Use this skill when making changes to Squirrel, a macOS InputMethodKit frontend for librime. Squirrel is an input method, so correctness depends on event ordering, session lifetime, marked text behavior, candidate window geometry, and clean handoff between the macOS text client and librime.

The Xcode project is organized around one app target, Squirrel.app, plus bundled resources and librime plugins.

• Squirrel/Sources/Main.swift: process entry point, command-line maintenance commands, IMK server creation, app setup, and global librime startup.
• Squirrel/Sources/SquirrelApplicationDelegate.swift: app-wide state. Owns the candidate panel, global SquirrelConfig, status item, Sparkle update integration, distributed notifications, and librime setup/finalization.
• Squirrel/Sources/SquirrelInputController.swift: the main InputMethodKit controller. Owns one active librime session per controller instance, receives key events, translates macOS events to Rime key events, commits text, updates marked text, and drives the candidate panel.
• Squirrel/Sources/MacOSKeyCodes.swift: maps AppKit/Carbon key codes and modifier flags to librime/X11 key symbols and masks.
• Squirrel/Sources/SquirrelConfig.swift: thin typed wrapper over RimeConfig, with base config/schema fallback and cached option reads.
• Squirrel/Sources/SquirrelTheme.swift: converts Rime/Squirrel style configuration into fonts, colors, layout flags, candidate formatting, and drawing attributes.
• Squirrel/Sources/SquirrelPanel.swift: nonactivating candidate/status panel. Builds attributed candidate text, positions the panel near the text cursor, handles paging/candidate mouse events, and delegates selection actions back to the input controller.
• Squirrel/Sources/SquirrelView.swift: custom AppKit drawing surface for candidate/preedit backgrounds, highlights, paging affordances, vertical text, and hit testing.
• Squirrel/Sources/ReservedProperty.swift: reserved librime plugin property protocol for frontend UI hints such as comment highlighting and UI refresh.
• Squirrel/Sources/BridgingFunctions.swift: Swift helpers for C bridge structs, persistent C strings, optional assignment, and geometry utilities.
• Squirrel/Sources/InputSource.swift: Text Input Source registration, enable/disable/select helpers, and current input source lookup.
• Squirrel/Resources/Info.plist: InputMethodKit registration metadata, input modes (Hans, Hant), IMK controller class names, connection name, Sparkle metadata, and input-source properties.
• Squirrel/Resources/Squirrel.entitlements: disables App Sandbox, enables network client access, and disables library validation for bundled dylibs/frameworks.
• Squirrel/SharedSupport: bundled Rime data, default schemas, OpenCC data, and squirrel.yaml.
• Squirrel/librime-*.dylib, Squirrel/Frameworks/Linked Frameworks/librime.1.dylib: backend libraries and plugins used by the frontend.

SquirrelApp.main() is the only entry point.

1. It first checks command-line arguments and exits early for maintenance commands:
   • --quit, --reload, --sync
   • --install / --register-input-source
   • --enable-input-source, --disable-input-source, --select-input-source
   • --build
   • --ascii, --nascii, --getascii
2. If no maintenance command is handled, it creates an IMKServer using InputMethodConnectionName from Info.plist.
3. It creates NSApplication.shared, assigns SquirrelApplicationDelegate, sets accessory activation policy, and changes the current directory to Bundle.main.sharedSupportPath. This is important because OpenCC/librime configuration may use relative dictionary paths.
4. It runs a quick problematic-launch detector to avoid repeated crash/freeze loops from bad configuration.
5. Normal startup calls:
   • setupRime()
   • startRime(fullCheck: false)
   • loadSettings()
   • app.run()
6. On app-run return, it calls rimeAPI.finalize().

SquirrelApplicationDelegate owns global librime setup.

