Select specific IPC messages

[ehmicky]

Background

A very common IPC pattern is to retrieve a message, but only when it matches a specific shape. For example, if messages have a type field, only events of a specific type.

This is currently easy to do with subprocess.getEachMessage().

for await (const message of subprocess.getEachMessage()) {
  if (message.type === 'example') {
    // ...
  }
}

When async iterator helpers will be available in Node, the following functional alternative will be available too:

for await (const message of subprocess
  .getEachMessage()
  .filter(message => message.type === 'example')) {
  // ...
}

Problem

However, it is currently not simple to do the same with subprocess.getOneMessage(), i.e. when retrieving only one message this way.

Using a while loop is slightly cumbersome. But more importantly, if multiple messages are received at the same time (more precisely, if the message event if emitted multiple times in a single tick), then some messages will be lost and never received.

let message;
while (true) {
  message = await subprocess.getOneMessage();
  if (message.type === 'example') {
    break;
  }
}

Solution

Adding a filter option to subprocess.getOneMessage().

const message = await subprocess.getOneMessage({filter: message => message.type === 'example'});