Bluetooth

This feature is only available at request - get free access by emailing us at ble@despia.com

Drive a full BLE central stack from JavaScript, scan for peripherals, connect to them, discover services and characteristics, read and write values, and subscribe to notifications. Connection state and notification data are delivered through global window callbacks, with automatic persistence and replay when the app is backgrounded or closed.

Foreground BLE works out of the box. Background scanning, background connections, and background notifications require the Bluetooth addon to be enabled in Despia Editor > App > Addons, followed by a fresh native build.

Installation

Bundle
CDN

npm install despia-native

pnpm add despia-native

yarn add despia-native

import despia from 'despia-native';

<script src="https://cdn.jsdelivr.net/npm/despia-native/index.min.js"></script>

<script type="module">
    import despia from 'https://cdn.jsdelivr.net/npm/despia-native/+esm'
</script>

How it works

Commands are dispatched through despia(). Results, events, and data come back through global window callbacks that you define before issuing any command. The native side never buffers foreground events, so a callback defined after the event fires misses that event.

const isDespia = navigator.userAgent.toLowerCase().includes('despia')

if (isDespia) {
    window.onBleDevice = device => {
        console.log('found:', device.name, device.id, device.rssi)
    }

    window.onBleScanEnd = () => {
        console.log('scan complete')
    }

    despia('bluetooth://scan?duration=10000')
}

The first BLE call triggers the iOS Bluetooth permission prompt. After the user responds, the bridge sets window.bluetoothActive to true on grant and fires onBleState with the current power state.

Permission and power state

Permission is exposed as window.bluetoothActive (boolean). Power and permission combined come through onBleState, which fires on every change without polling, including mid-session permission revoke from the system Settings app.

window.onBleState = s => {
    // s.state: 'on' | 'off' | 'unauthorized' | 'unsupported'
    if (s.state === 'on')           enableScanUI()
    if (s.state === 'off')          showTurnOnBluetoothHint()
    if (s.state === 'unauthorized') showPermissionDeniedUI()
    if (s.state === 'unsupported')  hideBleFeatures()
}

function bleReady() {
    return window.bluetoothActive === true
}

if (isDespia) despia('bluetooth://state')

scan and connect issued while the radio is off are deferred internally and run automatically once Bluetooth is powered on. Every other command silently no-ops if its precondition is not met (read before connect, write before discover, etc).

Scanning for devices

Filter by service UUID rather than by name, iOS does not always include the advertised name. Pass a comma-separated UUID list, omit it to scan everything. Bound every scan with a duration (milliseconds) so it stops automatically.

const HR_SERVICE = '180d'

window.onBleDevice = device => {
    // device.id is the opaque peripheral UUID, not a MAC address
    // device.services may be empty even when the device has services
    addToList(device.id, device.name, device.rssi)
}

window.onBleScanEnd = () => {
    setScanningUI(false)
}

if (isDespia) {
    despia(`bluetooth://scan?services=${HR_SERVICE}&duration=10000`)
}

// stop early
function cancelScan() {
    if (isDespia) despia('bluetooth://stopscan')
}

A new scan clears the previous scan’s discovered-device cache. iOS suspends scanning in the background, so always keep scans short and foreground.

Connecting and discovering services

Pass the id from an onBleDevice event into connect. After connected fires, call discover to retrieve the full service and characteristic tree. Use the properties array on each characteristic to decide between read, write, and subscribe.