• setupRime() creates the user data directory (~/Library/Rime) and temporary log directory, sets RIME_LOG_DIR, installs librime's notification handler, fills RimeTraits, and calls rimeAPI.setup(&traits).
• Important trait paths and identity fields:
  • shared_data_dir: app bundle shared support path.
  • user_data_dir: ~/Library/Rime.
  • log_dir: temporary rime.squirrel directory.
  • distribution code/name/version and app_name = rime.squirrel.
• startRime(fullCheck:) calls rimeAPI.initialize(nil), then start_maintenance(fullCheck). On successful maintenance it deploys squirrel.yaml with the config_version marker.
• loadSettings() opens base squirrel config, refreshes notification/status-icon settings, and loads light/dark panel themes.
• loadSettings(for schemaID:) opens the active schema config and, when it has a style section, overlays schema-specific panel style. Otherwise it falls back to base config.
• shutdownRime() closes config and calls rimeAPI.finalize().
• applicationShouldTerminate(_:) calls cleanup_all_sessions() before termination.

Do not initialize/finalize librime from individual input controllers. Controllers own sessions; the application delegate owns the backend lifetime.

SquirrelInputController subclasses IMKInputController and is the core input-method object.

• init(server:delegate:client:) stores the initial IMKTextInput client, calls createSession(), and registers local notification observers for ASCII-mode set/report requests.
• createSession() chooses a client bundle identifier, creates a librime session with rimeAPI.create_session(), clears schemaId, and applies app-specific options.
• destroySession() calls rimeAPI.destroy_session(session) and clears chord typing state.
• deinit destroys the session.
• activateServer(_:) refreshes the current client, optionally overrides the keyboard layout from keyboard_layout, clears local preedit cache, and updates the menu-bar status label from ascii_mode if a session already exists.
• deactivateServer(_:) hides palettes, commits the current composition to the client, and releases the client reference.
• commitComposition(_:) commits raw pending librime input via client.insertText, then clears the librime composition.

The controller keeps client weak. Always guard client access. An input method may be activated, deactivated, or retargeted by macOS at awkward times.

The critical loop is handle(_:client:) -> Bool in SquirrelInputController.

1. Ensure there is a valid librime session. If session == 0 or find_session(session) fails, call createSession().
2. Update the weak IMKTextInput client from sender when possible.
3. Detect client app bundle ID changes and apply app_options/<bundle-id> from squirrel.yaml.
4. For .flagsChanged:
   • Compute changed modifier flags by comparing with lastModifiers.
   • Convert modifiers with SquirrelKeycode.osxModifiersToRime.
   • Validate or infer modifier keycode. This protects against remote desktop tools sending bogus keycode 0 for modifier events.
   • Handle caps lock specially because librime expects XK_Caps_Lock before lock-mask state changes.
   • Process modifier releases before presses to handle delayed release events.
   • Update lastModifiers and call rimeUpdate().
5. For .keyDown:
   • Ignore Command-modified shortcuts so the client application receives them.
   • Choose charactersIgnoringModifiers or characters depending on modifiers and ASCII/non-ASCII behavior.
   • Convert keycode/character/modifiers to librime keycode and masks.
   • Call processKey(...).
   • Call rimeUpdate() when a valid rime keycode was processed.
6. Return true only when the event was handled and should not continue to the client application.

recognizedEvents(_:) returns key-down and flags-changed masks only.

processKey(_:, modifiers:) is the narrow frontend/backend key boundary.

• Before calling librime, it synchronizes _linear and _vertical options from the current panel theme. Arrow-key behavior can depend on candidate layout and text orientation.
• It calls rimeAPI.process_key(session, keycode, modifiers).
• If librime does not handle a Vim-like command-mode escape (Esc, Ctrl-C, Ctrl-[) and vim_mode is set, it forces ascii_mode on unless already in ASCII mode.
• If librime handles a key while _chord_typing is active, printable keys and modifiers are recorded and later released by a timer. Non-chording keys clear the chord buffer.

MacOSKeyCodes.swift is intentionally centralized. Add key translations there rather than scattering keycode conditionals through the controller.

rimeUpdate(clearReservedComments:) consumes all frontend-visible librime state after key processing, paging, selection, caret movement, or plugin UI refresh.

