Writing a pane provider

A pane kind is a provider. Terminals, the file browser, hosted applications and empty panes are all providers behind the same public contract, and so is anything you write. Adding one touches no part of WinMux: not the layout engine, not the session format, not the CLI.

The contract

WinMux.Panes is the whole surface. A provider says what kind of pane it makes and how to make one:

public sealed class ClockPaneProvider : IPaneProvider
{
    public PaneKind Kind { get; } = PaneKind.Create("com.example.clock");

    public ValueTask<IPaneRuntime> CreateAsync(PaneProviderContext context, CancellationToken token = default)
        => ValueTask.FromResult<IPaneRuntime>(new ClockPaneRuntime(context, Kind));

    public ValueTask<IPaneRuntime> RestoreAsync(PaneProviderContext context, CancellationToken token = default)
        => CreateAsync(context, token);
}

The runtime owns its view, its lifetime, and what it writes into the session file so it can be restored. samples/WinMux.SampleProvider in the repository is a complete working example.

Installing one

1. Build it as a library.

2. Put it in providers/<AssemblyName>/ beside WinMux.exe — the directory is named after the assembly:

   WinMux.exe
   providers/
     Acme.Provider/
       Acme.Provider.dll
       SomeDependency.dll

3. Start WinMux. The status bar says what loaded, or why something did not.

One directory per provider, not a flat folder, because two providers wanting different versions of the same library is the normal case and a flat folder makes it unresolvable. Each gets its own AssemblyLoadContext.

Do not ship the contract assemblies

WinMux.Panes, WinMux.Core and Avalonia must be WinMux’s copies, not yours. If your provider brings its own, your IPaneProvider is a different type from the one the registry wants and nothing will load — the classic plugin failure, and a confusing one because the type names match.

In a project reference, that is Private="false".

When it does not load

Every refusal is reported with its reason, because a plugin that does not appear and does not say why is the worst outcome available:

• no public parameterless constructor;
• threw while being constructed, or when asked its kind;
• claims a pane kind something else already provides — two providers for one kind would make restoring a session ambiguous;
• the directory does not contain the assembly its name promises.

One bad provider never costs the good ones.

Choosing a kind

Use reverse-DNS — com.example.clock — and collisions become impossible by accident. The built-in kinds are terminal, file-browser, foreign-app and empty.

A session referring to a kind no installed provider handles is not destroyed: the descriptor round-trips through the file untouched and the pane opens as unavailable, saying which kind is missing. Install the provider and it comes back.