Integration Guide for WASM Plugins

Static File Serving

Plugins can serve HTML, CSS, JavaScript and other static files:

Structure:

@signalk/my-plugin/
├── public/           # Automatically served at /plugins/my-plugin/
│   ├── index.html
│   ├── style.css
│   └── app.js
├── plugin.wasm
└── package.json

Access: http://localhost:3000/plugins/my-plugin/ serves public/index.html

Resource Providers

WASM plugins can act as resource providers for Signal K resources like weather data, routes, waypoints, or custom resource types.

Enabling Resource Provider Capability

Add resourceProvider: true to your package.json:

{
  "wasmCapabilities": {
    "network": true,
    "dataRead": true,
    "dataWrite": true,
    "resourceProvider": true
  }
}

Registering as a Resource Provider

AssemblyScript

import { registerResourceProvider } from '@signalk/assemblyscript-plugin-sdk/assembly/resources'

// In plugin start():
if (!registerResourceProvider('weather-forecasts')) {
  setError('Failed to register as resource provider')
  return 1
}

Rust

#[link(wasm_import_module = "env")]
extern "C" {
    fn sk_register_resource_provider(type_ptr: *const u8, type_len: usize) -> i32;
}

pub fn register_resource_provider(resource_type: &str) -> bool {
    let bytes = resource_type.as_bytes();
    unsafe { sk_register_resource_provider(bytes.as_ptr(), bytes.len()) == 1 }
}

// In plugin_start():
if !register_resource_provider("weather-forecasts") {
    // Registration failed
    return 1;
}

Implementing Resource Handlers

After registering, your plugin must export these handler functions:

`resources_list_resources` - List resources matching a query

AssemblyScript:

export function resources_list_resources(queryJson: string): string {
  // queryJson: {"bbox": [...], "distance": 1000, ...}
  // Return JSON object: {"resource-id-1": {...}, "resource-id-2": {...}}
  return '{"forecast-1": {"name": "Current Weather", "type": "weather"}}'
}

Rust:

#[no_mangle]
pub extern "C" fn resources_list_resources(
    request_ptr: *const u8, request_len: usize,
    response_ptr: *mut u8, response_max_len: usize,
) -> i32 {
    // Parse query, build response
    let response = r#"{"forecast-1": {"name": "Current Weather"}}"#;
    write_string(response, response_ptr, response_max_len)
}

`resources_get_resource` - Get a single resource

AssemblyScript:

export function resources_get_resource(requestJson: string): string {
  // requestJson: {"id": "forecast-1", "property": null}
  return '{"name": "Current Weather", "temperature": 20.5, "humidity": 0.65}'
}

`resources_set_resource` - Create or update a resource

AssemblyScript:

export function resources_set_resource(requestJson: string): string {
  // requestJson: {"id": "forecast-1", "value": {...}}
  // Return empty string on success, or error message
  return ''
}

`resources_delete_resource` - Delete a resource

AssemblyScript:

export function resources_delete_resource(requestJson: string): string {
  // requestJson: {"id": "forecast-1"}
  return ''
}

Accessing Resources via HTTP

Once registered, resources are available at:

GET  /signalk/v2/api/resources/{type}           # List all
GET  /signalk/v2/api/resources/{type}/{id}      # Get one
POST /signalk/v2/api/resources/{type}/{id}      # Create/update
DELETE /signalk/v2/api/resources/{type}/{id}    # Delete

Standard vs Custom Resource Types

Signal K defines standard resource types with validation:

routes - Navigation routes
waypoints - Navigation waypoints
notes - Freeform notes
regions - Geographic regions
charts - Chart metadata

Custom types (like weather-forecasts) have no schema validation and can contain any JSON structure.

Weather Providers

WASM plugins can act as weather providers for Signal K's specialized Weather API.

Weather Provider vs Resource Provider

Feature	Weather Provider	Resource Provider
API Path	`/signalk/v2/api/weather/*`	`/signalk/v2/api/resources/{type}`
Methods	getObservations, getForecasts, getWarnings	list, get, set, delete
Use Case	Standardized weather data	Generic data storage
Capability	`weatherProvider: true`	`resourceProvider: true`
FFI	`sk_register_weather_provider`	`sk_register_resource_provider`

Enabling Weather Provider Capability

{
  "wasmCapabilities": {
    "network": true,
    "dataWrite": true,
    "weatherProvider": true
  }
}

Implementing Weather Handler Exports

Your plugin must export these handler functions:

`weather_get_observations` - Get current weather observations

export function weather_get_observations(requestJson: string): string {
  // requestJson: {"position": {"latitude": 60.17, "longitude": 24.94}, "options": {...}}
  return (
    '[{"date":"2025-01-01T00:00:00Z","type":"observation","description":"Clear sky",' +
    '"outside":{"temperature":280.15,"relativeHumidity":0.65,"pressure":101300,"cloudCover":0.1},' +
    '"wind":{"speedTrue":5.0,"directionTrue":1.57}}]'
  )
}

`weather_get_forecasts` - Get weather forecasts

export function weather_get_forecasts(requestJson: string): string {
  // requestJson: {"position": {...}, "type": "daily"|"point", "options": {"maxCount": 7}}
  return '[{"date":"...","type":"daily","outside":{...},"wind":{...}}]'
}

`weather_get_warnings` - Get weather warnings/alerts

export function weather_get_warnings(requestJson: string): string {
  // requestJson: {"position": {...}}
  return '[]'
}

