Gum Docs Writing Reference

Organize by Functionality, Not API Type

A page's home is the functional section a user browses when they have the problem it solves, not a type-indexed folder chosen because that's where the class lives. A new feature's primary explanation goes in its task-based section (e.g. cursor hit-testing for scaled/render-target rendering under events-and-interactivity/); the gum-code-reference/<Type>/ folders hold only terse per-member stubs that link back to it.

Verify Behavior Claims Before Writing Them

Before writing a claim like "X isn't available on backend Y" or "X requires package Z," verify it against the relevant domain skill (e.g. gum-runtime-fonts, gum-kernsmith-integrations, gum-cross-platform-unification) or source — this skill covers GitBook mechanics only, not product behavior. A missing per-platform package is not proof of a capability gap: the platform may get the capability another way (Silk.NET has no KernSmith package because it renders through SkiaSharp, which rasterizes fonts natively — parity, not a gap).

File Location and SUMMARY.md

All docs live under docs/. docs/SUMMARY.md is the master table of contents — GitBook navigation is built entirely from it. A file not listed in SUMMARY.md is unreachable from the nav even if it exists on disk.

Every new file must be added to SUMMARY.md with a 2-space indent per nesting level:

* [Section](gum-tool/section/README.md)
  * [Page Title](gum-tool/section/page.md)
    * [Sub-page](gum-tool/section/sub/page.md)

Paths in SUMMARY.md are relative to docs/.

README.md as Section Landing Pages

A README.md inside a folder is the section's landing page. In SUMMARY.md, link it as folder/README.md. Sub-pages of that section are indented under it.

Image Paths

All images live in docs/.gitbook/assets/. Reference them using a depth-relative path from the page file:

• From docs/gum-tool/gum-elements/general-properties/x-units.md → ../../../.gitbook/assets/filename.png
• From docs/gum-tool/gum-elements/skia-standard-elements/arc/README.md → ../../../../.gitbook/assets/filename.png

Image filename convention: date-time prefix like 15_06 10 06.png (day_hour minute second) or 08_14 21 31.gif. Spaces in filenames require wrapping the path in <> angle brackets in standard Markdown image syntax: ![](<../../../.gitbook/assets/15_06 10 06.png>). In <figure> tags the src attribute handles spaces without angle brackets.

Some images use a generic image (N).png naming pattern with numeric suffixes like image (13) (1).png — these are GitBook-uploaded files; don't rename them.

GitBook-Specific Syntax

Hint blocks — must always have a closing {% endhint %} tag or the page breaks:

{% hint style="info" %}
Content here.
{% endhint %}

{% hint style="warning" %}
Content here.
{% endhint %}

Figure blocks — used for images with captions. Always include alt attribute and <figcaption>:

<figure><img src="../../../.gitbook/assets/filename.png" alt=""><figcaption><p>Caption text</p></figcaption></figure>

For inline images without captions, use standard Markdown: ![](<path>).

HTML entities —   (space) and d (d) appear in GitBook-generated content for spacing and special characters. Do not strip them; GitBook uses them intentionally.

Internal Links

Always use relative paths — no absolute URLs. Link to a section landing page using its README.md:

[Container](../container/README.md)

Do not use anchor-only links to other files (e.g., [foo](other-file.md#section) is fine, but [foo](#section) only works within the same page).

Tool Docs vs Code Docs

The docs have two top-level sections — Gum Tool (docs/gum-tool/) and Code (docs/code/). These must never be mixed:

• Tool docs (gum-tool/) describe the Gum UI tool — properties, editors, menus, and workflows within the tool. They should not contain C# code samples or reference runtime APIs.
• Code docs (docs/code/) describe using Gum at runtime in game code — C# APIs, Forms controls, layout in code, etc. They should not describe how to use the Gum tool UI.

When writing a tool doc page (e.g., a property page under gum-elements/), focus entirely on the tool experience: what the property does visually, where it appears in the UI, and screenshots showing the effect. Do not add code examples showing how to set the property in C#. Conversely, code doc pages should not walk through the tool's UI.

Cross-Cutting Topics (Binding, Localization, Theming)

Some features cut across many controls — binding, localization, theming, code generation — and individual controls have control-specific behaviors that intersect with them. For example, TextBox.Text can be bound to a numeric ViewModel property and the binding implicitly converts string ↔ number; ListBox interacts with BindingContext differently than other controls; localized labels behave differently in PasswordBox.

The convention for these:

• Document the system-level behavior in the feature's own section. Someone hitting the feature for the first time will look there. Binding rules go in docs/code/binding-viewmodels/, localization rules in the localization section, etc. This is the single source of truth.
• Add a brief note on the control's page with a short example and any control-specific gotchas, then link back to the feature page for the general rule (e.g. [Implicit Type Conversion](../binding-viewmodels/advanced-binding-options.md#implicit-type-conversion)). Do not duplicate the system-level explanation — trust the reader to follow the link.

