Creating AssemblyScript Plugins

AssemblyScript is the recommended language for developers familiar with TypeScript. It produces the smallest binaries (3-10 KB) and has the fastest development cycle.

Step 1: Install SDK

npm install @signalk/assemblyscript-plugin-sdk
npm install --save-dev assemblyscript
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Step 2: Create Plugin File

Create assembly/index.ts:

import {
  Plugin,
  Delta,
  Update,
  PathValue,
  emit,
  setStatus
} from '@signalk/assemblyscript-plugin-sdk/assembly'

class MyPlugin extends Plugin {
  name(): string {
    return 'My AssemblyScript Plugin'
  }

  schema(): string {
    return `{
      "type": "object",
      "properties": {
        "updateRate": {
          "type": "number",
          "default": 1000
        }
      }
    }`
  }

  start(config: string): i32 {
    setStatus('Started')

    // Emit a test delta
    const pathValue = new PathValue('test.value', '"hello"')
    const update = new Update([pathValue])
    const delta = new Delta('vessels.self', [update])
    emit(delta)

    return 0 // Success
  }

  stop(): i32 {
    setStatus('Stopped')
    return 0
  }
}

// Export for Signal K
const plugin = new MyPlugin()
export function plugin_name(): string {
  return plugin.name()
}
export function plugin_schema(): string {
  return plugin.schema()
}
export function plugin_start(configPtr: usize, configLen: usize): i32 {
  const configBytes = new Uint8Array(configLen)
  for (let i = 0; i < configLen; i++) {
    configBytes[i] = load<u8>(configPtr + i)
  }
  const configJson = String.UTF8.decode(configBytes.buffer)
  return plugin.start(configJson)
}
export function plugin_stop(): i32 {
  return plugin.stop()
}
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Note on Plugin IDs: The plugin ID is automatically derived from your package.json name. For example:

• @signalk/example-weather-plugin → _signalk_example-weather-plugin
• my-simple-plugin → my-simple-plugin

This ensures unique plugin IDs (npm guarantees package name uniqueness) and eliminates discrepancies between package name and plugin ID.

Step 3: Configure Build

Create asconfig.json:

{
  "targets": {
    "release": {
      "outFile": "plugin.wasm",
      "optimize": true,
      "shrinkLevel": 2,
      "converge": true,
      "noAssert": true,
      "runtime": "incremental",
      "exportRuntime": true
    },
    "debug": {
      "outFile": "build/plugin.debug.wasm",
      "sourceMap": true,
      "debug": true,
      "runtime": "incremental",
      "exportRuntime": true
    }
  },
  "options": {
    "bindings": "esm"
  }
}
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Important: exportRuntime: true is required for the AssemblyScript loader to work. This exports runtime helper functions like __newString and __getString that the server uses for automatic string conversions.

Step 4: Build

npx asc assembly/index.ts --target release
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Step 5: Create package.json

{
  "name": "my-wasm-plugin",
  "version": "0.1.0",
  "keywords": ["signalk-wasm-plugin"],
  "wasmManifest": "plugin.wasm",
  "wasmCapabilities": {
    "dataRead": true,
    "dataWrite": true,
    "storage": "vfs-only"
  }
}
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Important: What makes a WASM plugin?

The wasmManifest field is the key identifier that tells Signal K this is a WASM plugin (not a Node.js plugin). It must point to your compiled .wasm file.

The package name can be anything - scoped (@myorg/my-plugin) or unscoped (my-wasm-plugin). Choose a name that makes sense for your plugin and avoids conflicts on npm.

Step 6: Test Install

Option 1: Symlink (Recommended for Development)

Symlinking your plugin directory allows you to make changes and rebuild without copying files:

# From your Signal K node_modules directory
cd ~/.signalk/node_modules
ln -s /path/to/your/my-wasm-plugin my-wasm-plugin

# Now any changes you make and rebuild will be picked up on server restart
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Option 2: Direct Copy

mkdir -p ~/.signalk/node_modules/my-wasm-plugin
cp plugin.wasm package.json ~/.signalk/node_modules/my-wasm-plugin/

# If your plugin has a public/ folder with web UI:
cp -r public ~/.signalk/node_modules/my-wasm-plugin/
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Option 3: NPM Package Install

# If you've packaged with `npm pack`
npm install -g ./my-wasm-plugin-1.0.0.tgz

# Or install from npm registry (if published)
npm install -g my-wasm-plugin
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Note: Symlinking is the most efficient method for development - changes are picked up on server restart without copying files. Use npm install for production deployments or when distributing plugins.