let deviceId = null

window.onBleConnect = e => {
    if (e.state === 'connected') {
        despia(`bluetooth://discover?id=${encodeURIComponent(e.id)}`)
    } else if (e.state === 'failed') {
        console.error('connect failed:', e.error)   // 'timeout' | 'unknown device' | system error
    } else if (e.state === 'disconnected') {
        console.warn('disconnected:', e.error)
    }
}

window.onBleDiscovered = tree => {
    tree.services.forEach(svc => {
        svc.characteristics.forEach(chr => {
            if (chr.properties.includes('notify')) {
                subscribe(svc.uuid, chr.uuid)
            }
        })
    })
}

function connect(id) {
    deviceId = id
    if (isDespia) {
        despia(`bluetooth://connect?id=${encodeURIComponent(id)}&timeout=10000`)
    }
}

function disconnect() {
    if (isDespia && deviceId) {
        despia(`bluetooth://disconnect?id=${encodeURIComponent(deviceId)}`)
    }
}

Pass auto_connect=true to have the bridge re-issue the connection automatically after an unexpected disconnect. An explicit disconnect clears this.

despia(`bluetooth://connect?id=${encodeURIComponent(id)}&auto_connect=true&timeout=10000`)

Reading and writing characteristics

A read resolves to onBleData with source: 'read'. Writes accept three payload formats, pick exactly one, the bridge never guesses. Precedence if more than one is supplied is text then hex then value.

Format	Parameter	Example
UTF-8 plain string	`text`	`text=Hello%20world`
Hex string, tolerates `:-` spaces and optional `0x`	`hex`	`hex=DE%3AAD%3ABE%3AEF`
Base64, the binary contract	`value`	`value=3q2%2B7w%3D%3D`

const SVC = '12345678-1234-1234-1234-1234567890ab'
const CHR = 'abcd1234-ab12-cd34-ef56-abcdef123456'

window.onBleData = p => {
    if (p.source === 'read') {
        // p.value is base64, p.valueHex is upper-case hex
        const bytes = Uint8Array.from(atob(p.value), c => c.charCodeAt(0))
        console.log('read bytes:', bytes)
    }
}

window.onBleWriteComplete = w => {
    console.log(w.success ? 'sent' : `write error: ${w.error}`)
}

function readChar(id) {
    despia(`bluetooth://read?id=${encodeURIComponent(id)}&service=${SVC}&char=${CHR}`)
}

function writeText(id, text) {
    despia(
        `bluetooth://write?id=${encodeURIComponent(id)}` +
        `&service=${SVC}&char=${CHR}` +
        `&text=${encodeURIComponent(text)}` +
        `&with_response=false`
    )
}

Write with response acknowledges only after the peripheral confirms. Write without response fires onBleWriteComplete immediately on dispatch, BLE has no transport-level ack for this mode, so success: true confirms the bytes left the radio, not that the peripheral received them. If the characteristic does not support write-with-response, the bridge falls back automatically.

Subscribing to notifications

Notifications are the only event source that survives the app being backgrounded or terminated. Subscribe after connected, and define a handler that works for both live events and replayed ones.

const HR_SVC = '0000180d-0000-1000-8000-00805f9b34fb'
const HR_CHR = '00002a37-0000-1000-8000-00805f9b34fb'

function handleHeartRate(p) {
    const bytes = Uint8Array.from(atob(p.value), c => c.charCodeAt(0))
    const bpm   = bytes[1]
    renderBpm(bpm, p.background, p.timestamp)
}

window.onBleData = p => {
    if (p.source === 'notification') handleHeartRate(p)
}

function subscribeHeartRate(id) {
    despia(
        `bluetooth://subscribe?id=${encodeURIComponent(id)}` +
        `&service=${HR_SVC}&char=${HR_CHR}`
    )
}

function unsubscribeHeartRate(id) {
    despia(
        `bluetooth://unsubscribe?id=${encodeURIComponent(id)}` +
        `&service=${HR_SVC}&char=${HR_CHR}`
    )
}

Background delivery and replay

When the app is not active, window.onBleX callbacks do not fire live. The bridge persists every event to disk (capped at the most recent 500, oldest dropped) and replays them in FIFO order through window.onBleEvent on the next app activation. Each replayed payload carries background: true and event: '<original callback name>'.