Weather Data Format

Observation/Forecast Object

{
  "date": "2025-12-05T10:00:00.000Z",
  "type": "observation",
  "description": "light rain",
  "outside": {
    "temperature": 275.15,
    "minTemperature": 273.0,
    "maxTemperature": 278.0,
    "feelsLikeTemperature": 272.0,
    "relativeHumidity": 0.85,
    "pressure": 101300,
    "cloudCover": 0.75
  },
  "wind": {
    "speedTrue": 5.2,
    "directionTrue": 3.14,
    "gust": 8.0
  }
}

Units:

Temperature: Kelvin
Humidity: Ratio (0-1)
Pressure: Pascals
Wind speed: m/s
Wind direction: Radians

Warning Object

{
  "startTime": "2025-12-05T10:00:00.000Z",
  "endTime": "2025-12-05T18:00:00.000Z",
  "details": "Strong wind warning",
  "source": "Weather Service",
  "type": "Warning"
}

Accessing Weather Data via HTTP

# List providers
curl http://localhost:3000/signalk/v2/api/weather/_providers

# Get observations for a location
curl "http://localhost:3000/signalk/v2/api/weather/observations?lat=60.17&lon=24.94"

# Get daily forecasts
curl "http://localhost:3000/signalk/v2/api/weather/forecasts/daily?lat=60.17&lon=24.94"

# Get point-in-time forecasts
curl "http://localhost:3000/signalk/v2/api/weather/forecasts/point?lat=60.17&lon=24.94"

# Get weather warnings
curl "http://localhost:3000/signalk/v2/api/weather/warnings?lat=60.17&lon=24.94"

Radar Providers

WASM plugins can act as radar providers for Signal K's Radar API at /signalk/v2/api/vessels/self/radars.

Enabling Radar Provider Capability

{
  "signalk": {
    "wasmCapabilities": {
      "radarProvider": true,
      "network": true
    }
  }
}

Registering as a Radar Provider

// Declare the host function
@external("env", "sk_register_radar_provider")
declare function sk_register_radar_provider(namePtr: usize, nameLen: i32): i32;

export function start(configJson: string): i32 {
  const name = "My Radar Plugin";
  const nameBytes = String.UTF8.encode(name);
  const result = sk_register_radar_provider(
    changetype<usize>(nameBytes),
    nameBytes.byteLength
  );

  if (result === 0) {
    sk_set_plugin_error("Failed to register as radar provider", 38);
    return 1;
  }

  return 0;
}

Required Handler Exports

// Return JSON array of radar IDs this provider manages
export function radar_get_radars(): string {
  return JSON.stringify(['radar-0', 'radar-1'])
}

// Return RadarInfo JSON for a specific radar
export function radar_get_radar_info(requestJson: string): string {
  const info = {
    id: 'radar-0',
    name: 'Furuno DRS4D-NXT',
    brand: 'Furuno',
    status: 'transmit',
    spokesPerRevolution: 2048,
    maxSpokeLen: 1024,
    range: 2000,
    controls: {
      gain: { auto: false, value: 50 },
      sea: { auto: true, value: 30 }
    }
  }
  return JSON.stringify(info)
}

RadarInfo Interface

interface RadarInfo {
  id: string // Unique radar ID
  name: string // Display name
  brand?: string // Manufacturer
  status: 'off' | 'standby' | 'transmit' | 'warming'
  spokesPerRevolution: number // Spokes per rotation
  maxSpokeLen: number // Max spoke samples
  range: number // Current range (meters)
  controls: RadarControls // Current control values
  legend?: LegendEntry[] // Color legend for display
  streamUrl?: string // Optional external WebSocket URL
}

Streaming Radar Spokes

Radar spoke data arrives at ~60Hz (2048 spokes/rotation × 30-60 RPM). Plugins stream binary protobuf data directly to clients:

import { sk_radar_emit_spokes } from './signalk-api'

// Called when spoke data received via UDP multicast
function processSpokeData(radarId: string, spokeProtobuf: Uint8Array): void {
  sk_radar_emit_spokes(radarId, spokeProtobuf.buffer, spokeProtobuf.byteLength)
}

Clients connect to the WebSocket stream:

const wsUrl = `ws://${location.host}/signalk/v2/api/vessels/self/radars/radar-0/stream`
const ws = new WebSocket(wsUrl)
ws.binaryType = 'arraybuffer'

ws.onmessage = (event) => {
  const spokeData = new Uint8Array(event.data)
  // Decode and render spoke
}

Integration Guide for WASM Plugins

Static File Serving

Resource Providers

Enabling Resource Provider Capability

Registering as a Resource Provider

AssemblyScript

Rust

Implementing Resource Handlers

resources_list_resources - List resources matching a query

resources_get_resource - Get a single resource

resources_set_resource - Create or update a resource

resources_delete_resource - Delete a resource

Accessing Resources via HTTP

Standard vs Custom Resource Types

Weather Providers

Weather Provider vs Resource Provider

Enabling Weather Provider Capability

Implementing Weather Handler Exports

weather_get_observations - Get current weather observations

weather_get_forecasts - Get weather forecasts

weather_get_warnings - Get weather warnings/alerts

Weather Data Format

Observation/Forecast Object

Warning Object

Accessing Weather Data via HTTP

Radar Providers

Enabling Radar Provider Capability

Registering as a Radar Provider

Required Handler Exports

RadarInfo Interface

Streaming Radar Spokes

On This Page

`resources_list_resources` - List resources matching a query

`resources_get_resource` - Get a single resource

`resources_set_resource` - Create or update a resource

`resources_delete_resource` - Delete a resource

`weather_get_observations` - Get current weather observations

`weather_get_forecasts` - Get weather forecasts

`weather_get_warnings` - Get weather warnings/alerts