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The Cosmos object also exposes a ws(network) method that allows you to interact with the WebSocket API of a full node. Some WebSocket endpoints have overlap with the REST API, but others are unique to the WebSocket API. Most importantly, it abstracts block and transaction subscriptions through the .onBlock and .onTx methods. Beware that most public nodes enforce a strict rate limit of 5 subscriptions per client. If you require more, you will need to run your own isolated full node. However, typically, 5 subscriptions is enough for most Dapps. 
import { Cosmos, TendermintQuery } from '@apophis-sdk/cosmos';

const network = await Cosmos.getNetworkFromRegistry('neutrontestnet');

const ws = Cosmos.ws(network);
ws.onBlock(block => {
  console.log('New block:', block.header.height);
});

let query = new TendermintQuery().exact('tx.hash', '<my-hash>');

ws.onTx(query, tx => {
  console.log('Tx confirmed:', tx.hash);
});
It also exposes an expectTx method which you can use to await the confirmation of a transaction: 
const tx = Cosmos.tx([/* ... */]);

const result = await ws.expectTx(tx);
console.log(result);

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Tendermint Queries

Tendermint is underlying the Cosmos SDK and provides a way for full nodes to index and query transactions based on events and their attributes emitted by the Cosmos SDK modules and smart contracts. The TendermintQuery class is a builder for Tendermint queries. It provides a fluent interface for building queries and is a first-class citizen in Apophis’ WebSocket API. 
const address = 'neutron1...';
const query = new TendermintQuery().exact('message.sender', address);

ws.onTx(query, tx => {
  console.log(`Tx sent by ${address}:`, tx.hash);
});
There are some built-in query keys:
  • block.height
  • message.sender
  • TODO: List built-in query keys
But you can also query by attributes emitted on the Response of your smart contract: TODO: Example Sorry for the incomplete state of this documentation. The missing parts I need to re-discover myself. :)