function handleHeartRate(p) {
    const bytes = Uint8Array.from(atob(p.value), c => c.charCodeAt(0))
    renderBpm(bytes[1], p.background, p.timestamp)
}

// foreground path
window.onBleData = p => {
    if (p.source === 'notification') handleHeartRate(p)
}

// replay path, fires on next app activation for everything that happened while away
window.onBleEvent = e => {
    if (e.event === 'onBleData' && e.source === 'notification') handleHeartRate(e)
    if (e.event === 'onBleConnect') updateConnectionUI(e)
}

Make handlers idempotent and key them off id, char, and timestamp, an event captured right at the foreground-to-background boundary can occasionally surface through both paths. Always implement onBleEvent if anything that happens while backgrounded matters to your app. Scanning generally does not progress in the background on iOS, connections and notifications do once the Bluetooth addon is enabled.

Backend mirroring with server POST

Pass server=<url> to connect and subscribe to have the bridge POST every connection-state change and every notification to your backend as JSON. This runs independently of the web layer and continues to fire when the app is backgrounded or closed, so your server receives data even when no callback can run.

const SERVER = 'https://api.example.com/ble'

function startMonitoring(id) {
    despia(
        `bluetooth://connect?id=${encodeURIComponent(id)}` +
        `&auto_connect=true` +
        `&server=${encodeURIComponent(SERVER)}`
    )
}

window.onBleConnect = e => {
    if (e.state === 'connected') {
        despia(
            `bluetooth://subscribe?id=${encodeURIComponent(e.id)}` +
            `&service=${HR_SVC}&char=${HR_CHR}` +
            `&server=${encodeURIComponent(SERVER)}`
        )
    }
}

Two payload shapes hit your endpoint, both as application/json POSTs:

{
  "event": "ble_data",
  "deviceId": "E2C56DB5-...",
  "service": "0000180d-0000-1000-8000-00805f9b34fb",
  "characteristic": "00002a37-0000-1000-8000-00805f9b34fb",
  "value": "Bkg=",
  "valueHex": "0648",
  "timestamp": 1747391284.523,
  "background": true,
  "battery": 83
}

{
  "event": "ble_state",
  "deviceId": "E2C56DB5-...",
  "state": "disconnected",
  "timestamp": 1747391284.523,
  "background": true,
  "battery": 83,
  "error": "The connection has timed out unexpectedly."
}

battery is the device battery as an integer 0 to 100, or -1 if unavailable. Reads are intentionally not mirrored, only notifications and connection-state changes. The iOS POST is fire-and-forget with no retry, treat it as best-effort telemetry rather than a guaranteed-delivery channel and rely on the in-app event plus replay for correctness.

RSSI and signal strength

window.onBleRssi = r => {
    updateSignalBars(r.id, r.rssi)   // r.rssi is dBm, negative
}

function pollRssi(id) {
    if (isDespia) despia(`bluetooth://rssi?id=${encodeURIComponent(id)}`)
}

End-to-end, ESP32 text display

Scan filtered by service, connect to the first matching peripheral, discover, write a UTF-8 string without response.

const SVC = '12345678-1234-1234-1234-1234567890ab'
const CHR = 'abcd1234-ab12-cd34-ef56-abcdef123456'
let deviceId = null

window.onBleDevice = d => {
    if (d.name === 'PDLOC-SCREEN' && !deviceId) {
        deviceId = d.id
        despia('bluetooth://stopscan')
        despia(`bluetooth://connect?id=${encodeURIComponent(deviceId)}&timeout=10000`)
    }
}

window.onBleConnect = e => {
    if (e.id !== deviceId) return
    if (e.state === 'connected') {
        despia(`bluetooth://discover?id=${encodeURIComponent(deviceId)}`)
    } else if (e.state === 'failed') {
        console.error('connect failed:', e.error)
    }
}

window.onBleDiscovered = () => {
    send('ROAD=Cliftmont Avenue|BLOCK=3500 block')
}

