Runtime Hot Reload Reference

What It Is

Hot reload lets a running game pick up changes saved in the Gum tool without restarting. GumService.EnableHotReload(absoluteGumxSourcePath) starts a FileSystemWatcher on the source project directory; file changes trigger a debounced rebuild of GumService.Default.Root's children from freshly loaded save data.

User-facing docs: docs/code/hot-reload.md. User docs are the source of truth for the public API; keep them in sync when behavior changes.

Key Files

File	Purpose
MonoGameGum/GumHotReloadManager.cs	IGumHotReloadManager + GumHotReloadManager
MonoGameGum/GumService.cs	EnableHotReload, per-frame Update, Uninitialize stop
docs/code/hot-reload.md	Public documentation

Platform Gating

The entire file is wrapped in #if !IOS && !ANDROID. The EnableHotReload method on GumService is likewise gated. File compiles for MonoGame, KNI, FNA (under XNALIKE) and Raylib — namespace switches via #if. Any new API surface must respect both gates.

Source vs Bin Paths — Critical Distinction

The watched path is the source .gumx (the file the Gum tool edits), not the bin/Content copy. Two directories matter:

• _projectSourcePath / sourceDirectory — where files change, where FontCache/ is read from
• _binGumDirectory — snapshot of FileManager.RelativeDirectory at Start(); where fonts are copied to and where the runtime loads from

During PerformReload, animations are reloaded by enumerating *Animations.ganx under the source directory through a LooseFileGumFileProvider rooted there (via GumService.LoadAnimationsFromProvider). FileManager.RelativeDirectory is not swapped — the provider's root is the source directory. (This replaced an older approach that swapped RelativeDirectory and probed FileManager.FileExists per element.) If you add asset-resolving logic to the reload path, root it at the source directory yourself rather than relying on a global-state swap.

Reload Pipeline

FileSystemWatcher event
  → HandleFileChange filters by extension (.gumx/.gucx/.gusx/.gutx/.fnt/.ganx)
  → sets _pendingReload + _lastChangeTime
  → (.fnt paths also appended to _changedFontFiles under _fontFileLock)

GumService.Update → _hotReloadManager.Update(Root)
  → if _pendingReload && 200ms elapsed since last change → PerformReload

The 200 ms debounce coalesces the Gum tool's multi-file save burst into one reload. Don't shorten it without testing against a real tool save — partial saves will otherwise rebuild against an inconsistent on-disk state.

PerformReload — What Actually Happens

1. CopyAndUnloadChangedFonts() — copies changed .fnt files plus matching <basename>*.png texture pages from source FontCache/ to bin FontCache/, then LoaderManager.Disposes both so they reload from disk.
2. GumProjectSave.Load + Initialize; swap into ObjectFinder.Self.GumProjectSave.
3. Reload animations: enumerate *Animations.ganx under the source directory via a LooseFileGumFileProvider and load each through GumService.LoadAnimationsFromProvider into the new project (no RelativeDirectory swap — the provider is rooted at the source dir).
4. ApplyDiff(roots, newProject, SystemManagers.Default) — applies the in-place reconciliation walk described below.
5. Fire ReloadCompleted.

ApplyDiff — In-Place Reconciliation Walk

Defined on GumHotReloadManager and exposed public static for direct test use. Walks every root and, at each visual whose ElementSave.Name matches an element in the new project:

1. Re-point element.ElementSave to the new project's element.
2. Structural diff against the new Instances list (DiffDesignTimeChildren):
   • Partition the visual's children into design-time (those with Tag is InstanceSave) and everything else (runtime-added or Tag-cleared).
   • For each existing design-time child not present in the new Instances: Parent = null + RemoveFromManagers().
   • For each new InstanceSave:
     • If a matching design-time child exists, compare its visual's ElementSave.Name against the new BaseType. Mismatch → remove+recreate (retype). Match → refresh the Tag to point at the new InstanceSave instance.
     • If a same-named non-design-time child exists, leave it alone (runtime owns that slot). Do not create a duplicate.
     • Otherwise call instance.ToGraphicalUiElement(systemManagers) and attach via Parent = parent + ElementGueContainingThis = parent.
   • ReorderDesignTimeChildren walks the design-time slots in Children and Moves items so the design-time subsequence matches newEs.Instances order. Non-design-time children keep their slots.
3. SetVariablesRecursively(newEs, newEs.DefaultState) — re-applies the new default-state values. Qualified-name variables (MyInstance.X, MyInstance.Parent, etc.) flow into the children by Name, which also handles reparenting and animates new instances into position.
4. Recurse into runtime-added children only. Design-time children are skipped — their variables were already set via the parent's qualified-name walk.

Non-Obvious Behaviors / Gotchas

• Tag is the design-time marker. InstanceSave.ToGraphicalUiElement sets Tag = instanceSave (GumRuntime/InstanceSaveExtensionMethods.GumRuntime.cs:39). If user code nulls or replaces that Tag, the diff treats the visual as runtime-owned: not removed, not retyped, not duplicated. Variable application via Name still works. Documented limitation, see docs/code/hot-reload.md.
• Retype detection uses ElementSave.Name, not the old Tag.BaseType. This is the visual's actual built-from type, which stays stable across diffs whether the caller passed a fresh project or mutated the existing one in place — important for tests.
• Root.Children and any roots passed to Update(IEnumerable<GraphicalUiElement>) are the reload surface. PopupRoot / ModalRoot are untouched.
• Runtime state on design-time visuals is overwritten by SetVariablesRecursively. Games that mutate UI in code need to rerun that logic on ReloadCompleted.
• Children added with no Tag (or with a Tag that isn't an InstanceSave) are runtime-owned and preserved. This includes ItemsControl-generated rows and anything the user constructed programmatically.
• Reparenting flows through the qualified <instance>.Parent variable — there is no explicit reparent step in the diff. The parent state's Foo.Parent = "Bar" line is what re-attaches Foo under Bar during SetVariablesRecursively. This is why both old and new parent visuals must exist before that step runs.
• Textures (non-font .png) and .ganx are watched but not reloaded in the cache-eviction sense. .ganx is only re-read by enumerating the source directory into the new project's ElementAnimations (via GumService.LoadAnimationsFromProvider); .png edits require a restart.
• _changedFontFiles is mutated off the game thread (watcher callback). It's protected by _fontFileLock; any new shared state added must be similarly synchronized.
• Stop() is idempotent-safe but does not reset other state — GumService.Uninitialize just nulls the manager reference afterward.
• Font cache path is built with FileManager.Standardize(..., preserveCase: true, makeAbsolute: true). The loader's cache keys are case-preserving absolute paths; any eviction added must match that exact shape or it will silently miss.

Extending

• For new watched extensions: add to HandleFileChange's extension check. If the asset type has a cache, add eviction to PerformReload (follow the font pattern — copy source→bin, then LoaderManager.Dispose the standardized path).
• Prefer injecting a custom IGumHotReloadManager rather than adding game-specific logic to GumHotReloadManager. (Currently EnableHotReload hardcodes new GumHotReloadManager() — if a test/custom manager seam is needed, add an overload accepting an IGumHotReloadManager.)
• Subscribe to ReloadCompleted from game code to reapply runtime state after a rebuild.