The control-page note exists so that a reader troubleshooting "why doesn't my int binding take" while reading the TextBox page finds what they need without bouncing around. The feature-page section exists so that a reader learning the binding system isn't surprised later. Both audiences are real; the split avoids both blind spots without duplicating content.

Date-Specific Content Goes in Hint Blocks

Anything that is only true relative to a release date — "as of ", "before ", "in X changed" — must live in a {% hint %} block, not in the body paragraphs of a section. Phrase the surrounding prose in terms of current behavior so it stays correct after the dated note is removed.

Why: dated notes eventually become noise once enough time has passed (everyone reading the docs is on the new behavior). Keeping them in hint blocks gives a single, visually obvious anchor to delete later instead of forcing the author to re-read every paragraph hunting for embedded dates.

Pattern:

The Font Generator project property controls ... There are two options:

* **KernSmith** — ... This is the default for new projects.
* **BMFont** — ... a separate external tool.

{% hint style="info" %}
**As of May 2026:** New projects default to **KernSmith**. Before this, the default was **BMFont**. Existing projects keep their current setting.
{% endhint %}

Not this — date leaks into body bullets, so removing the dated context later means rewriting prose:

* **KernSmith** — ... Default for new projects starting in May 2026.
* **BMFont** — ... Was the default before May 2026.

This applies to: "as of ", "starting in ", "before ", "in version X", "the May 2026 release", and any "what changed in " section heading. When in doubt: if deleting the sentence later would leave the surrounding paragraph still accurate, it's safe in the body; if deleting it would leave a contradiction or dangling reference, lift it into a hint.

Flag Release Status for New Features

Before documenting a new feature or behavior, ask the user whether it has already shipped, is out in preview, or hasn't shipped yet — they usually know offhand, which is faster than investigating. Only investigate yourself when they don't know: walk the feature's commits with git merge-base --is-ancestor <commit> <sha> against the commit each candidate NuGet build was made from, then confirm against the live NuGet feed.

"Shipped" means published to NuGet (stable or preview), not merged to main. Runtime NuGet packages publish on a separate, manually-triggered workflow (dotnet-nuget.yaml) from the Tool's near-daily GitHub Releases (build-and-release.yml) — a feature on main, or even bundled into a dated Tool release, can sit unpublished to NuGet for days.

Then flag accordingly in a {% hint style="info" %} block (same pattern as dated content above):

• Not yet shipped — Available in <next calendar month> <year>, or now if building Gum from source. Next month is always the target; don't ask the user which release.
• Shipped in preview — note that it's a preview feature.
• Fully shipped — no flag; write it as current behavior.

Prose Style: Clarity Beats Brevity

Cut length that adds reading, never length that adds understanding. Density and clarity are separate axes, and a longer sentence that lands the first time beats a compressed one the reader has to decode. Trimming a page is not the same as making it clearer, and a reader who has to re-read a sentence has not been saved anything.

Three habits that cost words without buying understanding:

• Restating a claim as its own negation. "The font is generated automatically, not something you opt into" is one fact wearing two coats. Keep the positive half and delete the rest.
• Naming the mechanism instead of the effect. A reader wants to know what changes on screen and which property to set. The internal class, cache, or algorithm that does it belongs in a skill or a code comment, not in a user-facing page, unless the reader has to call it.
• Imperatives that hide the subject. "Set Font to a .ttf path to bypass family lookup" makes the reader the subject and the property an object. Make the property the subject so the line reads as reference material a reader can scan.

Read-aloud test. If a clause repeats the clause before it, or the sentence runs out of breath, rewrite it. Do not shorten it.

❌ "Set text.Font to a path ending in .ttf to bypass family name resolution and route the value through the file backed rasterization path rather than the operating system font lookup. This happens automatically, and there is nothing to enable."

✅ "Gum loads a Font value ending in .ttf as a font file rather than looking it up as an installed font family. It decides this from the extension alone, so there is no setting to turn on."

Run this as a pass before committing, not only while drafting. The mechanical rules on this page (em dashes, {% endhint %} pairs, placement comments, SUMMARY indentation) can be grepped, and the prose rules cannot, so they are the half that silently gets skipped. Re-read every new or edited paragraph against the three habits above and against the passive-voice rule under Tone and Style.

Tone and Style

• Second person ("you"), present tense, instructional tone.
• Use numbered lists for procedures; bullet lists for non-sequential items.
• Bold UI element names: menu items, button labels, tab names, category names shown in the tool (e.g., Source category, Variables tab).
• Backticks for property names, variable names, property values, file names, and code: Width Units, Relative To Children, Is Tiling Middle Sections.
• Avoid passive voice, prefer "Gum displays a label" over "a label is displayed."
• Screenshots (.png) for single states; animated GIFs (.gif) for multi-step interactions. Use them liberally.
• Never use an em dash (—). Use a comma, period, or parenthetical instead. Check new/edited text for the character before finishing a doc pass.
• Prefer plain language over jargon when a plainer phrasing says the same thing; keep Gum's own terms (control names, property names) since those are what readers need to look up.
• Avoid hyphenated compound phrases where a plainer wording works.
• After writing, re-read a passage and ask whether someone with no technical background could follow it; simplify if not, without dropping needed Gum keywords.

