Gum Tool Output System Reference

Architecture

IOutputManager (Tools/Gum.Presentation/Managers/MainOutputViewModel.cs):

• AddOutput(string) — appends a timestamped line
• AddError(string) — appends a timestamped line prefixed with "ERROR: " (two spaces), then raises ErrorAdded
• ErrorAdded — event MainOutputPlugin subscribes to, selecting the Output tab so an error is seen without a dialog

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.
• AddError steals the tab: it selects the Output tab, so it is the wrong call for a routine or repeated message. Use AddOutput unless the user genuinely needs to look now.
• No per-line color: severity is carried by the "ERROR: " text prefix. The view binds one TextBox to one concatenated OutputText string, so coloring a single line needs the model changed to a line collection first.

Key Files

File	Purpose
Tools/Gum.Presentation/Managers/MainOutputViewModel.cs	IOutputManager interface + MainOutputViewModel implementation
Gum/Commands/GuiCommands.cs	PrintOutput — dispatcher-safe wrapper
Tools/Gum.Presentation/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