Event Watching Web3 v1 in Truffle test

Question

I want to test off chain data input in truffle testing. So basically I kick off a test and then need to wait for another program to do something. In the old web3, this worked great:

function promisifyLogWatch(_event) {
  return new Promise((resolve, reject) => {
    _event.watch((error, log) => {
      _event.stopWatching();
      if (error !== null)
        reject(error);
      resolve(log);
    });
    });
}

And I test it in my js file like this:

await promisifyLogWatch(oracle.NewValue({ fromBlock: 'latest' }));

But now I get that _event.watch is not a function. How do I update this for web3 v1?

Answer 1

This is a pattern I use as well to test Oraclize callback functions. I'm using something like this in a presentation at EthCC next week, so today I had to figure out how to do this in newer versions of Truffle. I ended up doing the following:

const getFirstEvent = (_event) => {
  return new Promise((resolve, reject) => {
    _event.once('data', resolve).once('error', reject)
  });
}

I remembered this post from a while back, so I came back to add this as an answer.