1function DataStream({ source }) {
2  const [data, setData] = useState([]);
3  const [isActive, setIsActive] = useState(true);
4
5  useEffect(() => {
6    let subscription;
7    
8    if (isActive) {
9      subscription = source.subscribe({
10        next: (value) => {
11          setData(prev => [...prev, value]);
12        },
13        error: (err) => console.error(err)
14      });
15    }
16    
17    // Complete the cleanup
18    return /* MISSING CODE */
19  }, [source, isActive]);
20
21  return (
22    <div>
23      <button onClick={() => setIsActive(!isActive)}>
24        {isActive ? 'Stop' : 'Start'}
25      </button>
26      <ul>{data.map((item, i) => <li key={i}>{item}</li>)}</ul>
27    </div>
28  );
29}