Main sequence:

1. Clear reserved comment UI hints unless the caller explicitly preserves them.
2. rimeConsumeCommittedText() calls get_commit, inserts committed text into the client, frees the commit struct, resets local preedit, and hides the panel.
3. get_status detects schema changes:
   • reloads schema-specific settings through the app delegate;
   • calculates inlinePreedit and inlineCandidate using panel config plus librime options (no_inline, inline);
   • sets librime soft_cursor to the inverse of inline preedit.
4. get_context reads composition and menu state:
   • preedit string;
   • selected segment byte offsets converted to Swift indices;
   • cursor position;
   • candidate texts, comments, labels, page number, last-page flag, highlighted index.
5. It updates marked text through show(preedit:selRange:caretPos:).
6. It updates the candidate panel through showPanel(...) unless no context is available, in which case it hides palettes.
7. It frees the librime context.

The text path is:

NSEvent -> SquirrelInputController.handle -> processKey -> rimeAPI.process_key -> rimeUpdate -> get_commit/get_status/get_context -> client.insertText and/or client.setMarkedText plus SquirrelPanel.update.

• Committed text must go through client.insertText(_, replacementRange: .empty).
• Active composition should go through client.setMarkedText(_, selectionRange:, replacementRange: .empty).
• show(preedit:selRange:caretPos:) caches the last marked preedit, caret, and selected range to avoid redundant marked-text calls.
• When non-inline preedit is configured, the controller may set a full-width space (U+3000) as marked text so clients such as iTerm2 do not echo every raw preedit character.
• commitComposition(_:) commits raw pending librime input during deactivation. This matters when macOS switches input sources or the focused text client changes.

Input methods must be conservative about when they consume events. Incorrect true returns drop app shortcuts or text; incorrect false returns can duplicate input.

The app delegate creates one shared SquirrelPanel during applicationWillFinishLaunching. The active input controller assigns itself to panel.inputController before updating the panel.

showPanel(...) gets cursor geometry from client.attributes(forCharacterIndex:lineHeightRectangle:), stores it in panel.position, and calls panel.update(...).

SquirrelPanel.update(...):

• stores the latest preedit/candidate state;
• builds a single attributed string containing preedit and candidate rows;
• applies theme attributes, candidate labels, comments, semantic comment colors, no-break hints, and paragraph styles;
• updates the NSTextView storage and layout orientation;
• forces TextKit layout before measuring geometry;
• calls SquirrelView.drawView(...) for background/highlight paths;
• calls show() to position and display the panel.

SquirrelPanel.show():

• chooses screen based on cursor position;
• sets effective appearance;
• measures text with TextKit 2;
• constrains oversized panels to most of the screen and scales via content-view bounds;
• positions normal panels near the cursor, with special handling for vertical text;
• applies content-view rotation for vertical mode;
• configures translucency background (NSGlassEffectView on macOS 26+, NSVisualEffectView otherwise);
• orders the nonactivating panel front.

Mouse and scroll events on the panel are forwarded back to the input controller:

• click candidate -> selectCandidate(_:) -> rimeUpdate();
• click/scroll paging controls -> page(up:) -> rimeUpdate();
• click preedit position -> moveCaret(forward:) -> rimeUpdate().

SquirrelView owns the drawing and hit-testing model.

• It uses an NSTextView with TextKit 2 layout to measure actual rendered text segments.
• contentRect and contentRect(range:) enumerate text layout segments to compute bounds.
• draw(_:) builds Core Graphics paths for panel background, preedit background, candidate backgrounds, highlighted candidate, highlighted preedit range, border, shadow, and paging controls.
• shape is also used as the panel background mask and hit-test region.
• click(at:) maps mouse points back into TextKit offsets and candidate/preedit ranges.

When changing panel layout, preserve the order: set attributed text, set layout orientation, force layout, measure, draw paths, then show/reposition.

SquirrelConfig is a typed facade over RimeConfig.

• openBaseConfig() opens squirrel config.
• open(schemaID:baseConfig:) opens schema config and falls back to base config for missing values.
• getBool, getDouble, getString, and getColor cache successful reads.
• getAppOptions(_:) reads boolean options under app_options/<bundle-id>.

SquirrelTheme.load(config:dark:) reads global style/*, then optional preset color scheme settings. Per-color-scheme values can override style values for layout, color, fonts, alpha, spacing, and candidate formatting.

Important theme flags:

• candidate_list_layout: linear vs stacked candidate list.
• text_orientation: horizontal vs vertical.
• inline_preedit, inline_candidate: marked text vs panel display strategy.
• translucency, mutual_exclusive, memorize_size, show_paging.
• candidate_format: template using [label], [candidate], [comment]; legacy %c and %@ are normalized.

