Rx.js, Chrome Messaging API

Question

After another part of the extension has sent a message using this code

chrome.runtime.sendMessage({greeting: "hello"});

Any Rx.js experts out there who can take this messaging API

chrome.runtime.onMessage.addListener(
  function(request, sender, sendResponse) {
    console.log(sender.tab ?
                "from a content script:" + sender.tab.url :
                "from the extension");
    if (request.greeting == "hello")
      sendResponse({farewell: "goodbye"});
});

and wrap it in an Observable that emits new messages and allows a sendResponse?

Simply emitting incoming messages as an Observable is very easy.

const MessagingObservable = Rx.Observable.create(observer => {      
  chrome.runtime.onMessage.addListener(listener);
  function listener(request, sender, sendResponse) {
    observer.next(request); 
  }
  return () => {
    chrome.runtime.onMessage.removeListener(listener);
  };
});

But how to bind the sendResponse callback?

Answer 1

You can use the built-in fromEventPattern function to create an observable, like this:

const messages = Rx.Observable.fromEventPattern(
  handler => chrome.runtime.onMessage.addListener(handler),
  handler => chrome.runtime.onMessage.removeListener(handler),
  (request, sender, sendResponse) => ({ request, sender, sendResponse })
);

Note that the call to fromEventPattern includes a result selector, so that values emitted by the observable contain the request, the sender, and the sendResponse, which you'd use like this:

messages.subscribe(({ request, sender, sendResponse }) => {
  console.log(request);
  sendResponse(/* ... whatever ... */);
});

To support calling sendResponse asynchronously, the listener needs to be able to return true. This can be done by wrapping the handler, like this:

const messages = Rx.Observable.fromEventPattern(
  handler => {
    const wrapper = (request, sender, sendResponse) => {
      const event = { async: false, request, sender, sendResponse };
      handler(event);
      return event.async;
    };
    chrome.runtime.onMessage.addListener(wrapper);
    return wrapper;
  },
  (handler, wrapper) => chrome.runtime.onMessage.removeListener(wrapper)
);

And you'd use it like this:

messages.subscribe(event => {
  console.log(event.request);
  event.async = true;
  setTimeout(() => event.sendResponse(/* ... whatever ... */), 1000);
});