window.onBleWriteComplete = w => {
    console.log(w.success ? 'sent' : `write error: ${w.error}`)
}

function send(text) {
    despia(
        `bluetooth://write?id=${encodeURIComponent(deviceId)}` +
        `&service=${SVC}&char=${CHR}` +
        `&text=${encodeURIComponent(text)}` +
        `&with_response=false`
    )
}

if (isDespia) {
    despia(`bluetooth://scan?services=${SVC}&duration=15000`)
}

Editor setup for background BLE

Foreground scan, connect, read, write, and subscribe work without any Editor configuration. Enable the addon when you need notifications or connections to survive the app being backgrounded or closed.

Enable the Bluetooth addon

Open the Despia Editor, go to App → Addons, and toggle Bluetooth on.

Rebuild the native app

Trigger a fresh build from the Despia Editor and reinstall on device. The addon is compiled into the binary, it cannot be applied over the air.

Verify background delivery

Connect to a peripheral with auto_connect=true, subscribe to a notifying characteristic, background the app, then trigger a notification from the peripheral. On next app open, onBleEvent should replay the queued events with background: true.

Skipping the rebuild is the most common silent failure. The Editor toggle appears enabled, foreground BLE still works, but notifications stop arriving the moment the app is backgrounded and onBleEvent returns an empty queue on next open. Any change to the Bluetooth addon, the integration credentials, or any other Editor setting requires a fresh native build to take effect.

UUID formats and case

Three input formats are accepted, 16-bit (180d), 32-bit (8 hex), and 128-bit dashed (12345678-1234-1234-1234-1234567890ab). Other shapes cause the command to silently no-op. UUIDs returned in callback payloads are always lower-cased 128-bit strings, compare case-insensitively. valueHex is the one exception, it is upper-case hex.

Gotchas

id is an opaque per-iOS-device UUID, not a MAC address, and can change across reinstalls. Re-scan to obtain a fresh id rather than hard-coding it.
Always define window.onBleX callbacks before issuing any command. There is no foreground buffering.
Always call discover before read, write, or subscribe. Pre-discovery calls no-op.
Write-without-response success: true confirms dispatch, not delivery. Use write-with-response or an application-level ack when correctness matters.
iOS auto-negotiates ATT MTU on connect, small writes (tens of bytes) are fine. Chunk at the application layer for payloads beyond roughly 180 bytes.
Server POST from iOS is fire-and-forget. Treat it as best-effort and rely on the in-app event plus replay for guaranteed delivery.
Always bound scan with duration or call stopscan. An unbounded scan drains the battery.

Resources

NPM Package

despia-native

Support

support@despia.com

Deployment

Best Practices

Local Ecosystem

Authentication

Payments

Analytics

Connectivity

Machine Learning

Wallets

Native Features

Editor

Roadblocks

Store Rejections

Platform Support

Installation

How it works

Permission and power state

Scanning for devices

Connecting and discovering services

Reading and writing characteristics

Subscribing to notifications

Background delivery and replay

Backend mirroring with server POST

RSSI and signal strength

End-to-end, ESP32 text display

Editor setup for background BLE

UUID formats and case

Gotchas

Resources

NPM Package

Support

​Installation

​How it works

​Permission and power state

​Scanning for devices

​Connecting and discovering services

​Reading and writing characteristics

​Subscribing to notifications

​Background delivery and replay

​Backend mirroring with server POST

​RSSI and signal strength

​End-to-end, ESP32 text display

​Editor setup for background BLE

​UUID formats and case

​Gotchas

​Resources

NPM Package

Support

Installation

How it works

Permission and power state

Scanning for devices

Connecting and discovering services

Reading and writing characteristics

Subscribing to notifications

Background delivery and replay

Backend mirroring with server POST

RSSI and signal strength

End-to-end, ESP32 text display

Editor setup for background BLE

UUID formats and case

Gotchas

Resources