Add electron

backend/src/DataSource/DataSource.ts

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+import { DataSourceState } from './'
+
+type MessageCallback = (topic: string, payload: Buffer) => void
+
+// A DataSource should automatically reconnect if connection was broken
+interface DataSource<DataSourceOptions> {
+  connect(options: DataSourceOptions): DataSourceState
+  disconnect(): void
+  onMessage(messageCallback: MessageCallback): void
+  topicSeparator: string
+}
+
+export { DataSource, MessageCallback }

backend/src/DataSource/DataSourceState.ts

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+interface InternalState {
+  connecting: boolean
+  connected: boolean
+  error?: Error
+}
+
+export class DataSourceState {
+  private state: InternalState = {
+    error: undefined,
+    connected: false,
+    connecting: false
+  }
+
+  public setConnected(connected: boolean) {
+    this.state = {
+      error: undefined,
+      connected: connected,
+      connecting: false
+    }
+  }
+
+  public setError(error: Error) {
+    this.state = {
+      error: error,
+      connected: false,
+      connecting: false
+    }
+  }
+
+  public setConnecting() {
+    this.state = {
+      error: undefined,
+      connected: false,
+      connecting: true
+    }
+  }
+
+  public toJSON() {
+    return this.state
+  }
+}

backend/src/DataSource/MqttSource.ts

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+import { Client, connect as mqttConnect } from 'mqtt'
+import { DataSource, DataSourceState } from './'
+
+export interface MqttOptions {
+  url: string
+}
+
+export class MqttSource implements DataSource<MqttOptions> {
+  private client: Client | undefined
+  private messageCallback?: (topic: string, message: Buffer) => void
+  private rootSubscription = '#'
+  public topicSeparator = '/'
+
+  public onMessage(messageCallback: (topic: string, message: Buffer) => void) {
+    this.messageCallback = messageCallback
+  }
+
+  public connect(options: MqttOptions): DataSourceState {
+    const state = new DataSourceState()
+
+    const client = mqttConnect(options.url, {
+      resubscribe: false
+    })
+
+    this.client = client
+
+    client.on('error', (error: Error) => {
+      state.setError(error)
+    })
+
+    client.on('close', () => {
+      state.setConnected(false)
+    })
+
+    client.on('reconnect', () => {
+      state.setConnecting()
+    })
+
+    client.on('connect', () => {
+      state.setConnected(true)
+      client.subscribe(this.rootSubscription, (err: Error) => {
+        if (err) {
+          throw new Error('mqtt subscription failed')
+        }
+      })
+    })
+
+    client.on('message', (topic, message) => {
+      this.messageCallback && this.messageCallback(topic, message)
+    })
+
+    return state
+  }
+
+  public disconnect() {
+    this.client && this.client.end()
+  }
+}

backend/src/DataSource/index.ts

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+import { DataSource } from './DataSource'
+import { DataSourceState } from './DataSourceState'
+import { MqttOptions, MqttSource } from './MqttSource'
+
+export {
+  DataSource,
+  DataSourceState,
+  MqttOptions,
+  MqttSource,
+}