Important: If your plugin includes static files (like a web UI in the public/ folder), make sure to copy that folder as well. Static files are automatically served at /plugins/your-plugin-id/ when the plugin is loaded.

Step 7: Verify Plugin Configuration in Admin UI

After installing your plugin, verify it appears in the Admin UI:

1. Navigate to Plugin Configuration: Open the Admin UI at http://your-server:3000/@signalk/server-admin-ui/ and go to Server → Plugin Config

2. Check Plugin List: Your WASM plugin should appear in the list with:

   • Plugin name (from name() export)
   • Version (from package.json)
   • Enable/Disable toggle
   • Configuration form (based on schema() export)

3. Verify Configuration Persistence:

   • Configuration is saved to ~/.signalk/plugin-config-data/your-plugin-id.json
   • Changes are applied immediately (plugin restarts automatically)
   • The file structure is:

     {
       "enabled": true,
       "enableDebug": false,
       "configuration": {
         "updateRate": 1000
       }
     }
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4. Troubleshooting:

   • If plugin doesn't appear: Check package.json has the signalk-wasm-plugin keyword and wasmManifest field
   • If configuration form is empty: Verify schema() export returns valid JSON Schema
   • If settings don't persist: Check file permissions on ~/.signalk/plugin-config-data/

Important: The Admin UI shows all plugins (both Node.js and WASM) in a unified list. WASM plugins integrate seamlessly with the existing plugin configuration system.

API Reference

Base Classes

Plugin

Abstract base class for all plugins.

Methods to implement:

• id(): string - Unique plugin identifier
• name(): string - Human-readable name
• schema(): string - JSON schema for configuration
• start(config: string): i32 - Initialize plugin
• stop(): i32 - Clean shutdown

Signal K Types

Delta

Represents a Signal K delta message.

const delta = new Delta('vessels.self', [update])
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Update

Represents an update within a delta. The server automatically adds $source and timestamp.

const update = new Update([pathValue])
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PathValue

Represents a path-value pair.

const pathValue = new PathValue('navigation.position', positionJson)
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Position

GPS position with latitude/longitude.

const pos = new Position(60.1, 24.9)
const posJson = pos.toJSON()
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Notification

Signal K notification.

const notif = new Notification(NotificationState.normal, 'Hello!')
const notifJson = notif.toJSON()
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API Functions

emit(delta: Delta): void

Emit a delta message to Signal K server.

emit(delta)
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Requires capability: dataWrite: true

setStatus(message: string): void

Set plugin status (shown in admin UI).

setStatus('Running normally')
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setError(message: string): void

Report an error (shown in admin UI).

setError('Sensor connection failed')
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debug(message: string): void

Log debug message to server logs.

debug('Processing data: ' + value.toString())
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getSelfPath(path: string): string | null

Read data from vessel.self.

const speedJson = getSelfPath('navigation.speedOverGround')
if (speedJson !== null) {
  const speed = parseFloat(speedJson)
}
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Requires capability: dataRead: true

getPath(path: string): string | null

Read data from any context.

const posJson = getPath('vessels.self.navigation.position')
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Requires capability: dataRead: true

readConfig(): string

Read plugin configuration.

const configJson = readConfig()
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saveConfig(configJson: string): i32

Save plugin configuration.

const result = saveConfig(JSON.stringify(config))
if (result !== 0) {
  setError('Failed to save config')
}
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Helper Functions

import {
  createSimpleDelta,
  getCurrentTimestamp
} from '@signalk/assemblyscript-plugin-sdk'

// Quick delta creation
const delta = createSimpleDelta('my-plugin', 'test.value', '"hello"')
emit(delta)
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JSON Parsing

The SDK includes assemblyscript-json for parsing JSON data. This is useful when working with configuration, API responses, or resource provider requests.