The app uses distributed notifications for process-to-running-instance commands.

• SquirrelReloadNotification -> deploy: shutdown Rime, reinitialize, reload settings.
• SquirrelSyncNotification -> sync_user_data().
• SquirrelToggleASCIIModeNotification -> posts local SquirrelSetASCIIModeNotification with Bool.
• SquirrelGetASCIIModeNotification -> posts local report request; active controller responds with SquirrelASCIIModeResponse (ascii or nascii).
• kTISNotifySelectedKeyboardInputSourceChanged -> updates status item visibility and finalizes stranded compositions.

The finalization fallback is important: some macOS/input-source switch paths may not call deactivateServer. When the selected input source no longer starts with im.rime.inputmethod.Squirrel, the app delegate calls deactivateServer on the panel's current input controller to avoid orphaned composition/panel state.

notificationHandler(...) is installed by setupRime() and receives backend notifications.

• deploy/start, deploy/success, deploy/failure: show user notifications.
• option: parses enabled/disabled option names, gets abbreviated and long state labels from librime, updates status icon for ascii_mode, and optionally shows a status message on the panel.
• property where the value starts with _ and contains =: treats it as a reserved frontend property and calls handleReservedProperty(...) on the current panel input controller on the main actor.
• schema: when notifications are enabled, extracts and shows schema name.

Reserved properties currently include:

• _comment_highlight: comma-separated candidate indices to draw with accent_text_color.
• _comment_warning: comma-separated candidate indices to draw with warning_text_color.
• _refresh_ui: requests rimeUpdate(clearReservedComments: false).

Reserved-property values are query-string compatible; bare comma lists are parsed under the value field.

SquirrelInstaller wraps Text Input Source Services.

• Input modes are im.rime.inputmethod.Squirrel.Hans and im.rime.inputmethod.Squirrel.Hant; Hans is the primary default.
• register() calls TISRegisterInputSource for /Library/Input Library/Squirrel.app when no Squirrel modes are already enabled.
• enable, disable, and select operate on TIS input sources.
• currentInputSourceID() reads TISCopyCurrentKeyboardInputSource() and is used to control status item visibility and stranded-composition cleanup.

Info.plist must stay consistent with InputSource.swift: input mode identifiers, InputMethodConnectionName, and controller class names are part of macOS input method registration.

The Swift/C boundary uses generated librime types plus helpers in BridgingFunctions.swift.

• Initialize librime structs with .rimeStructInit() so memory is zeroed and data_size is set correctly.
• Free librime-owned structs after successful reads: commits with free_commit, statuses with free_status, contexts with free_context.
• setCString(_:to:) duplicates Swift strings for C fields. Be mindful that duplicated C strings are manually allocated.
• RimeStringSlice.asString must respect .length; do not replace it with String(cString:) for abbreviated labels.

Follow the existing Swift/AppKit style unless there is a strong local reason to do otherwise.

• Types use PascalCase: SquirrelInputController, ReservedPropertyValue, SquirrelTheme.
• Methods, properties, local variables, and enum cases use camelCase.
• Keep local acronym style consistent with nearby code: rimeAPI, schemaId, currentApp, asciiMode.
• Boolean names should read naturally with is, has, can, should, or a clear state noun when the existing API already uses one.
• Generated C bridge fields may keep snake_case names such as data_size; use narrow SwiftLint suppressions rather than renaming generated API concepts.
• Prefer let for values that do not change, private for implementation details, and private(set) when other types need read-only state.
• Keep IMK lifecycle and event handling in SquirrelInputController; keep global Rime/app lifetime in SquirrelApplicationDelegate.
• Keep config access in SquirrelConfig and configurable visual state in SquirrelTheme.
• Keep key translation in MacOSKeyCodes; do not scatter raw Carbon/Rime key mappings through input handling code.
• Keep candidate panel state and positioning in SquirrelPanel; keep drawing, geometry, and hit testing in SquirrelView.

Reuse existing helpers before adding new abstractions.

