-bson:https://{host}:{port}/ uri to the previewer

--session-id allows you to distinguish between previewer instances that are connecting to the same listening socket on the IDE side. Specifying that argument will make the previewer to send you session id as a part of the initial message set

--method sets the preview presentation method.

• win32 previewer will send you an HWND it renders its contents to
• avalonia-remote previewer will act as Avalonia Remote widget protocol server and will send rendered frames using the same connection to the IDE
• html previewer start an http server on address specified by --html-url that would display rendered XAML to any browser that's compatible with websockets and HTML5 canvas

app - the path to the final app executable (e. g. App.dll (.NET Core) or App.exe (.NET Framework)). Should be after other parameters

Example previewer invocation:

~/Projects/ControlCatalogStandalone/ControlCatalog.NetCore/bin/Debug/netcoreapp3.0$ dotnet exec --depsfile ControlCatalog.NetCore.deps.json --runtimeconfig ControlCatalog.NetCore.runtimeconfig.json  ~/.nuget/packages/avalonia/0.9.0/tools/netcoreapp2.0/designer/Avalonia.Designer.HostApp.dll --method avalonia-remote --transport tcp-bson:https://127.0.0.1:26434 ControlCatalog.NetCore.dll 

Messages are encoded in BSON and are defined in:

• https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.Remote.Protocol/ViewportMessages.cs
• https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.Remote.Protocol/InputMessages.cs
• https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.Remote.Protocol/DesignMessages.cs

A NuGet package Avalonia.Remote.Protocol contains a reference implementation in BsonTcpTransport class.

Previewer connects to the IDE and sends StartDesignerSessionMessage with session id from --session-id command line argument.

With avalonia-remote transport IDE sends ClientSupportedPixelFormatsMessage with supported pixel formats and ClientRenderInfoMessage with DPI settings. IDE is expected to send ClientRenderInfoMessage on DPI change. IDE should not send those messages with transports other than avalonia-remote.

IDE sends UpdateXamlMessage with:

• Xaml - Full XAML text
• XamlFileProjectPath - path to the XAML file in the project leading / and with / as path separator char. E. g. /MyDir/MyFile.xaml. Is required for IUriContext.BaseUri.
• AssemblyPath - the full path of the assembly that was generated from the project that contains the XAML file in the previewed application path directory

Previewer responds with a UpdateXamlResultMessage. If win32 transport is used and XAML is loaded successfully, Handle property will contain an HWND.

If XAML is loaded successfully, previewer will update the rendered image using the current method

Previewer will send HWND with UpdateXamlResultMessage, IDE is required to reparent that HWND to its own window

IDE is required to pass an local url with a free port number in --html-url argument. IDE is expected to feed said url to a WebView or a standalone browser of IDE's choice

IDE is required to send ClientSupportedPixelFormatsMessage and ClientRenderInfoMessage as a part of the handshake.

Previewer will send rendered XAML in FrameMessages. IDE is required to respond with FrameReceivedMessage with the last SequenceId from a FrameMessage it has received.

If IDE wishes to allow mouse and keyboard interaction with the previewer, IDE may opt to send messages defined in https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.Remote.Protocol/InputMessages.cs