name: gum-tool-output
description: Gum Output tab. Triggers: IOutputManager, MainOutputViewModel, GuiCommands.PrintOutput, adding output/error messages.
Gum Tool Output System Reference
Architecture
IOutputManager (Gum/Managers/MainOutputViewModel.cs) — interface with two methods:
AddOutput(string) — appends a timestamped lineAddError(string) — appends a timestamped line prefixed with "ERROR: " (two spaces)
MainOutputViewModel — implements IOutputManager. Stores all output as a single OutputText string (not a list). Registered as a singleton in DI, aliased as both MainOutputViewModel and IOutputManager.
MainOutputPlugin — PriorityPlugin that creates the Output tab at TabLocation.RightBottom.
How to Write Output
Inject IOutputManager and call AddOutput or AddError directly — this is the standard approach. Callers must already be on the UI thread.
IGuiCommands.PrintOutput is a thin wrapper around AddOutput that marshals to the UI dispatcher — use this when calling from a background thread. There is no PrintError equivalent in IGuiCommands; callers needing AddError from a background thread must dispatch manually.
Non-Obvious Behaviors
- Buffer cap:
OutputText is capped at 50,000 chars. When exceeded, it is trimmed to the last 25,000 chars — oldest output is silently discarded.
- No dispatcher in
IOutputManager: AddOutput/AddError write directly to the OutputText property with no thread marshaling. Calling from a background thread will throw.
- Auto-scroll: The
TextBox in the view uses TextBoxAutoScroll.AutoScrollToEnd="True" — new output scrolls into view automatically.
Key Files
| File | Purpose |
|---|
Gum/Managers/MainOutputViewModel.cs | IOutputManager interface + MainOutputViewModel implementation |
Gum/Commands/GuiCommands.cs | PrintOutput — dispatcher-safe wrapper |
Gum/Commands/IGuiCommands.cs | PrintOutput declaration |
Gum/Plugins/InternalPlugins/Output/MainOutputPlugin.cs | Registers the Output tab |
Gum/Plugins/InternalPlugins/Output/MainOutputPluginView.xaml | Output tab view (TextBox + clear button) |
Gum/Services/Builder.cs | DI registration of MainOutputViewModel as IOutputManager |