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

// Parse a JSON string
const jsonStr = '{"name": "My Boat", "speed": 5.2}'
const parsed = JSON.parse(jsonStr)

if (parsed.isObj) {
  const obj = parsed as JSON.Obj

  // Get string values
  const nameValue = obj.getString('name')
  if (nameValue !== null) {
    const name = nameValue.valueOf() // "My Boat"
  }

  // Get number values
  const speedValue = obj.getNum('speed')
  if (speedValue !== null) {
    const speed = speedValue.valueOf() // 5.2 (as f64)
  }
}
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Available methods on JSON.Obj:

• getString(key) - Returns JSON.Str | null
• getNum(key) - Returns JSON.Num | null
• getBool(key) - Returns JSON.Bool | null
• getObj(key) - Returns JSON.Obj | null
• getArr(key) - Returns JSON.Arr | null
• getValue(key) - Returns JSON.Value | null

Note: Plugins using resource providers or parsing complex JSON should add assemblyscript-json to their dependencies:

npm install assemblyscript-json
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JSON Value Encoding

Values must be JSON-encoded strings:

// Numbers
const pathValue = new PathValue('temperature', '25.5')

// Strings (note the quotes)
const pathValue = new PathValue('name', '"My Boat"')

// Objects
const pathValue = new PathValue(
  'position',
  '{"latitude":60.1,"longitude":24.9}'
)

// Use helper classes
const pos = new Position(60.1, 24.9)
const pathValue = new PathValue('position', pos.toJSON())
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Resource Providers

WASM plugins can register as resource providers to serve data via the Signal K REST API.

Setup

1. Add capability to package.json:

{
  "wasmCapabilities": {
    "resourceProvider": true
  }
}
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2. Register in your plugin's start():

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

start(config: string): i32 {
  if (registerResourceProvider('weather')) {
    debug('Registered as weather resource provider')
  }
  return 0
}
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3. Export handler functions:

// List all resources - GET /signalk/v2/api/resources/weather
export function resources_list_resources(queryJson: string): string {
  return '{"current":' + cachedData.toJSON() + '}'
}

// Get specific resource - GET /signalk/v2/api/resources/weather/{id}
export function resources_get_resource(requestJson: string): string {
  const req = ResourceGetRequest.parse(requestJson)
  if (req.id === 'current') {
    return cachedData.toJSON()
  }
  return '{"error":"Not found"}'
}
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API Access

Once registered, your resources are available at:

curl http://localhost:3000/signalk/v2/api/resources/weather
curl http://localhost:3000/signalk/v2/api/resources/weather/current
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Network Requests with Asyncify

AssemblyScript plugins can make HTTP requests using the as-fetch library with Asyncify support.

Setup

1. Add dependencies:

npm install as-fetch @signalk/assemblyscript-plugin-sdk
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2. Enable the Asyncify transform in asconfig.json:

{
  "options": {
    "bindings": "esm",
    "exportRuntime": true,
    "transform": ["as-fetch/transform"]
  }
}
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3. Declare network capability in package.json:

{
  "wasmCapabilities": {
    "network": true
  }
}
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Making Requests

import { fetchSync } from 'as-fetch/sync'

const response = fetchSync('https://api.example.com/data')

if (response && response.status === 200) {
  const data = response.text()
  // Process data...
}
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How Asyncify Works

Asyncify enables synchronous-style async code in WASM:

1. WASM execution pauses when fetchSync() is called
2. HTTP request happens in JavaScript
3. When response arrives, WASM execution resumes
4. Your code continues with the response

The Signal K runtime handles all state transitions automatically.

Troubleshooting Network Requests

fetchSync hangs or doesn't work:

• Ensure "transform": ["as-fetch/transform"] is in asconfig.json
• Use correct import: import { fetchSync } from 'as-fetch/sync'
• Verify "network": true in wasmCapabilities

Request fails:

• Check Node.js version >= 18 (required for native fetch)
• Verify the URL is accessible
• Check API keys/authentication

See the example-weather-plugin for a complete implementation.

AssemblyScript Limitations

AssemblyScript is a strict subset of TypeScript. Notable differences:

• No any type
• No union types (use tagged enums)
• No dynamic arrays (use fixed-size or manual memory)
• No standard library (console, setTimeout, etc.)
• Manual memory management

See AssemblyScript documentation for details.

Troubleshooting

Plugin doesn't load

Check that:

• wasmManifest points to correct file
• signalk-wasm-plugin keyword is present
• WASM binary is valid: file plugin.wasm

Compilation errors

Common issues:

• Using disallowed TypeScript features
• Missing type annotations
• Incorrect memory operations

Runtime errors

Check server logs:

DEBUG=signalk:wasm:* npm start
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Additional Resources

• AssemblyScript Documentation
• Example Plugins