Code Sample Placement Comments

Every \``csharp` block must have a placement comment as its first line so readers and agents know where the code belongs.

Comment	Meaning
// Initialize	General initialization — could be Initialize(), CustomInitialize(), a constructor, etc. Use this by default.
// Update	Code that runs each frame in the update loop.
// Draw	Code that runs each frame in the draw loop.
// Class scope	Field or property declarations at class level.
// In CustomInitialize	Code specifically in a generated partial CustomInitialize() method.

When to omit the comment:

• Tutorial pages that show a complete Game/app class — leave as-is.
• Non-game types shown as their own type definition (ViewModels, enums, partial generated classes).
• using statements — self-evident, no comment needed.

Mixed-scope snippets (class members + method bodies): use // Class scope for the member declarations, then show the actual method block(s) with full signatures and indented bodies — no outer class wrapper:

// Class scope
int clickCount;

protected override void Initialize()
{
    var button = new Button();
    button.Click += (_, _) => clickCount++;
}

Platform-agnostic by default: prefer // Initialize over MonoGame-specific phrasing unless the page is explicitly MonoGame-only. This supports Raylib and other runtimes.

Irreducible per-platform syntax → tabs, don't ask. First try to collapse a sample to a single platform-neutral form. When the syntax genuinely cannot collapse — most often because a host skeleton differs (a Game subclass vs a Program.Main loop) — wrap just those blocks in GitBook {% tabs %} with one {% tab %} per platform (e.g. MonoGame / Raylib). This is the standard, expected treatment for irreducible divergence; use it without asking. Keep the surrounding prose shared above/below the tabs — only the code that truly differs goes inside. Don't assume a member's type differs per backend from memory (e.g. KeyCombo.PushedKey, KeyEventArgs.Key) — check current source, since Gum has been steadily unifying input types onto Gum.Forms.Input.Keys across backends.

XnaFiddle Links

For adding or updating XnaFiddle interactive links, see xnafiddle.md.

Code Sample Rules

• One-time initialization goes first, never mid-sample — put all startup/registration calls (e.g. GumUI.UseKeyboardDefaults(), GumUI.UseGamepadDefaults(), input-device registration, service configuration) at the top of the sample, right under the // Initialize comment where GumUI is set up — before the feature code. Readers copy a sample as one block, so a setup call buried between feature lines reads as part of the feature logic and is confusing. Group setup first, then the feature it enables. Bad: create the control, add items, then UseKeyboardDefaults(), then wire focus. Good: UseKeyboardDefaults() first, then create the control and wire focus.
• No reflection — never use GetType(), typeof(), MemberInfo, or any System.Reflection APIs in doc code samples. Use is pattern matching instead: if (device is Gum.Input.Keyboard). The one exception is typeof(T) on a generic type parameter, which is cannot replace, as in an IContentLoader sample branching on the requested asset type.
• Fully qualify non-Gum types — any type that is not part of the Gum API must use its fully qualified name. For example, use System.DateTime not DateTime. Do not rely on implicit using statements for non-Gum types.
• Prefer visible output over console output — by default, surface a sample's result in the UI itself (update a visible label, change a property on a displayed object) rather than logging it. This keeps the sample XnaFiddle-runnable and lets a reader see the effect — e.g. show "You clicked the button" in a Label instead of writing it to the output window. Avoid System.Console.WriteLine (requires opening the diagnostics window) and System.Diagnostics.Debug.WriteLine (breaks in NetFiddle) on any page intended to be fiddle-able. The one exception is a page whose subject is debugging — where the sample's output is a developer-facing diagnostic that nobody will fiddle (e.g. the events troubleshooting page). There, a breakpoint-and-inspect or Debug.WriteLine is the natural tool and is acceptable; visible-label output would be contrived.

Gotchas

• SUMMARY.md indentation is 2 spaces per level — wrong indentation breaks nesting visually in GitBook.
• A missing {% endhint %} closing tag breaks all content after it on the page.
• <figure> tags without alt and <figcaption> may not render correctly in GitBook.
• The <figcaption> content must be wrapped in <p> tags: <figcaption><p>Caption</p></figcaption>.
• Code blocks in docs use csharp language identifier: ```csharp.
• Some README.md section pages contain only a brief intro paragraph and a figure — that is intentional; sub-pages provide detail.
• Page titles use # Title (H1) at the top. Sections within a page use ## and ### — avoid jumping heading levels.
• Image/gif placeholders must be visible — never use HTML comments (<!-- TODO -->) for placeholders because they are invisible in the rendered docs. Instead use a warning hint block so the placeholder is obvious when browsing the docs.