# Step 1 — GUI Skeleton

## What Was Built

The main window layout for the LIN Master Simulator using PyQt6 (Python).

## Architecture

```
MainWindow (QMainWindow)
├── Menu Bar
│   ├── File → Load LDF... (Ctrl+O), Exit (Ctrl+Q)
│   └── View → Toggle Connection panel
├── LDF Toolbar (top, fixed)
│   ├── LDF File path (read-only QLineEdit)
│   ├── Browse button → opens file dialog
│   └── Auto-reload checkbox (default: on)
├── Central Widget
│   └── QSplitter (vertical, resizable)
│       ├── Tx Panel (QGroupBox)
│       │   └── QTableWidget: Frame Name | Frame ID | Length | Interval (ms) | Data | Signals | Action
│       └── Rx Panel (QGroupBox)
│           └── QTableWidget: Timestamp | Frame Name | Frame ID | Data | Signals
├── Connection Dock (QDockWidget, left side, detachable)
│   ├── Device dropdown + Refresh button
│   ├── Baud Rate label (read-only, auto-detected from LDF's LIN_speed field)
│   ├── Connect / Disconnect buttons
│   ├── Status label (red "Disconnected")
│   └── Device info label
├── Control Bar (bottom toolbar, fixed)
│   ├── Schedule table dropdown
│   ├── Global Rate (ms) spinbox (1-10000ms, default 50ms)
│   ├── Start / Stop / Pause buttons (disabled)
│   └── Send Selected Frame button (disabled)
└── Status Bar
    ├── Temporary messages (left)
    └── Connection indicator (right, permanent)
```

## Send Rate Hierarchy
When the scheduler runs, each frame's send interval is determined by priority:
1. **Per-frame interval** (Tx table "Interval (ms)" column) — highest priority, user override
2. **Global send rate** (Control bar spinbox) — default fallback for frames with no per-frame interval
3. **LDF schedule table** — auto-fills per-frame intervals when LDF is loaded

## Key Concepts Introduced

### PyQt6 Widget Tree
Every visible element is a QWidget. Widgets contain other widgets via layouts.
The root widget is QMainWindow which provides menu, toolbars, dock areas, and status bar for free.

### Layouts
- **QVBoxLayout**: stacks widgets vertically
- **QHBoxLayout**: stacks widgets horizontally
- **QSplitter**: like a layout but with a draggable divider

### Signals & Slots
Qt's event system. A signal (e.g., `button.clicked`) connects to a slot (any Python function).
Example: `self.btn_browse_ldf.clicked.connect(self._on_load_ldf)`

### QDockWidget
A panel that can be dragged to any window edge, floated, or hidden.
Used for the Connection panel so users can maximize table space during monitoring.

### Initial Widget States
Controls that require prerequisites (LDF loaded, device connected) start **disabled**.
This prevents users from triggering actions that would fail. We enable them as preconditions are met.

## Files

| File | Purpose |
|------|---------|
| `python/src/main.py` | Application entry point — creates QApplication, shows MainWindow |
| `python/src/main_window.py` | MainWindow class — all panel creation and layout |
| `python/tests/test_main_window.py` | 26 tests verifying widget tree structure and initial states |

## Running

```bash
# Launch the GUI
cd python/src && python main.py

# Run tests
cd python && python -m pytest tests/test_main_window.py -v
```

## Test Results
- 32 tests, all passing
- Tests cover: window properties, menu bar, toolbar, Tx/Rx tables (incl. Interval column), connection dock (incl. baud rate label), control bar (incl. global rate), status bar