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Lanterna

lanterna monitor

Synopsis

Terminal window
lanterna monitor [options]

Description

The monitor command starts a WebSocket server and renders a live terminal dashboard showing connected apps and their real-time performance metrics. As apps connect via the @lanternajs/react-native in-app module, the dashboard updates continuously with current FPS, CPU, memory, and navigation data.

This is useful during development for keeping an eye on performance as you interact with your app, without needing to run discrete measurement sessions.

Options

FlagDefaultDescription
--port <number>8347WebSocket server port

Requirements

The app must include the @lanternajs/react-native in-app module with LanternaProvider configured to connect to the monitor’s WebSocket server. Without instrumentation, the monitor will start but no data will appear.

See the In-App Module overview for setup instructions.

Dashboard

Once an instrumented app connects, the terminal renders a live-updating dashboard:

╭─────────────────────────────────────────────╮
│  lanterna monitor                           │
│  Port: 8347  Status: running                │
├─────────────────────────────────────────────┤
│  com.example.myapp                          │
│  Pixel 6 (android)                          │
│  Screen: ProfileScreen                      │
│  UI FPS: 58.2  Drops: 3                     │
│  JS FPS: 55.1                               │
│  CPU: 28.3%                                 │
│  Memory: 245 MB                             │
╰─────────────────────────────────────────────╯

The dashboard shows:

  • App identifier — The connected app’s package name or bundle ID.
  • Device info — Device name, platform, and whether it is an emulator or physical device.
  • Current screen — The active React Navigation screen name (when available).
  • UI FPS — Native UI thread frame rate and drop count.
  • JS FPS — JavaScript thread frame rate.
  • CPU — Current CPU usage percentage.
  • Memory — Current memory consumption.

Multiple apps can connect simultaneously. Each connected app appears as a separate section in the dashboard.

Examples

Start with default port

Terminal window
lanterna monitor

Start on a custom port

Terminal window
lanterna monitor --port 9000

When using a custom port, make sure the LanternaProvider in your app is configured to connect to the same port:

<LanternaProvider wsPort={9000}>
  <App />
</LanternaProvider>