Subscribe and unsubscribe
Subscribe
Subscribe to a topic
In order to start receiving messages from your provider, you need to subscribe to a topic as follows;
pubsub.subscribe({ topics: 'myTopic' }).subscribe({ next: (data) => console.log('Message received', data), error: (error) => console.error(error), complete: () => console.log('Done')});Following events will be triggered with subscribe()
| Event | Description |
|---|---|
next | Triggered every time a message is successfully received for the topic |
error | Triggered when subscription attempt fails |
complete | Triggered when you unsubscribe from the topic |
Subscribe to multiple topics
To subscribe for multiple topics, just pass a String array including the topic names:
pubsub.subscribe({ topics: ['myTopic1', 'myTopic1'] }).subscribe({ //...});Unsubscribe
To stop receiving messages from a topic, you can use unsubscribe() method:
const sub1 = pubsub.subscribe({ topics: 'myTopicA' }).subscribe({ next: (data) => console.log('Message received', data), error: (error) => console.error(error), complete: () => console.log('Done')});
sub1.unsubscribe();// You will no longer get messages for 'myTopicA'Subscription connection status updates
Now that your application is setup and using pubsub subscriptions, you may want to know when the subscription is finally established, or reflect to your users when the subscription isn't healthy. You can monitor the connection state for changes via Hub.
import { CONNECTION_STATE_CHANGE, ConnectionState } from '@aws-amplify/pubsub';import { Hub } from 'aws-amplify/utils';
Hub.listen('pubsub', (data: any) => { const { payload } = data; if (payload.event === CONNECTION_STATE_CHANGE) { const connectionState = payload.data.connectionState as ConnectionState; console.log(connectionState); }});Connection issues and automated reconnection
const fetchRecentData = () => { // Retrieve recent data from some sort of data storage service}
let priorConnectionState: ConnectionState;
Hub.listen("pubsub", (data: any) => { const { payload } = data; if ( payload.event === CONNECTION_STATE_CHANGE ) {
if (priorConnectionState === ConnectionState.Connecting && payload.data.connectionState === ConnectionState.Connected) { fetchRecentData(); } priorConnectionState = payload.data.connectionState; }});
pubsub.subscribe('myTopic').subscribe({ next: data => // Process incoming messages})