---
phase: 04-timeline-editor
plan: 02
subsystem: ui
tags: [canvas, timeline, drag-drop, undo-redo, command-pattern, fastapi, crud]
dependency_graph:
  requires:
    - phase: 04-timeline-editor/04-01
      provides: TimelineCanvas class with coordinate helpers, tracks data structure, render loop
    - phase: 03-communication-protocol
      provides: Show/Cue/Track models, show_store persistence
  provides:
    - CommandHistory with 50-step undo/redo stack
    - PlaceBlockCommand, MoveBlockCommand, ResizeBlockCommand, DeleteBlockCommand
    - Block CRUD REST endpoints (POST/PATCH/DELETE /api/shows/.../blocks)
    - Full block interaction: drag-from-palette, move, resize, delete
    - Overlap prevention on same track
    - Snap-to-beat on mouseup (100ms threshold)
  affects:
    - 04-03 (block inspector wiring will use selectedBlock and _onSelectBlock)
    - 04-04 (playback engine reads cues from tracks which are now populated)

tech-stack:
  added: [HTML5 drag-and-drop API, crypto.randomUUID(), command pattern]
  patterns:
    - Command pattern with execute/undo/redo for all timeline mutations
    - Fire-and-forget fetch for backend sync (no await, no blocking UI)
    - Live drag preview with revert-then-command on mouseup
    - Document-level mousemove/mouseup listeners during drag (not canvas-level)

key-files:
  created:
    - lightsync/frontend/timeline/history.js
    - lightsync/frontend/timeline/commands.js
    - lightsync/api/timeline.py
  modified:
    - lightsync/frontend/timeline/timeline.js
    - lightsync/frontend/app.js
    - lightsync/main.py

key-decisions:
  - "_startMove revert-then-command pattern: live drag updates block.timestamp directly for visual feedback, mouseup reverts to oldTimestamp then executes MoveBlockCommand — ensures history always has the correct before/after state"
  - "Circular import with main.py is expected: timeline.py imports lightsync.main as state at module level, same as shows.py — works at runtime, breaks only on isolated python -c import (non-issue)"
  - "Resize handles drawn as 8px highlighted zones on selected block for discoverability"
  - "Ghost preview during dragover uses trackIdx + time from _dragState rather than raw pixel coordinates — decoupled from render"

patterns-established:
  - "Timeline command: stores references to tracks array + block object, mutates block in-place, syncs to backend fire-and-forget"
  - "setCurrentShowId() module-level function for routing backend sync without coupling commands to app state"

requirements-completed: [TL-02, TL-03, TL-06]

duration: 9min
completed: 2026-04-06
---

# Phase 04 Plan 02: Block Interactions + Command Pattern Summary

**Command-pattern block editing with drag-from-palette, move/resize/delete interactions, undo/redo (50 steps), overlap prevention, snap-to-beat, and backend CRUD endpoints.**

## Performance

- **Duration:** ~9 min
- **Started:** 2026-04-06T23:35:38Z
- **Completed:** 2026-04-06T23:44:50Z
- **Tasks:** 2
- **Files modified:** 5 (3 created, 3 modified)

## Accomplishments

- `CommandHistory` class: 50-step max, execute/undo/redo, redo cleared on new action (branching protection)
- 4 command classes: `PlaceBlockCommand`, `MoveBlockCommand`, `ResizeBlockCommand`, `DeleteBlockCommand` — each with backend sync via fire-and-forget fetch
- REST API: POST/PATCH/DELETE `/api/shows/{show_id}/tracks/{device_id}/blocks` with auto-create track if missing
- Full mouse interaction: hitTest with 8px resize zones, live drag preview, overlap rejection, snap-to-beat on release
- Keyboard: Delete key removes selected block, Ctrl+Z undo, Ctrl+Shift+Z / Ctrl+Y redo
- Drag-and-drop from animation palette to canvas with ghost block preview

## Task Commits

1. **Task 1: Command pattern + history + backend block CRUD** - `658c92f` (feat)
2. **Task 2: Wire mouse interactions into TimelineCanvas** - `7e18ac6` (feat)