• Use .rimeStructInit() for librime structs that need zeroed memory and data_size.
• Use setCString(_:to:) when assigning Swift strings into Rime trait/config structs.
• Use the ?= operator for optional config overrides in theme/config-loading code.
• Use NSRange.empty for the project's sentinel empty range.
• Use RimeStringSlice.asString for Rime slices because it respects the slice length.
• Use SquirrelKeycode for macOS-to-Rime key conversion.
• Extend ReservedPropertyValue for reserved-property parsing instead of adding one-off string parsing.
• Add a shared helper only when multiple call sites need the same non-trivial behavior. Avoid wrapping a single straightforward expression.

Comment style is intentionally sparse.

• Keep the simple file headers already used by the project.
• Keep SwiftLint directive comments exactly where they are needed.
• Use English for retained comments.
• Comments should explain why, ordering constraints, ownership, or platform/backend quirks. Do not restate what the next line does.
• Remove commented-out debug prints and temporary tracing instead of preserving them in source.
• Keep comments near IMK/librime event ordering, TextKit measurement constraints, vertical-mode coordinate transforms, C memory ownership, and plugin/frontend contracts.
• Prefer one compact explanatory comment over long branch-by-branch examples unless the example prevents a likely regression.

Keep these invariants in mind for any change:

• Global librime lifetime belongs to the app delegate; session lifetime belongs to input controllers.
• Every key event path that changes librime state should call rimeUpdate() exactly when frontend state needs to be consumed.
• Do not consume Command shortcuts in normal text input; let client applications handle them.
• Deactivation must hide the panel and commit or clear active composition so no marked text or panel is stranded.
• Always guard against nil or stale IMKTextInput clients.
• Convert librime byte offsets into Swift string indices before building NSRange values.
• Keep get_context, get_status, and get_commit free calls paired with successful reads.
• Preserve app-specific options on session creation and when the focused client bundle changes.
• Candidate panel geometry depends on TextKit layout results. Avoid measuring before layout is forced.
• Vertical text affects key behavior, layout orientation, content rotation, panel positioning, and scroll paging direction.
• inlinePreedit and inlineCandidate are determined jointly by theme config and librime options.
• Shared panel state should always point to the active input controller before candidate updates or mouse actions.

For key handling changes:

1. Start in SquirrelInputController.handle and processKey.
2. Put reusable key mappings in MacOSKeyCodes.swift.
3. Preserve modifier ordering and caps-lock behavior.
4. Verify event consumption semantics.

For candidate UI changes:

1. Start in SquirrelPanel.update for text/attributes/data shaping.
2. Use SquirrelTheme for configurable style values.
3. Use SquirrelView for geometry, drawing, and hit testing.
4. Test horizontal, linear, vertical, paging, inline preedit, and no-candidate states.

For config changes:

1. Add reads in SquirrelTheme or SquirrelConfig only where the value belongs.
2. Keep base config and schema-specific fallback behavior intact.
3. Consider dark/light theme loading separately.

For lifecycle or command changes:

1. Start in Main.swift for command-line behavior.
2. Start in SquirrelApplicationDelegate for app-global observers, Rime setup, status item behavior, and termination.
3. Keep distributed notification names stable unless all callers are updated.

For librime plugin/frontend coordination:

1. Add reserved keys to ReservedPropertyKey.
2. Parse values in ReservedPropertyValue or in handleReservedProperty.
3. Apply UI effects in SquirrelInputController or SquirrelPanel, depending on whether the state belongs to the session or rendering.
4. Preserve _refresh_ui behavior for plugin-driven redraws.

When possible, validate with Xcode build diagnostics or a full Xcode build. For behavior changes, manually exercise:

• input activation/deactivation in multiple apps;
• typing, committing, cancelling, and switching input sources mid-composition;
• ASCII mode toggle and status reporting;
• schema switching and schema-specific style reload;
• candidate selection by number key and mouse;
• paging by key, mouse, and scroll;
• inline and non-inline preedit;
• vertical and linear candidate layouts;
• deployment/reload and sync commands;
• app quit/log out cleanup.

Input-method bugs often appear as duplicated text, dropped shortcuts, orphaned candidate panels, stale marked text, or session-specific state leaking between client apps. Test around those failure modes first.