## Files Created/Modified

- `lightsync/frontend/timeline/history.js` — CommandHistory class (execute, undo, redo, canUndo, canRedo, maxSteps=50)
- `lightsync/frontend/timeline/commands.js` — PlaceBlockCommand, MoveBlockCommand, ResizeBlockCommand, DeleteBlockCommand + setCurrentShowId() + syncBlock()
- `lightsync/api/timeline.py` — Block CRUD FastAPI router (POST/PATCH/DELETE)
- `lightsync/frontend/timeline/timeline.js` — hitTest, _hasOverlap, snapToBeat, _setupInteractions, _startMove, _startResize, handleDrop added; CommandHistory + 4 command imports wired
- `lightsync/frontend/app.js` — dragstart/dragover/dragleave/drop listeners for palette-to-canvas drag
- `lightsync/main.py` — timeline router registered with prefix="/api"

## Decisions Made

| Decision | Rationale |
|----------|-----------|
| Revert-then-command pattern in _startMove | Live drag updates block.timestamp for visual feedback; mouseup reverts to oldTimestamp, then MoveBlockCommand.execute() sets final value — history always has correct before/after state |
| Document-level mousemove/mouseup during drag | Canvas-only listeners lose events when mouse moves fast outside canvas bounds |
| Ghost preview via _dragState.trackIdx + time | Decoupled from raw pixels — render() computes screen position, maintaining coordinate abstraction |
| setCurrentShowId() module-level function | Commands need showId for backend sync but shouldn't be coupled to app state — module-level var injected by app.js |

## Deviations from Plan

### Auto-fixed Issues

**1. [Rule 1 - Bug] Plan's _startMove had duplicated/contradictory draft code**
- **Found during:** Task 2 implementation
- **Issue:** The plan's `_startMove` action block contained two conflicting implementations (onDrag/onUp followed by onDrag2/onUp2) with a comment "IMPORTANT: The executor MUST implement it cleanly as a single version"
- **Fix:** Implemented clean single version following the stated logic: record old, live-update during drag, revert + command on mouseup
- **Files modified:** `lightsync/frontend/timeline/timeline.js`
- **Committed in:** 7e18ac6

---

**Total deviations:** 1 auto-fixed (Rule 1 - Bug)
**Impact on plan:** Fix was necessary — the draft code in the plan would not compile/run correctly as-is. Clean implementation follows exact logic specified in the IMPORTANT note.

## Issues Encountered

- Circular import warning during `python -c "from lightsync.api.timeline import router"` — same pattern as existing `shows.py`. Not a bug: `import lightsync.main as state` at module level causes partially-initialized module error on isolated import, but works correctly at runtime when app bootstraps normally. Docker build and startup confirmed clean.

## Known Stubs

None — block placement, move, resize, delete are all fully wired. Backend CRUD endpoints accept and persist mutations. Undo/redo history tracks all operations.

## Self-Check

- `lightsync/frontend/timeline/history.js` exists: YES (658c92f)
- `lightsync/frontend/timeline/commands.js` exports 4 command classes: YES (658c92f)
- `lightsync/api/timeline.py` has POST/PATCH/DELETE routes: YES (658c92f)
- `lightsync/main.py` includes timeline router: YES (658c92f)
- `lightsync/frontend/timeline/timeline.js` has hitTest, _startMove, _startResize, handleDrop: YES (7e18ac6)
- `lightsync/frontend/app.js` has dragstart/dragover/drop: YES (7e18ac6)
- Docker build: SUCCESS (fa66387)
- App startup: CLEAN (no errors, uvicorn running)

## Self-Check: PASSED

## Next Phase Readiness

- Block placement and editing is fully functional — timeline is now interactive
- Plan 04-03 (block inspector panel) can wire into `timeline._onSelectBlock` callback and `timeline.selectedBlock`
- Plan 04-04 (playback/live engine) reads cues from `timeline.tracks[*].cues` which are now populated by user interactions
- No blockers

---
*Phase: 04-timeline-editor*
*Completed: 2026-04-06*